Insulating Attics and Cathedral Ceilings

In the first article of this series, I discussed a lot of the theory behind attic insulation that you should know before taking on your insulation project. If you haven’t read it, please, click the link and do your best to understand the basics before proceeding.

Condensation – killing houses slowly but surely

Snowy energy audit

This house definitely has problems

This winter, most of the questions I’ve received have been about moisture buildup in attics or cathedral ceilings. And all have the same answer – humid air from the house is rising up through all the little cracks and holes in your ceiling. Once it gets into the attic space (or the space between the roof and the ceiling of a cathedral ceiling), the air cools rapidly and the water held in the air condenses on to a near-by cold surface – usually the underside of the roof or the roofing nails.

Once this happens, the water drips  into your insulation or soaks into the roof sheathing. If this happens once and dries out, it’s no big problem. But during the course of the winter, this happens again and again, and before you know it, it’s raining inside the house as water leaks through holes in the ceiling.

There’s a saying – “there’s no such thing as a small water problem.” But often people will try to put a band-aid on them. Unfortunately, every day a water problem is neglected, is another day closer to serious structural problems and tens of thousands of dollars of repairs. So it is in your best interest to deal with water problems as seriously as if your house was on fire. Deal with it today, or you’ll be really sorry tomorrow.

What a moisture problem looks like

Let’s look at the underside of a roof that has experience this repeated wetting from condensation:

Insulation against roof deck leads to roof failure

Fiberglass insulation against roof deck in an attic leads to roof failure

moldy roof rec lights

Moldy roof deck above a cathedral ceiling

The first thing knowledgeable people ask is: “will adding more ventilation eliminate the problem?”

We’ve all been told that you have to have air flowing through the channels, from the eaves (soffit vents) up to the peak (ridge vent) in order to flush out the moisture. But this is only part of the story and it doesn’t guarantee anything. In some cases, it makes the problems worse.

The purpose of insulation in winter is to slow the heat loss from the house and out the roof. Take a look at the photo at the top of this article. Half the roof has snow, the other half does not. The half where the snow has melted is a clear sign of heat loss from the house. The heat warms the roof sheathing and melts the snow.

Recall what you know about condensation – condensation occurs when moisture in the air comes in contact with a cold surface. The roof is above the insulation, so it’s supposed to be cold. If it were warm (like the right half of the photo of snow), then condensation would be less likely. The situation is a Catch-22 – good insulation equals cold roof. Cold roof plus humid air equals condensation. Ugh!

Given these conflicting issues, what do you do?

Preventing moisture problems in attics and cathedral ceilings


U.S. EPA Air infiltration poster

The answer is simple – reduce the amount of humid air that can come in contact with the cold roof.

In the winter, almost all the moisture comes from inside the house. It rises up, along with the warm air, finds every tiny crack in the ceiling, and gets up into the attic or inside the cavity under a cathedral ceiling.

Step 1: reduce moisture in the house

The first thing you want to do is reduce the amount of water getting into the air from inside the house.

Ventilate when showering

humidistatThe most common source of large amounts of moisture is the shower. When you shower, you create a “worst case” scenario – hot water saturates the air. That “super saturated” air will immediately dump the humidity onto any cooler surface it touches.

When you shower, you must run the bath fan during and after your shower until the all the excess moisture in the bathroom is flushed out. Usually, this means allowing the bath fan to run for approximately 30 minutes after you shower.

If you have a door on the bathroom, keep it closed until all the moisture is flushed out. Never shower with the bathroom door open because the moisture will flow out into other areas of the house that are not designed to eliminate the moisture.

Bath fan tips:
  • Ensure the fan actually works. Put a piece of paper under the fan. It should suck strongly to the fan’s intake. If it does not, then the fan may be improperly vented.
  • Check the fan installation. Pull down the grill under the fan and look for gaps between the ceiling and the fan. Use canned foam to seal these gaps.
  • Make sure the fan vents through the roof. Venting the fan into the attic is common and exceedly stupid as it virtually guarantees a rotten roof. The duct from the fan should be as short as possible and go straight up through the roof. Your fan is worthless if the duct runs 50 feet across the the attic then lays on the soffit.
  • Install a humidistat that automatically runs the fan based on humidity levels. There are even bath fans with humidistats built in now but I prefer separate switches with humidistats like the one linked to above or this one. Why? Because you are less likely to forget to turn it on. The humidistat is wired in parallel with the normal switch so either one can activate the fan.

