Cathedral Ceilings – Mold and Moisture – Problems and Solutions


Rotten roof deck

Disclaimer: while I think all the information presented here is accurate and scientifically valid, you are advised to consult a *professional before changing your home. This article covers just one component of your home. Your specific home may have conditions that override the comments contained herein.

*By professional, I mean an experienced building scientist, not your local carpenter or roofer or even a structural engineer or architect. While many of these people are artists in what they do, most have no training in building science or engineering and cannot be trusted to properly design a roof assembly. Likewise, you wouldn’t hire a building scientist to swing a hammer and build your roof!
Cathedral ceilings are very popular – they give rooms a feeling of openness and an added aesthetic dimension. At the same time, they are responsible for a variety of building problems and homeowner heartbreak. What causes these problems and how do you avoid them?

There are a variety of climate zones. The south-eastern United States is hot and humid, while the north east is cold. The mid-Atlantic states, where I live, is mixed – during the summer it is hot and humid, during the winter it is cold. The south west is mostly hot and dry and the northwest is moderate in temperature but very humid! Each of these climate zones has its own particular building details. However, all must follow the laws of physics.

Physics tells us that moisture moves from areas of high humidity to areas of low humidity. If it’s more humid outside, moisture wants to come in. And when it’s more humid inside, the moisture will move toward the outside. Simple!

The trick is, the amount of moisture that air can hold depends on the temperature of the air. Warm air holds more moisture than cold air. And, at some temperature, the air reaches a point where it can’t hold more moisture. This temperature is called the dew point.

The next thing to know is that moisture in air is water vapor. Water vapor is much smaller than liquid water, so vapor moves through building materials much more easily than liquid. Water vapor is also lighter than air, so it rises up to the ceiling and through any cracks or holes. Warm air also rises, so there is a tendency for warm, humid air to exert lots of pressure on the ceiling. These concepts are also pretty simple. Just remember – humid air and warm air rise.

So what happens during the winter when you heat your house? The warm air in the house contains lots of water vapor. That warm air and water vapor rises up to the ceiling. If there are holes, like electrical boxes, recessed lights or ceiling fans, the vapor easily moves up into the space above the ceiling. What looks like a little hole to you looks like an open window to the tiny water vapor molecules.

Let’s look at a diagram of a typical cathedral ceiling during the winter….

Typical fiberglass insulated ceiling

The blue lines represent moisture trying to move from the inside of the house, where the air is warm, to the outside, where the air is cold and dry.

For most of the ceiling, the moisture encounters painted sheet rock, which stops air flow and most of the water vapor. However, a small amount of the vapor travels through the sheet rock slowly (thin blue lines). Where there are any holes, such as at recessed lights, air moves through the holes, carrying water vapor right up into the insulation. In fact, a small hole can transmit tens or hundreds of times more water into the ceiling cavity than moves through the the painted sheet rock.

As the water vapor moves through the insulation towards the cold roof and outside air, it encounters colder and colder temperatures. If the temperature reaches the dew-point, the water vapor condenses  into liquid water and can drip back down through the insulation and back onto the ceiling, causing water damage. Some people have experienced it literally raining in their living room when this happens!

In order to combat this effect, builders created the vented ceiling. This is a space between the insulation and the roof deck. At the bottom, there would be a soffit vent and at the top, a ridge vent. Such a construction allows airflow through the cavity. This airflow is supposed to carry away moisture that builds up inside the cavity. It is also intended to keep the roof deck cold to avoid snow melt and ice dams

Problems with the conventional vented roof

Problem 1: Moisture Buildup Within the Insulation

Thermal image showing air leaks

In moderate and sunny climates the roof may heat up during the day, warming it enough to carry away the water vapor and dry out the insulation. However, what if the roof doesn’t get much sun or the moisture moves through holes in the ceiling faster than it can get carried away?

The moisture in the ceiling cavity can accumulate until it leads to ceiling damage. This is prevalent when the ceiling has many recessed lights or, uses tongue and groove boards instead of sheet rock.

The thermal image shown here demonstrates just how leaky a tongue and groove ceiling is. The dark areas show places in the ceiling that are colder than the surrounding areas. The lightest areas are where the insulation is intact and the ceiling is warm.

Problem 2: Moisture Buildup on the Back of the Roof Deck

If the moisture makes it through the insulation without condensing, it tries to move up and out of the cavity. However, this space is like a refrigerator. As the air moves through, it gets colder. Eventually, it may form ice on the back of the cold roof deck. I’ve seen many roofs rotten and covered with mold because of this.

The next photo shows an example of this problem. The home owner was replacing a recessed light when the electrician found mold. After deciding to tear off all the ceiling sheetrock, they found that almost all of the plywood roof sheathing was moldy. It should be noted that this roof was built strictly to code….using fiberglass insulation and ridge/soffit vents and a gap above the fiberglass to allow air flow.

Moldy roof deck

Problem 3: Improper Venting

Almost every roof put on these days includes a ridge vent, even if  it is not necessary. This is done because roofing manufacturers have stated that they will not honor the warranty if the roof isn’t properly vented. Unfortunately, putting a ridge vent on an old house often creates these types of problems!

Older homes used gable vents, not ridge vents. Most older home do not have soffit vents. What do you suppose happens if a ridge vent is installed on roof that has no place for air to come from?

The ridge vent still pulls air out of the attic or, in the case of a cathedral ceiling, out of ceiling cavity. That air has to come from somewhere, and that somewhere is the house. Many of the problems I have seen are caused by the ridge vent.

Building rule #1: if it ain’t broke, don’t fix it!

Building rule #2: understand what you’re trying to accomplish before doing it.
I have no problem with ridge vents IF they’re implemented along with a proper amount of soffit vents. I have huge problems with ridge vents when installed without soffit vents. I also have a problem with improper use of ridge vents where they are inappropriate.

For my own projects, I use Cor-A-Vent. They make excellent products backed up by proper engineering and supported by excellent technical documentation. If you plan on doing any roof work, I strongly recommend visiting their site and reviewing their technical literature. If you only read one document, make it this one. I’ve also copied this document so that it is available below.

Solutions

Now that you understand how and why things go wrong with cathedral ceilings (and roofs in general), let’s look at how to do them right. First, we need to know exactly what problem we’re trying to solve (building rule #2).

