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.


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

  1. This is great advice, going to change my old 1920 rowhouse in Philly from uninsulated air space between roof joist and ceiling. To cathedral. Seem like hot roof is best with poly foam. as it is a super low slope almost flat roof. Do you could add foil faced foam board insulation to the bottom of roof deck then add the poly foam to the edge of the rafter? Would be furring 1 inch down for electrical, (second 370 ish SQ ft so not a lot of electrical stuff up there).
    I am assuming as long as no moisture it making it’s way up there, no problem just thinking about being able to reflect more uv from the house.

    • The first thing I would do is not assume that moisture won’t get up there. The only way I would trust that to be the case is if you use spray foam directly against the ceiling from the top. Moisture always seems to find its way into spaces. I just had another writer note that his poly iso foam was compromised by moisture which can happen. So in many cases blue board/pink board which is XPS foam is better to use in situations where moisture could be an issue Because it is much more moisture tolerant. I would be very cautious about doing a hot roof construction in a rehab situation. I wouldn’t want to create any situation that could lead to long-term roof rot. Would you be working from the inside, tearing out the ceilings or from the roof side, removing the roof and working down? In either case, it would be much easier and safer I think to have a spray foam job done. The upfront cost may seem much higher with spray foam but it is so much easier to do a thorough job since it expands and gets in all the little nooks and crannies where air could work its way through. And in the long run, small mistakes could lead to big expenses down the road.

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