Ensure the dryer vents outside

Far too often, dryer vents take long paths to get outside. This leads to inefficient or even dangerous operation. If your dryer takes forever to dry clothes, chances are good that it’s not venting effectively.

Don’t even think about getting a dryer “heat reclamation” device. These pump gallons of humid hot air into your home, greatly increasing your odds of mold and roof rot.

Turn off humidifiers

If you have moisture problems and you use a humidifier, stop using the humidifier. Humidifiers can dump gallons of water into the air every day. Central humidifiers are especially bad because they use the homes leaky ductwork to spread the moisture.

If you absolutely must use a humidifier, use it only in the room where you sleep and turn it off when you’re not using it.

Keep in mind that the primary reason you need a humidifier is because your house is leaking in cold, dry air. The need for a humidifier is a clear sign that your house is inefficient and you could save a lot of money and be much more comfortable by tightening your home. I know – when I moved into my home, our lips and skin cracked, our noses bled and we were really uncomfortable. We had to use humidifier round the clock to be comfortable. After replacing our old windows and air sealing the house, we haven’t had to use the humidifiers once.

Ventilate when cooking

If you cook on the stove a lot, make sure you use an outdoor venting range hood. This is especially important if you have a gas stove or oven because these generate carbon monoxide and water vapor.

Conservatively water plants

Much of that water you use to water plants ends up inside the house.

Avoid “ventless” gas fireplaces

These should be outlawed. Ventless gas fireplaces generate tons of water vapor and, if not running cleanly, carbon monoxide, which can kill you. No matter what Bob Villa might say, these units should never be installed or used. It’s physics. A byproduct of combustion is water and carbon dioxide. And if the combustion isn’t *perfect*, you also get carbon monoxide.

Look for hidden water sources

Basements and crawlspaces

If you have a crawl space or damp basement, this could lead to huge amounts of moisture entering your home. Many articles have been written on this topic. For now, be aware of this and check your basement and deal with any moisture problems under the house. If you don’t there’s a good chance you’ll end up with attic/roof moisture problems.

Indoor ponds and fountains

Self explanatory.

Any other place water is used in the house

Step 2: Air seal the ceiling between the living space and the insulated attic/cathedral ceiling


I’ll save myself 1000 words – refer to this excellent diagram and ensure that your home has an airtight seal between the living space and the attic or the inside of your cathedral ceiling.

Avoid traditional recessed light fixtures!

moldy roof rec lightsIf you’re considering (or already have) recessed light fixtures, use air tight LED fixtures. The manufacturers completely lie when they label traditional fixtures as ICAT (insulation contact, air tight). I dare you – turn one of these fixtures upside down and fill them with water. Water will rush through all the holes in the fixture. Now picture your ceiling during the winter – air flows right through the fixture, carrying moisture up into your attic or ceiling. You are virtually guaranteed roof rot if you have a ceiling filled with recessed lights.

Don’t believe me? remember this photo from earlier? This builder swore up and down that he did everything right and the roof was still rotting. Why? Because he installed about 20 of these fixtures in the ceiling. You can’t argue with physics.

Fortunately, you can cut your energy usage in quarter, never have to change a bulb again, and eliminate the moisture infiltration problem by using flush-mount LED lights like these. You can also retrofit existing recessed light fixtures using these types of lights.

One word of advice – use a thin bead of caulk around the lip of the fixture to seal it to your ceiling. This is the only way to ensure an air-tight seal. Yes, it will be a pain in the butt to remove later, but these things are rated to last 36,000 hours. That’s 4-years of continuous use so 10-20 years in real use. LEDs often last much longer.

Seal all duct registers and especially bath fans

I have never seen a bath fan that’s properly air sealed, and they’re in the most critical location in the house. The installer will cut a hole about 1″ larger than the fan then cover up this big gap with the fan’s grill. Out of sight, out of mind.

All that humid air from your showers will go right up through these gaps and into your attic. This is the place you should start with your air sealing because it’s so easy and so effective.

After that, if you have heating/air conditioning vents in the ceiling between the living space and attic, remove the grills (called “registers” in industry lingo for some reason). These are installed the same way as bath fans. Big holes. Sloppy installation. You can use spray foam or caulk to seal the gaps. Or, use this incredible tape, called “foil mastic.” Clean off the sheet metal of the duct “boot” (that’s the metal thing the duct is attached to that screws to your ceiling). And use this tape to seal all around the perimeter where the boot attaches to the ceiling. Be careful to measure it so that it doesn’t show outside the register before sticking it to the ceiling because once it sticks, it’s on there for good!