Refer back to the first diagram to the left – what problems exist?

  1. Warm, moist air enters ceiling cavity through holes and cracks
  2. Small amounts of moisture moves through the sheet rock
  3. Air and moisture easily move through fiberglass insulation
  4. Moist air temperature drops below the dew point
  5. Intentional airflow from ridge vent draws moisture/humidity from the house

Solution 1 – don’t put holes in your ceiling

This is obvious, so why do people insist on poking so many holes in a perfectly good ceiling?

Typical recessed light fixture

The biggest offender is the recessed light fixture. Most of these fixtures, even those rated for insulation contact, are worthless in cathedral ceilings. You may as well just drill holes in your ceiling to let the heat and moisture out.

If you absolutely must install recessed lights, buy boxed fixtures rated ICAT – Insulation Contact Air Tight. Be warned that all ICAT fixture’s are not created equal. For example, The fixture to the right is rated ICAT, but I don’t recommend this style. Much better are the fully sealed box fixtures, like the next photo.

Boxed ICAT fixture

Next, be sure to seal the fixture to the sheet rock so that it is air tight. A continuous bead of high temperature caulk greatly reduces the air lost by mounting recessed lights.

Recessed light fixtures may be the most common hole in your ceiling, but there are others that cause similar problems.

Be careful of electrical boxes mounted in the ceiling. These are usually very leaky, and can lead to even greater problems than recessed lights. These can be found above ceiling fans, smoke detectors or ceiling mounted lights.

Most of these are sealed with caulk or foam. Note that you need to do this carefully so as not to interfere with the electrical wires and/or contacts in the box. Best, have a licensed weatherization contractor deal with it.

Solution 2 – Use Insulation that Stops Air Movement

Fiberglass is a cheap insulation product, but it is not a good one in all situations. It works by reducing conductive heat transfer, but it does almost nothing to slow air movement. It doesn’t matter whether you put three inches or three feet of fiberglass insulation in the cavity, it’s not going to stop air movement and the associated moisture movement.

Unfortunately, most contractors love fiberglass. Anyone can install it and it is cheap. Granted, almost everybody installs it incorrectly – they compress it (reducing the R-value) and they install it in areas with air movement, rendering it almost useless.

If you absolutely, positively must use fiberglass, then do it right. Install it flush to the sheet rock so no air can come between the fiberglass and the ceiling. Do not compress it – if wires must be routed, split the fiberglass so the wire runs through the middle. If it has to go around obstructions, don’t compress it – cut it to the exact size and shape needed. And finally, make sure everything is sealed air tight so no air is tempted to flow through the fiberglass.

A much, much better solution is to use dense packed cellulose insulation. This is cellulose insulation that is installed to approximately 3.5 lbs./cubic foot density. At this density, cellulose does not allow air movement under normal conditions. The way it is blown in also forces it into all the nooks and crannies – around wires and pipes and fixtures. It also can be used in a “hot roof” design. With this installation, soffit and ridge vents are not used. The entire cavity is filled with cellulose. For details, see this link. A PDF of the Applegate Insulation technical bulletin is also available below.

What about moisture? Cellulose insulation can hold a tremendous amount of water, dispersing it throughout the material. In this way, like a sponge, any small amount of moisture that gets into the cellulose spreads out rather than puddling up. In a properly made roof assembly, this moisture then moves towards the shingles and out. Note – there is some controversy about using dense packed cellulose in ceiling cavities. Read this building science article by Joe Lstiburek. The problem arises when you have very porous ceilings, like the tongue and groove ceilings. Because so much moisture passes into the ceiling cavity, it can overcome the moisture flushing capacity of the insulation and roof. This is especially problematic if you have a completely vapor impermeable roof, like a metal roof or a roof covered with a rubber membrane (like all flat roofs). To avoid problems, listen to Joe and don’t dense pack your cathedral ceiling if you have any doubts.

Even better than cellulose insulation is high density sprayed polyurethane foam. This foam creates an air-impervious barrier and is also very effective at slowing vapor movement when applied at adequate thicknesses (greater than about 2 inches). Because of these properties, foam is usually applied using the hot roof (no venting) method. Note that the amount of insulation needed depends upon your climate zone. Colder climates need more insulation.

Polyurethane foam also has an excellent R-value, about twice that of fiberglass or cellulose.

Solution 3: Understand the Physics

Usually, when I explain the unvented roof to people, they ask “where does all the moisture that gets in there go?” To this I reply – where does all the moisture in your house go? Are your walls rotting out? Has your floor collapsed recently? Is there mold growing anywhere? Mostly, they say “no – but that’s not the same.

In fact, it is exactly the same! The reason that your house doesn’t rot or have mold growing everywhere is because the humidity of the air isn’t high enough to cause condensation on normal surfaces. In the same way, when you apply foam, the air cannot come in contact with a surface cold enough for condensation to occur (unless you keep your house like a greenhouse!) The humidity within the ceiling cavity is the same as it is inside the house so you have no problems.

Except for one….

There is an unfortunate technique called “flash and batt” that has become popular among builders because it allows them to air seal using spray foam but keep the cost down by providing the majority of the R-value with cheap fiberglass. Unfortunately, many of these insulation contractors do not understand physics. In the cathedral ceiling example, they spray a thin layer of foam to the bottom of the roof deck, air sealing the cavity from the top. Then, they fill the cavity with fiberglass.

What’s wrong with this picture? The fiberglass allows the warm air from the house to move into the cavity, through the fiberglass. But, the air cools as it gets close to the thin layer of foam on the roof deck. If the foam is not thick enough (usually the case), then it will be very cold. Now, the moisture in the air condenses on the inner surface of the spray foam. Even worse, the air and moisture sealing properties of the foam then locks this liquid water into the ceiling cavity. Before you know it – rotten ceiling or rotten roof.

Now, you can do flash-and-batt in a way that it works, more or less. Remember the physics – we want to stop the air and moisture movement and prevent the water vapor from coming in contact with cold surfaces. To do this, you build your ceiling, install wires and fixtures and then spray the back side of the ceiling with foam. This seals everything and keeps the moisture in the house. The problem is, this means putting the roof on last! This is never done because you need the roof on as soon as possible to keep the weather out while you’re building the house.