Cover the attic hatch

AtticAccess375x327.jpegAttic hatches are responsible for a huge amount of energy loss and the associated moisture damage in attics. Make sure yours is air-tight (which is almost none except for these Rainbow attic stairs). These are the best I’ve ever seen and worth the money.

For the other 99% of you who already have attic stairs, install the ESS Energy attic stair hatch. This thing is awesome. Or, you can build your own out of foam-board. But seriously, the ESS Energy system is so much better than the other solutions out there that’s it’s worth it.

Get a blower door test before and after air sealing

The best way to go about air sealing is to have an energy auditor come and do an infrared inspection with blower door test before and after the project. You do it before the air sealing to locate the air leaks so you can prioritize the work. You do it after to ensure the work was done right!

When I did these inspections on a daily basis, I found that the combination of blower door test and infrared thermography was the only reliable way of finding the problems. Some folks will say they can find all the problems without the right tools, but, frankly, they’re misguided. I don’t care if you’ve been doing this for 50 years, your eyes are not going to find hidden problems.

Let me give you an example. Suppose you go to the doctor. You’re not feeling well. Strange pains in your abdomen. If the doctor prodded you a couple times, told you “it’s nothing, you’ve got gas” then said “I don’t need to use a newfangled MRI machine” – would you trust your life to them?

For a few hundred bucks, are you going to trust someone’s eyes or a proven tool?

Ready? Insulate your attic!

After you’ve reduced humidity problems in your home, then air-sealed the ceiling between the living space and the attic/cathedral ceiling, then you can insulate.

How should you insulate?  Honestly, if you’ve done your prep work properly, you can use cellulose, spray foam, fiberglass, recycled denim or horse hair and your home will be relatively well insulated and energy efficient.

Obviously there are differences, but it should be clear that the prep work for insulation is key. Even the best insulation product can have issues if you don’t air seal and reduce humidity.

Some guidelines for insulation in colder climates

I have to start by saying this is all for winter insulation. Hot, humid climates have their own rules which are often reversed from cold climates.

Moisture barriers go towards the warm, inside of the house

In winter, the moisture barrier should be the first thing beyond the ceiling material. You absolutely don’t want to trap moisture on the cold side of the insulation.

Refer to your local building code for the use of moisture barriers. Many jurisdictions are changing their approach to moisture barriers.

Install insulation

Like I said, the specific insulation product usually doesn’t matter if everything to this point has been done right. The main differences between products are price, R-value per inch of thickness and convenience of use. These can be significant differences and you have to determine what works best for your own home.

Some things they should tell you about insulation but often don’t
  • Blown in products are messy. Cellulose or fiberglass blown in to an attic basically renders the attic useless for other purposes. If you think you’ll never go into the attic, then great, blow in a couple feet of cellulose and be done with it. But if you have heating/cooling equipment up there or you want to use the storage space, you will hate the decision. Trust me. I want to kill my insulation installer for blowing cellulose in my attic after I told them I didn’t want it.
  • If you’re doing it yourself, consider recycled denim like UltraTouch. This insulation doesn’t leave you itchy. It’s dense and fits into the cavities securely. I did my parent’s attic with it and it was wonderful to work with. The downside is that it’s pricey and hard to cut. But it’s really good stuff. Well worth looking into for a DIY project.
  • Mold on roof deck from humid air rising up from the house.

    Rotten roof deck caused by moisture trapped by insulation

    If you’re installing insulation directly under the roof, like in a cathedral ceiling or attic ceiling, make sure to leave about a 2″ gap between the insulation and the roof sheathing. If you don’t, there’s a good chance your roof will end up like this.

  • If you install insulation directly under the roof, you have to air seal underneath of it, like with an airtight sealing. In the attic, you can install rigid foam board to the rafters and essentially create a cathedral ceiling in there.
  • There are different types of spray foam. Open cell, which is like what couch cushions are made of, and closed cell, which is rock-hard. They have different characteristics that you need to research in order to make a good decision about their use.
  • You do not want to install insulation on the attic floor and under the roof. This is a case of “more is worse.” The insulation on the attic floor will make the attic cold, and the insulation above can trap the moisture in the attic, leading to moisture problems if the attic space isn’t ventilated. But if you ventilate the attic, then the insulation under the roof is doing nothing. Don’t do it! Choose one location – either the floor or the ceiling and insulate it properly.
  • If you insulate and air seal the attic floor, then ventilate the main attic space according to code. No matter how good your air sealing, moisture will enter the attic from the living space. If this is trapped in the cold attic, you can rot your roof. So the attic has to be ventilated. Personally, I’ve seen gable vents work fine in homes but it’s all the rage to use soffit and ridge vents. Whatever. Just ventilate the space well to flush out moisture.
  • If you insulate under the roof, be sure to air seal under the insulation just as well as you would the ceiling of your home. The attic becomes part of the living space when you insulate under the roof. Treat it as such. Fail to do so? See photo above.