So, if anybody says you can save a lot of money by doing flash-and-batt for your ceiling, send them packing – they don’t understand the physics of insulation and you’ll probably end up having to buy a new roof in a few years because your builder cheaped out on insulation.

Side note: Walls

Think about walls. They don’t require ventilation. They’re supposed to be sealed tight, and they don’t rot out (except when they leak, but that’s another story.) Why do we build ceilings differently than walls? Well, there is a slight difference – remember that warm, moist air rises. So ceilings are more likely to have warm, high humidity conditions than walls. However, the same physics applies.

As noted above, flash-and-batt is becoming more popular, but the same rules that apply to its installation in ceilings applies to it in walls – the foam must be sprayed against the backside of the sheet rock on the wall. It must not be applied to the outer wall sheathing with fiberglass on the inside. This is almost certain to lead to rotten, moldy walls. But, if you don’t do it the cheap way, then just spray the foam against the sheathing, but use enough of it so that the inner surface never gets cold.

Final Comments – Understanding R-Value

If you’re interested in more physics, see my notes on insulation and heat transfer.

Related articles and discussions

Caveat – if you read through discussion groups on this topic, you will find a lot of bad information. Information from builders, architects, engineers. I’ve sorted through the garbage and only included links to sources that appear correct. If you need to, go back and read “solution 3” above. The physics are simple. But the details can be complicated since the exact roof structures vary considerably and the devil’s in the details!

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111 thoughts on “Cathedral Ceilings – Mold and Moisture – Problems and Solutions

  1. Hello Ted, thank you very much for this wonderful information. I came across this site after a couple of weeks of trying to find answers to a unique situation. My home was built in 1958, and i believe my problems are have to do with the fact i have no attic and no ventilation.

    I have cathedral ceilings on a one story house and absolutely no attic. I have No intake or exhaust ventilation; I have a sandwich of tongue/groove ceiling, with a 4.5 inch Polyisocyanurate rigid foam that runs from ridge down to the eaves/fascia, followed by plywood, tar paper and asphalt shingles.

    I have lots of air bubbles/pockets and several water marks on ceiling with some rot on a couple of plywood boards. We purchased the home about 5yrs ago and passed a roof inspection. We started noticing some of the walls sweat and drips fall from ceiling rafters once in a while about 2 yrs ago. I’m attributing this to condensation.

    My plan is to reduce the foam from the eaves (approx 18-24 inches) so that i can put some intake vents, then add a ridge vent to accomplish a 50-50 NFVA. I will also replace any rotten wood and shingles. However, my question is – with the Polyisocyanurate rigid foam tightly packed throughout with very tiny gaps maybe 1/4 inch whats the best way to accomplish the NFVA to meet 1/300? or am i headed in the wrong direction?

    BTW: I’ll be happy to pay a consultation fee if needed or if you can recommend a Building Scientist in the bay area that would be great.
    Once again, great article!

    • Thanks, I appreciate the feedback.
      With that much polyiso, you shouldn’t be getting condensation the inner surface of the foam is getting cold.
      I’m not sure I understand your description. The “bubbles/pockets” are where? You noted that the interior is T&G ceiling on top of the polyiso insulation. Not sure where the bubbles are appearing. Where are the plywood boards you’re seeing the rot on? It sounds like that’s the roof sheathing. But if you’ve got 4.5″ of polyiso under the roof sheathing, unless it’s installed very loosely, it shouldn’t be allowing enough moisture to get up to the sheathing to cause moisture problems.
      With T&G, if you add ventilation, you actually run the risk of drawing even more moisture into the cavity. I would be more inclined to determine how the polyiso is allowing moisture to get up to your roof sheathing. Do you have recessed lights where the polyiso is cut away to make room for the lights? Any other way air/moisture could be getting into the cavity?

      Where are you located? I can search a bit in your area for someone who looks qualified to help you diagnose the problem. From your description, I suspect that your roof rot problems will continue until you reduce the amount of air that is flowing past the T&G ceiling and into the cavity. Unfortunately, this might require redoing your ceiling if there’s no drywall or solid air barrier under the T&G ceiling.

      • Thanks for the quick reply and my apologies for the confusion.
        – The air pockets are on the roof shingles.
        – I’m in Fairfield CA, between Sacramento and San Francisco. – Yes the sheathing is where I found thee rot. I found water stain in a closet and went up on the roof to remove shingles and found the rot. One thing I should note about this area of the roof is that it’s next to a pine tree that drops a ton of pine needles throughout so perhaps this is trapping the air.
        – not sure how old the roof
        – the slope is 2:12

        Thanks

  2. Ted,

    Very informative article. I wonder if I could ask you a few questions about my situation.

    Situation:
    I live in Seattle. I have cathedral ceilings with 2×6 rafters, filled with ~3 inches of rockwool and venting space at the top. The house was built in 1962 and unfortunately has T&G boards nailed directly to the trusses; therefore, no air barrier. There is an internal, non-structural chimney a few feet below the ridge. Recently, I discovered water dripping down the chimney. Upon opening up the roof, there was severe sheathing and rafter rot at the top of the ridge. I’m unsure if the rot was due to condensation or from rain due to inadequate flashing around the chimney.

    My plan:
    Because I want to remove the chimney from floor plan anyway, I will demo the chimney to remove one potential area to leak. For energy efficiency, my plan is to convert to an unvented roof. To do this, I will put in the following layers:
    1) R23 rockwool in the assembly to allow it to dry to the interior.
    2) A layer of 1/2″ CDX sheathing
    3) Ice and water shield as an air/moisture barrier (basic quality)
    4) 6″ of polyiso foam board insulation
    5) A layer of 1/2″ CDX sheathing
    6) A second layer of ice and water shield (more premium quality)
    7) Standing seam panel metal roof

    In my climate, the 6″ of polyiso will keep the underside of the lower sheathing layer warm enough to prevent condensation. My biggest concern is stopping the air leaks around the perimeter. To address this, I am doing the following:
    1) Putting spray foam on all joints from the inside of the cavity. See here for photo of what I’ve done already: https://tinyurl.com/yck86xy4.
    2) Putting caulk on top of the joists on the perimeter of the roof to create an air seal with the sheathing.
    3) Putting caulk on all joints from the outside of the cavity (i.e., the joints on the other side of the spray foam).

    A few questions:
    Q1: Any overall concerns with my plan?
    Q2: I don’t expect air leaks on the sheathing, but I am concerned about getting a tight seal on the perimeter. How forgiving are small air leaks in terms of moisture buildup?
    Q3: Would not putting any insulation in the cavity improve the ability for the cavity to dry? I know rockwool is good about not absorbing moisture, but I wonder if leaving it out might allow the entire assembly to dry out better. Would take me down from R62 to R39.
    Q4: Any other guidance you have?

    Regards,
    Evan

    • One of the key things to successful building is designing for imperfect implementation. Even if you seal perfectly today, it’s quite possible that seals will break over time with heating/cooling cycles. Not that they will happen but they’re quite possible. This is probably what happened with your chimney leak. It may have been fine at one time, but maybe it depended on caulk, which finally failed. I’d give this advice to anybody building.
      As for your specific implementation, in your climate, the layout appears to be highly moisture tolerant. If you have a roof leak, the ice and water shield will allow water to drain down an out at the soffits and the polyiso facing itself will be another line of defense against roof-side leaks. The edges would be the primary vulnerable points. On the inside, (Q3) it appears that you’ve considered controlling the moisture levels and minimized condensation potential through exterior insulation. So from a distance, I’m not seeing problems. I was wondering about drying also with the rockwool layer, but with that thick polyiso layer, I’m not seeing major risks. Especially with Seattle’s relatively mild winter.
      I would definitely run it by a local “green building” expert (of which you have many in the Seattle area) as they will have more intimate knowledge of challenges specific to your area.
      Q2: There’s always that risk. The key thing is that things need to be designed to dry quickly in case any small amounts of condensation do form. On the inside, I think you’re ok, as I understand the design. On the outside, you’d want plenty of airflow under the metal roof and particularly near “at-risk” areas, like the perimeter you mentioned. You want to avoid trapping moisture inside the structure where repeated condensation/wetting cycles could build up. Without actually seeing the construction, it’s hard for me to get more specific.
      Q4: Always keep in mind that water vapor is a tiny molecule whereas liquid water is much larger, so it is easy for vapor to get in and hard for liquid to get out. Also, consider thermal bridging of rafters. I think you’ve got this covered since, as I understand it, the polyiso will be outside the rafters, so you’ll have a highly insulated exterior, minimizing thermal bridging and the potential for condensation near the cold spots caused by and around them.

      • Thank you for your detailed and prompt reply. Overall, I am less concerned about a roof leak because I think the roof installation and ice and water shield installation are more likely to be implemented correctly, although I agree with you advice to plan for imperfect implementation.

        Above, I only called out one concern, which is is preventing air coming in from the 2 x 6 blocks between the joists (see picture linked above). If air leaks around this block into the cavity, I’m concerned about condensation.

        One concern I neglected to mention is condensation from humid interior air on the inside of the 2 x 6 block, since there won’t be any insulation on the exterior of this block to keep the inside-facing surface of the block above dew point. Are vertical surfaces at less risk of this type of condensation than horizontal surfaces (e.g., roof sheathing)? I know that the exterior polyiso is the only thing preventing condensation on the sheathing; is there a similar requirement for exterior insulation on the 2 x 6 blocks?

  3. Dear Ted,
    I recently purchased a vacation home which developed condensation on a cathedral ceiling. Before purchasing the home I noticed streaks along the ceiling but thought that it may have been from air movement from a ceiling fan. I had the ceiling painted then after a particularly humid/raining May the streaks reappeared and I noticed sweating on the drywall. There has been nothing since as its been fairly dry but still very humid. I’ve also been running the air conditioner when its humid outside. When away for long periods and not running the AC, I run the ceiling fan in reverse.
    I’ve read your blog with extreme interest but my situation differs from those in your examples as my problem surfaced in the spring without the air conditioner running. The only thing I can think of based on your excellent article on the physics of humidity is that the outside saturated air is cooler than the hot air along the ceiling of a hot roof. Does this make sense?
    Here are some cabin particulars:
    • The home is a 1958 log cabin with 2×4 rafters located directly on the Chesapeake Bay which I suspect contributes to the humidity levels.
    • It looks as if there was a gable vent at one time though that has been covered over with drywall.
    • Size of ceiling is 12×16 on each gable end (384 sq. feet). Ceiling is drywall with some small gaps along a structural beam.
    • It looks as if the insulation was placed directly in contact with the roof sheathing (though not confirmed) creating a hot roof with no air circulation. I believe the insulation has a foil vapor barrier facing the room.
    I’ve contacted a building envelope company and they recommend removing either the ceiling or roof sheathing and spraying in closed cell foam. They made this recommendation without visiting the house. I gathered from their website they are in the insulation business.
    A local Carpenter familiar with the community log cabins and their cathedral ceilings recommends increasing the R-Value by overlaying a Zip system roof sheathing over the existing sheathing. I need a new roof anyway so this is doable. My concern with this proposal is that there still would not be any air circulation thus I would still have a hot roof albeit a little cooler.
    Before I proceed I would like your opinion on whether either will solve the problem. The other solution would be to run a dehumidifier to maintain humidity levels.
    Help.
    Also, do you know how I could locate a competent consultant?