Ok, that’s enough. My fingers are tired from typing all morning. If you’ve read half of this and gotten this far, you should have most of the information you need to create a comfortable, efficient home that will last for decades.


20 thoughts on “Insulating Attics and Cathedral Ceilings

  1. Hey Ted: Would you install flush mount air tight led lights in bedroom ceilings where above the ceiling is the attic or would you just install one fixture in the middle of the room. Given the size of a middle fixture would it be the same possible air leak as 4″ or 6″ led flush mount lights? I am in the process of re insulating and air sealing the attic so not sure I want to install all those holes but need the light for the bedrooms. Hopefully you can give me your thoughts. Thank so much.

    • Hi Pam, I like the flush mount since they are truly airtight and provide more even light distribution if you use multiple fixtures. However, a single center fixture would be fine since that just involves a single electric box which could be sealed well. Each LED light would require some holes in the ceiling for wiring anyway so you’d want to air seal those in any case. So it really comes to personal preference. The only thing I’d avoid are traditional recessed lights since they’re much harder to air seal properly.

      • Thanks Ted for providing the information on flush mount led lights versus a single fixture when the attic is above the installation. Do you have a recommended product. Halo seems to have what they call air-tight fixtures.

        Thanks so much for your help in choosing the right flush mount led fixture.

        Have a great day. Pam

      • I think all the pancake style flush mount lights are completely airtight as long as they are installed with appropriate gaskets or otherwise sealed to the ceiling. I haven’t had any personal experience in the last several years. I would just go on the reviews and see what people have liked. The main things would be color, temperature and dimability. You may also need to change your wall dimmers for ones that are intended for use with low power fixtures. The old style ones expect normal incandescent bulbs and they get flaky with low wattage LED lights

  2. Great article! The house I bought converted the attic into a loft area, essentially making it a cathedral ceiling. The roof is failing and we’re having a new one installed. I removed the rooftop evaporative cooler and I found that the rafters are only 2×4 with foil backed fiberglass insulation. Are there any decent options aside from having new rafters installed? Is that even possible?

    • My preferred solution is to add a couple inches of board foam to the inside of the rafters. You can either take off the existing sheetrock and screw it through and add a new layer of sheetrock or if you’re doing extensive renovations rip out the existing ceiling and then add board foam on the inside of that and then resheet rock on top. Poly ISO board foam gives you great r-value compared to standard styrofoam. Doing this also seals up the ceiling quite well to minimize the amount of moisture that can get into the cavity and cause condensation issues. Just don’t cut through it to install recessed lights or anything as that will allow moisture into the cavity.

  3. I have a 1400 ft2 home built in 1943 and an unheated garage built in 1946. The roof shingles are about to be replaced on both. The roofing contractor pointed out there was no venting in the house from the attic. The plan is install static roof vents to exhaust air from the attic. The air intake for the attic is from the soffits. The exhaust vents will be added at 1 ft2 of exhaust area per 300 ft2 of floor space in the 2 level house. For the soffit intakes this I understand should match the area for exhaust. So for ~1400ft2/300 ft2 this gives 5 ft2 of exhaust venting. On the attic air intake side, the soffits have lathe under aluminium covers. It looks like at one point the soffits were lathe and plaster, with the plaster removed…possibly when the aluminium covers were put on. The questions I have are: 1) Should I remove the lathe under the aluminium covers in the soffits…I am having trouble with the estimation/calculation of the air intake area that I understand should be 5ft2? 2) The contractor recommends static exhaust vents, but is it better to install turbine vents? I understand some wind is needed for these to be justfied…the neighbours have turbine vents that I see are most often active. 3) The house is in the Pacific Northwest in a temperate climate. What color shingles be used for the house and garage? The house does get warm in the summers, I hope this will be partly resolved with the addition of the attic ventilation. Indoor house temperature in winters can be cold as the house is quite leaky. I understand a dark shingle will help giving radiant heating, whereas light color shingle helps to reflect heat.