    • Hi Tom, thanks for your comments. I agree – this is an unusual time to get condensation, so it’s a great discussion topic!
      Here’s what happens…
      Recall from my (probably overly complicated) discussion of the physics of condensation – you get condensation when an object is at a temperature below the “dew point” of the air. If the air is hot and humid, then the dew point can be relatively high. For example, during the summer, it can be 90F with 60% relative humidity. There’s so much moisture in this air that the dew point is 75F!
      On a clear night, your roof/ceiling can cool off pretty quickly. These are also the nights that the temperature drops quickly. Here in Eastern PA, it was just 90+ during the day, high humidity, but the temperature dropped to 58 degrees.
      Continuing with this situation, if you’ve got a high humidity in the house, and the roof or ceiling gets cool, the moisture is going to condense on any parts of the ceiling or roof that cooled off. Running the air conditioner, as you have, pretty much eliminates the problem because the AC is removing the excess moisture from the air, keeping the dew point temperature far below what would be necessary for condensation.
      Note – if the drywall is sweating, the moisture is inside the house, and ventilation under the roof will have little/no effect on this problem.
      Are the streaks near where the beams go through the drywall? Or are they streaks from the condensation on the drywall just dripping down the ceiling? That makes a big difference.
      Also, are the streaks in one area or all over the ceiling?
      When you noticed the sweating on the drywall, had you been at the house or did it just show up after having the house closed up for a while?
      Here’s one possible hypothesis…
      If you hadn’t been at the house and the heat/AC were off, then the interior in April would have settled to a cool temperature. It could easily be 40-50 degrees in the house. Drywall is pretty dense, and holds its temperature. So let’s say you showed up at the house and opened it up to air out a bit. With that humid air, the interior of the house would quickly rise up to a high humidity but the drywall would remain chilly for some time. This would cause sweating.
      As for insulation – no matter how good the insulation, the drywall would sweat in this situation. In fact, it might sweat MORE with good insulation because the heat from the sun on the roof wouldn’t warm up the drywall as much.
      I grew up in a stone house in Philly where the walls sweat all summer long. We didn’t have AC, and the heavy plaster walls stayed at some average temperature, say 75F. Then we’d open up the house to get some fresh air and things would really sweat!

      So what’s the solution?
      1 – use air conditioning to keep the humidity down.
      2 – when you’re not there in the cooler months, keep the heat on. Not high, but you might want to keep it at least 60F in order to keep the drywall and other parts of the house at a temperature that is much less likely to have a condensation problem.
      3 – this will be much less of a problem during the summer when the drywall is much warmer. don’t worry about it unless you experience an actual problem. Or, set the AC to 80F so that it runs from time to time to reduce the interior humidity. But really, unless you leave the windows open and let hot, humid air in, you’re not likely to have a problem.
      4 – during the winter when you’re not there, definitely keep the heat on. Again, 55-60 to be on the safe side.

      As for insulation, venting, etc. – I wouldn’t bother with that unless the problems seems chronic and these simple suggestions don’t improve the situation. But honestly, based on the physics, I think you’d be wasting many thousands of dollars unnecessarily.

      Final note – if you are going to redo the roof, adding insulation through the Zip system sheathing wouldn’t be a bad idea in general as it will give you added R-value, saving you on heating and cooling. It won’t necessarily fix your problems, but it will make your home more comfortable and efficient – something I’m always a proponent of!

      • Ted,

        Thanks so much for the quick response. To answer your questions the streaks start at the peak and slowly migrate down the cieling. In one case onto the adjoining wall. No dripping that I can tell. The streaks are located all across the cieling. In all cases the event seems to have occured after the house was vacant and I opened the windows. In one case I noticed sweating near the outside wall and immediately put the cieling fan on and it quickly dissipated.

        So based on your reply it looks as if I have to keep the indoor humidity levels down regardless of the outdoor tempatures.

  4. Good Afternoon Mr. Ted,
    Very good information you have on your website. I have a question, My house was build in 1985 by a general contractor, many things have been changed, like the Vynil Covering the whole house, also the ceiling was changed a year before I moved in. My electric bill is always high. I need to add more insulation, currently living in VA and have 3 inches of Rockwool as insulation. There is no air seal in the ceiling at all. I went up there to check the insulation. There are soffits in the front and back of the house, however, the front has been seal with a 2 x4 and has insulation over the soffit. The back has all the soffits on the floor but it looks like air is coming thru. There is a ridge vent and a gable vent. there is some wood that looks soft in the bottom of the gable. Also, there are 2 skylights on the same side as the working soffits, they are insulated with unfaced fiberglass. I have had 2 contractors come and do an estimated for the insulation. One of them did not mention air sealing at all. The other I felt was trying to push me to do spray foam and “have more storage”.
    Do I just air seal and spray some blow in insulation and call it quits? what would you recommend?

    • There can be many causes of high utility bills. Since you mention electric, I”m assuming you have electric heat. Maybe a heat pump? That would be fairly common in your area. I would definitely have it checked out and ensure that it is working up to spec before attacking other issues.
      Regarding insulation and ventilation. When you say there is no air seal in the ceiling, what are you referring to? Are the ceilings standard sheet-rock? Are there lots of recessed lights or is it a standard ceiling? Typically, you only have to worry about holes (like recessed lights) in the ceiling and where pipes come through from the house into the attic, and attic hatches. Otherwise, with a normal ceiling, you won’t really notice any particular sealing.
      I wonder why the previous owners covered the soffits with a 2×4? That’s odd. You definitely want the soffits open and clear all the way to the ridge for a standard construction.
      Also, you say “the back has all the soffits on the floor but it looks like air is coming through” – I don’t understand this. Air is supposed to come through the soffits into the attic. Or are you saying air is coming through the ceiling of the living space up into the attic? I’m afraid I don’t understand your description.
      The soft wood on the gable side – if a knife point easily penetrates the wood, then you’ve got a problem there that needs to be dealt with. You’d want to find out why that wood is soft – usually that means a water leak. Do you see any signs of leakage from the gable vent? It’s possible that during a heavy, blowing rain, water is getting in and causing the wood to get wet and rot. You want to deal with that before any insulation work.
      As for air sealing, the main thing is to close up openings between the living space and the attic, under the insulation. This means around any air vents, bath fans, recessed lights, electrical boxes and so forth. This can be easy or hard depending on the number of fixtures that need to be sealed.
      In VA, you should have people who are familiar with proper technique. Definitely keep looking until you find an insulation contractor who understands the importance of air sealing prior to insulation. They’ll be worth some added expense because if done incorrectly, added insulation could lead to a rotten roof.
      One option that deals with the air sealing very effectively is to have a thin layer of spray foam, maybe 1″, applied. Then blow in insulation on top of that. For foam, they charge by the square foot and inch of thickness, so you can get the benefits of the air sealing without breaking the budget. Then the blow in should be cheap and you can have a lot of that put in for a high R-value.

  5. I am considering remodeling and it would include my cathedral ceiling. How can I get in touch with a “building scientist” to hash out my ideas with?

    • Do a search on building science and energy audit for your area. You may have to talk to a few people before finding someone who is qualified to do consulting work. Check their websites to see how informed they seem. Rule out those that just seem like they want to sell you insulation or products like windows. Their site should emphasize their consulting work.

  6. https://polldaddy.com/js/rating/rating.jsI recently purchased my first 771 sq ft home, that was built in 1993. With our first warm temperatures…we have water leaking through our ceiling.

    The living room has a T&G cathedral ceiling with 6 recessed LED lights. After reading everyone’s comments, I realize I have the trifecta of potential moisture issues. My concern is what is happening in my ceiling or roof?

    The shingles on the roof were replaced in 2013. So I am guessing this might be a condensation problem? I can not access the attic to the rest of the house to see if there is mold up there. Mold is a little scary and I am sure it is extremely expensive to deal with. I am getting very nervous with the warmer temperatures, that it might start raining in my living room. I know this is a tough question, but do you have an idea roughly what it might cost to fix this situation? Do I need to replace the whole ceiling? Could it be ice damming? Should I get a thermal read of the house?

    Sorry for all of the questions. I just have first time homeowners jitters.

    • if you’re lucky, it might be okay except for moisture getting in through the recessed lighting fixtures. If that’s the case, you could probably remedy the situation for less than a couple hundred dollars by using sealed lighting retrofits.
      If the T&G has nothing behind it other than insulation, then the solution will be much harder as it will require a full retrofit of some sort. The least expensive is probably sheetrock right over the existing ceiling and using surface mount lights instead of recessed lights.
      The only way to know what it’s like inside the ceiling is to cut open a section and examine behind it.

    • To follow up on my first reply, where is the water dripping from relative to the recessed light fixtures? If the moisture is entering through the fixtures, then there should be a pattern of dripping that is aligned with the fixtures. It might be above them or below them depending on where the water froze and where it ran after it thawed.
      Also, the LED lights – are they just replacement bulbs or is the actual fixture an LED fixture that’s air-tight? The LED light fixtures will look like this:
      https://www.homedepot.com/b/Lighting-Recessed-Lighting-Recessed-Lighting-Trims/N-5yc1vZc7or

      • Thank you for the quick response. I was trying to send a photo of the water damage and the type of lights we have, but was unsuccessful. I am not super tech friendly. Lol

        The water did not come in through the light fixture. It came in through the area close to the the front of the house. Located near the bottom slope going towards the eaves. We found out that our attic access is outside the house. Hopefully we will be able to access the airflow space in that part of the ceiling.

        Our lights look like the retrofit dimmable lights on the Home Depot link you sent me. There is a glass covering of the bulb…so it’s are to say for sure. We are guessing that the leak may have come from one of the roof vents???

        Going to get a couple quotes and pray that there is no Mould or substantial repairs that need to be done 🤞🏼

        Thank you for all of your help. 😊

      • I should note that it’s usually water vapor that gets up into the ceiling cavity. It then condenses, like the moisture in the air in a shower that condenses on the mirror. Then, at some point, the water will drip and run down the slope of the ceiling and come out at some other random location where it can get out.
        If you do have sealed light fixtures, then it’s still possible that the moisture is getting into the cavity around the fixtures, but likely that it’s just going through the gaps between the tongue and groove ceiling.
        Moisture problems of this sort almost never occur from air getting in from outside the house during the winter because the moisture content of cold air is extremely low. Unless you have an actual roof leak where rain is getting in, you should be looking for how air gets into the ceiling cavity from inside your home.
        As for mold, the prevailing wisdom is that, since natural molds are everywhere, if there’s just a little blackening of the wood, the best bet is to encapsulate it with paint or foam and forget about it. Once it’s encapsulated, it cannot get into the air in your house. If you try scrubbing it, you’ll dislodge it which will make it airborne. Also, many people tell you to bleach mold, but that’s not good. Bleach is an extremely powerful agent. Mold is broken down with simple soap water and washes right away. The problem with a roof deck is that it’s filled with nails, making it difficult to clean, so encapsulation is often the easiest and most effective solution.

      • I found out I do not have a true cathedral ceiling. It has a 3 to 4ft clearance at the peak. There is T&G on the inside of the living space followed by drywall, then painted and topped with R30 fibreglass insulation. Also there are 6 small vents in the roof.
        I had 2 people come out to look at the attic space. One roofer told me it is condensation coming through the cut barrier around the led lights. He quoted me around $588 to seal 6 pot lights. The other person went through the entire attic space and found the bathroom fan/vent is not leading to the outside. The deck is completely wet around this area. He thinks this might be one of the main reasons why there is so much frost at the ends of the attic. He also thinks I need a whirlybird air vent to get some the moisture out.
        For now I have lots of frost that will begin to melt and find its way into my living space. Not sure what the source of this problem is and how I can prevent it from raining in my home until I find out. I am very confused as to who I need to go with? Maybe I need to call another person for the tie breaker? Sorry for all of the messages. Thank you so much for all of your help.

      • The bath fan has to be the top priority. That’s dumping huge amounts of moisture into the attic. Generally, attic vent fans are unnecessary and can cause more problems because they can actually suck more air from the house into the attic.
        The recessed lights should be sealed. you might be best off simply caulking the perimeter of the housing to the circle cutout and then replacing the normal bulbs with LED retrofits that seal tightly to the ceiling. You can do this for probably half or less than sealing the fixtures and you’ll cut your utilities too!

  7. Ted, we have an unheated sunroom that warms significantly on a winter day and gets very cool at night. Over the past couple years we have noticed some water staining which has gotten worst this year in New Jersey with very cold weather. I can access part of the ceiling from another attic and can see moisture and some black mold on the underside of the roof deck, even in areas two feet from the sheet rock and insulation. Do you think additional attic ventilation help solve the problem?

    • Hi Joe,
      Is the sunroom part of the living space of the house? What I mean is, is it exposed to the humidity in the house? If so, that would definitely lead to condensation during the cold we’ve been having.
      The question is, what is construction of the sunroom ceiling that leads to the attic? There has to be a way that moisture is getting up to the roof deck.
      The most important thing is tracking down the source of the moisture, which is carried by air escaping from the house up to the attic. If you can close off the air leakage, then the mold and condensation up there will stop and you (most likely) won’t need to worry about added ventilation. So track that down. You’ll likely find some holes or wall cavities that allow humid air to flow into the attic space and that’s why the roof is getting moldy.


  8. https://polldaddy.com/js/rating/rating.jsHi, I’m in the Chicago area and we just went through about 2-3 weeks of nothing over 20 degrees. Today it was about 35. We have cathedral ceilings in our 2nd floor master bedroom. Today we noticed water dripping from the 3 of the drywall seams.
    Ceiling is 2 x 12’s with R-38 fiberglass insulation. It has 6 recessed lights. The entire ceiling has the foam vent chutes below the roof sheathing going from lower soffit to upper soffit. Where the chute goes through the exterior wall soffit area it is spray foamed to be airtight.
    2 questions.:
    Are we probably only noticing the dripping now because condensation has been happening for the cold snap and is just now thawing out and dripping? It seems that the colder air would have held less moisture than the relatively warmer air we are in now, and we should have been seeing water all along?
    Also, there is about a 5 x 6 section of the ceiling that has a small attic area. In there, there is unfinished drywall with definite air gaps. If I get this sealed and go over each recessed light with caulk, sealer, foil tape and seal them perfectly, is there any hope of this ceiling working? Or am I doomed to water issues in the future?
    If its only the moisture that travels through the drywall and paint, is that enough to have issues?

    While searching the web for answers i came across your site. Best explanation on the web!
    Thank you very much,
    Steve

    • That definitely sounds like a moisture freeze-thaw issue. It’s very common and often misdiagnosed. Inside the house, even low humidity level are enough to freeze and build up over time inside a frigid roof cavity.
      With good air sealing of the recessed lights, you can greatly reduce the amount of moisture that will get in there as long as the rest of the ceiling is solid and well painted. Latex paint acts as a vapor retarder as well, so the amount of moisture that gets through sheetrock is miniscule in comparison to even a small hole (through the lights) that lets air through.
      I have had great luck with retrofit LED lighting fixtures sealed to the sheetrock. Combined with sealing the holes in and around the can, it greatly improves the odds of eliminating future moisture problems.
      Thanks for your feedback on my information. I try to keep things approachable but it can be hard since it’s inherently highly technical.
      Good luck! Feel free to ask more questions if anything crops up.

  9. Hi Ted – I have a two-story home in the Midwest with a drywall vaulted ceiling over the master bedroom and HVAC mechanicals in the attic over the rest of the second story (kids rooms etc.). The initial set-up was that the decking in attic portion was covered in batting and the cathedral ceilings were finished with drywall and I presume insulation. There was no soffit and two static vents for the entire house. We had the roof replaced in mid-2017 and the roofer insisted that we upgrade to a ridge vent and edge vent to create some airflow, though there was no vapor barrier or baffling installed. It appeared that there was some space between the fiberglass insulation and the decking in the HVAC room at least. Fast forward to January 2018 and -10′ temperatures and we are hearing a dripping in the celing after the HVAC cycles in the half of the house that has the cathedral ceiling. The peak of the cathedral ceiling was cracked (this is new) and I’ve gone and caulked the peak of the entire cathedral ceiling. I am hoping that this stops some of the hot air from rising and meeting the decking. There are no canned lights in this room. Do you have any other suggestions?

    • Wow, this is the week of vaulted ceiling issues! Must be the cold weather is bringing out all the problems!
      It’s odd that the dripping occurs after the HVAC runs. Are you seeing any signs of water staining on the ceiling where you’re hearing the dripping? With insulation in there, it would be extremely unusual to actually hear dripping because there’s no room for the water to fall. Instead, it usually saturates the insulation and slowly leaks out like a sponge.
      One thing to look for is ice-cicle formation at the soffit vents. You would think the water would run down the ceiling and drip out there if there’s that much water.
      I’m wondering if there might be something else leading to that sound, though water dripping is very distinctive, so if you hear dripping, I’m sure that’s what it is.
      If you have any more clues, please post another comment. I’m scratching my head on this one!

      • Ted – Thanks for the quick reply!
        We aren’t seeing any staining or water formation so not sure exactly where it is dripping but definitely sounds like it is hitting near where the wall meets the ceiling. The drips move around depending on time of day. These drips all seem to be near where the roofer installed the edgevent on top of the decking. We had the roofer and two accomplices out to inspect and said that the everything on top of the roof looks fine and there are no ice damns and no water coming into ridge/edge vent so they think the drip is coming from the inside of the house from condensation. They suggested putting a few turbine vents near the edge of the roof to draw out the moist air but that sounds absurd. I am thinking that installation of the edge/ridge vent on a previously minimally vented roof is now drawing up too much moist air and the insulation etc. was never made to be vapor blocked which is causing the issue.

  10. Hi Ted,
    your article and description of the problem with condensation and vaulted ceilings seems to be exactly what I am facing. We have a post and beam house undergoing renovations. I have 2 rooms with vaulted ceilings and exposed beams. We just put on a new roof and the roofer noted the roof deck was rotted and moldy when he tore up the roofing. We replaced the entire decking, We had batt insulation already in place under the decking and ceiling sheetrock was in good condition from previous renovation. We have soffit vents and he installed a new ridge vent. He also stated that he re-positioned the baffles that were in place. It is now freezing in New Jersey with snow cover on the roof and we have brand new water dripping and staining from the side beams in one room and mid beams in the other room – we never had this problem in the past – the new roof went up 2 months ago.

    We have recessed lighting in both rooms and additionally, the sheetrock is not sealed up against the exposed beams – but rather there is a 1/4″ gap and a corner bead reveal between the sheetrock and each and every beam.

    Why do you think this is first happening now that we have a brand new roof on? Should I caulk / seal the gap between the sheetrock and the beams and maybe try to seal around the recessed lighting fixtures?

    Do you think this will ever get better or do I have to rip out all the Sheetrock and spray foam????

  11. https://polldaddy.com/js/rating/rating.jsI have a cathedral ceiling dining room that was an add-on to an older home – all this occurred prior to our purchase of the home. There are exposed beams with drywall inbetween. We noted water stains near the outer wall and near the peak in previous years but a year ago we replaced the aged cedar shake roof with an architectural asphalt shingle roof. Last spring and summer we had a steady drip from several spots near the peak of the ceiling even for a few months after the end of the preciptation events in June. We live in the high mountain desert in Wyoming but do have a stream immediately adjacent to this room. Condensation seems the most likely problem but living in a rural area, have few options for local consultation so would like some advice for resolving this problem. What would you suggest?

    • With the weather conditions when the dripping is worst, it does sound like high air humidity is leading to the condensation.
      I think the difference between the cedar and asphalt shingles has to do with the fact that asphalt has greater and faster temperature swings than the cedar.
      Solutions will depend on your budget. But before trying to remedy the problem, it would be prudent to verify the problem.
      If it were my house, I’d cut a couple holes large enough to look through and maybe reach an arm into because you might have to push aside some insulation. The theory is that moisture is getting up under the roof deck. At night in the high desert, the roof will cool very quickly, leading to condensation. The question is where is the humid air coming from? It could be from the inside of the house,or, it could have moved into the roof cavities from the outside during the rain.
      If the moisture is from in the house, then you need to prevent indoor air from getting into the cathedral ceiling. This can happen if you have lots of recessed lights, for example.
      If the moisture is coming from outside, it’s likely just entering the soffit vents and getting trapped in there by insulation. Again, only direct inspection will shed light on this.
      The most surefire solution would be a pain to implement. It involves pulling down the sheetrock ceiling and using spray foam under the roof. This would reduce the temperature swings and greatly reduce the chance of any condensation.

      • https://polldaddy.com/js/rating/rating.jsHi Ted,
        your article and description of the problem with condensation and vaulted ceilings seems to be exactly what I am facing. We have a post and beam house undergoing renovations. I have 2 rooms with vaulted ceilings and exposed beams. We just put on a new roof and the roofer noted the roof deck was rotted and moldy when he tore up the roofing. We replaced the entire decking, We had batt insulation already in place under the decking and ceiling sheetrock was in good condition from previous renovation. We have soffit vents and he installed a new ridge vent. He also stated that he re-positioned the baffles that were in place. It is now freezing in New Jersey with snow cover on the roof and we have brand new water dripping and staining from the side beams in one room and mid beams in the other room – we never had this problem in the past – the new roof went up 2 months ago.

        We have recessed lighting in both rooms and additionally, the sheetrock is not sealed up against the exposed beams – but rather there is a 1/4″ gap and a corner bead reveal between the sheetrock and each and every beam.

        Why do you think this is first happening now that we have a brand new roof on? Should I caulk / seal the gap between the sheetrock and the beams and maybe try to seal around the recessed lighting fixtures?

        Do you think this will ever get better or do I have to rip out all the sheetrock and spray foam????

  12. Ted,
    I live is southeast Texas close to Houston. My house is a log home with all cathedral ceiling composed of T&G nailed to rafters. Above the ceiling is fiberglass insulation, air chutes then roof decking. There are ridge vents but hardly any soffit vents because of how the house was designed. Several years ago I decided to remove all the ridge vents. Several weeks after that during a nice spring day I noticed condensation on the exposed ridge beams, ceilings and walls close to the peak of the ceiling. Of course, mold and mildew follows. I have since reinstall the ridge vents and the condensation problem have not occurs again. The house is very drafty especially in the winter since the cold air entering the ridge vent have no place to go but inside the house. I have been thinking of applying clear silicone caulk to all the T&G seams to seal the ceiling then removing the ridge vents. I don’t want to sheetrock over the T&G. What are your thoughts and do you have any other solutions. Thank you very much.

    • I’m surprised that the condensation only occurred after closing the ridge vent. It might have been coincidental. On the other hand, I can imagine how that might happen. With a ridge vent, the warm air from your house gets sucked up through the gaps in the T&G and helped to keep the ceiling warmer. When you closed the vent, less air would flow up there so the ceiling could get colder, leading to more condensation risk. Just a guess however.
      You could try caulking all the seams, though that would be a lot of caulking! Plus, it might not hold since the T&G boards would expand and contract with temperature and humidity. Over time, this could break the seal with the caulk and all your work would be wasted.
      The easiest thing might be to build in the ceiling. Attach 1/2″ to 1″ of foil-faced board foam to the existing T&G ceiling, taping the seams so everything is air-tight. Then install a new, attractive, T&G ceiling, screwed through the insulation board to the old ceiling. You get the beauty of the T&G ceiling and the additional insulation while also greatly reducing the chance of any condensation/mold.
      While this might seem like overkill, it would ultimately be a lot less work than caulking every seam and would work much better and be a permanent solution.

      • Ted,
        Thanks for the quick reply. I did not notice any more condensation after I reopened the ridge vents. I have cleaned the beams and ceilings of all the mold and will keep and eye on it. Back to the sealing the T&G. I agree that redoing the ceiling is the best solution but not a viable solution right now financially for me (see Hurricane Harvey). Caulking the seams is less expensive since my labor is free and it will buy me some time if you think that will work. Do I remove the ridge vents after sealing the ceiling either by caulking the seams or redoing the ceiling completely?

      • I typically don’t like messing with parts of the house that don’t have problems. If there’s no great reason to remove the ridge vent, and you had problems before when you removed it, then I wouldn’t remove them again.
        The caulking could certainly be a stop-gap to greatly reduce airflow through the T&G. So if you’ve got the inclination, give it a go!
        One thing you might want to try – get an incense stick (or something else that creates wisps of smoke that you can see). Hold it up to the T&G cracks. If there’s air movement, you should see it suck the smoke right into the ceiling cavity. This will be strongest near the ridge. After you caulk, you can use the same test to ensure that you’ve sealed the cracks. Cheap and easy tests!
        Good luck. Hope you can get it sealed up and don’t have any more moisture problems.

  13. Our cabin has an 8′ front porch and 14′ back porch. Our metal roof is continuous over the porches and the cabin. 6:12 pitch on back porch, 12:12 pitch on cabin roof. The porches will have t&g ceilings. The soffit vents will be placed within the porch ceilings. Do you recommend they be placed at the edge (behind the gutters), or tucked closer to the cabin for a straight shot up to the ridge vent? I wish I could send a pic, hard to explain. THANK YOU!!

    • Whatever provides a good, unrestricted airflow. It’s important that they’re large enough or minimal air will flow. For example, those little round things sold as vents are trash. Ideally, use a product like the cor-a-vent which are properly designed.

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