    Thanks for your help,


    • Sorry for the delay in replying.
      Definitely make sure there is enough air intake space in the soffits. The lath itself wouldn’t be a problem, though if it is still clogged up with plaster, that would be an issue. It might be a real pain to remove.
      If you’re going to go through the trouble, you might want to look at the Cor-A-Vent products. They’re designed for proper venting, both at the soffits and the ridge.
      I’m not sold on turbine vents, though if your neighbors have them and they seem to be operating often, it could be worth a try. However, instead of the expense, added complexity of another mechanical device, and potential for leaks around the flashing, I would be much more inclined to just use a good ridge vent system.
      Tough call on the shingles. Usually, it’s only in very hot and sunny climates that light roofs are useful. In mixed climates like the Northwest, the overall effects will probably be minimal. Better attic insulation, ventilation, and a tighter house would be much more beneficial to your overall comfort and utility costs.

  4. Question: TED, What is the best way to insulate a cathedral ceiling [….with 6″-16″ of space between the drywall and roof sheathing; cold climate – Michigan] note: 16″ gap at the peak, 6″ gap down near the wall/top plate intersection; soffit vents and ridge vent are available.

    • Ideally, you want about 1.5″-2″ gap under the roof deck to make a smooth air flow channel between the soffit and ridge vents. You want to construct it in such a way that little/no warm, humid air from inside the house can make it into that channel. Then you want as high an R-value as possible between the ceiling and that channel.
      If I were doing it from scratch, I would install 1.5″ nailers along the edge of the rafters and up against the roof deck. I’d then install 2″ foil-faced foam board, with the shiny side towards the roof. Depending on the cost, I’d either add more layers of 2″ poly-iso OR I’d spray high density polyurethane foam to the back of the board foam, filling in the cavity. This would give you about R-6 per inch of poly-iso or spray foam. The side of the foam closest to the ceiling sheetrock would be warm, like the sheetrock itself, so there would be no risk of condensation if any moisture did get behind the ceiling. And, the layers of foam would minimize the chance of any moisture making its way to the cold side near the roof.
      This method would also give you some working space between the ceiling sheetrock and the foam so you could run wires.

      If you wanted to save some money, you could use somewhat less foam and some other insulation, but you’d want to construct it differently. In this case, you would have to build it from the in-side (ceiling) out towards the roof. You’d use spray foam or poly-iso board foam right to the back of the ceiling’s sheetrock. Use several inches of that. Then, fill the cavity with a few inches of rockwool or fiberglass or other insulation, being careful to leave a couple inches for air flow from the soffits to ridge. The reason you would construct it this way is that you want to have the loose fill on the cold side (towards the roof) just in case any moisture makes it past the foam. The moisture could then flow through the loose insulation and be flushed out the ridge vent.

      From the practical standpoint, I’d much rather do the first option as it allows you to put a roof on the house then deal with the inner insulation and ceiling without worrying about doing all the work before it rains.

      Hope that helps.

      • ted, thank you for the writeup!
        Option 1 looks good, but quick question about option 2….
        My roof is already attached, so could I possibly build option 2 {from the roof sheath toward the drywall as follows}:
        a) shingles and roof sheath (already in place)
        b) a 2″ baffle with holes in it
        c) 6″ fiberglass insulation
        d) 2″ polyiso board
        e) air gap (6″ airgap near ridge; tapered down to 0″ airgap near top plate)
        f) sheetrock

      • From your description, that should be good. You’re hitting the necessary points: keeping airflow directly under the roof-deck to flush out moisture; putting moisture-permeable insulation facing the fresh air flow to eliminate moisture that might get in there; and having a moisture impermeable layer towards the inside of the house to keep moisture at the warm part of the roof assembly to minimize the chance of condensation.
        Do your best to seal the polyiso so that moisture stays on the warm side. A bead of high-quality caulk is best.

    • it sounds like you probably have a big air leak in your system if you’re getting that much cold air through the duct. I’ve seen cases where ductwork has actually fallen off leaving the entire. System exposed to attic dust and cold air.
      do any of the other outlets have a lot of cold air coming out of them?
      one product that I found pretty effective is simply using glad press and seal plastic over vents. however I must caution you that you wouldn’t want to do this to more than one vent because constricting the flow of the entire system can have adverse effects even as serious as causing a furnace to run too hot posing a fire risk.

  5., I live in Illinois and am getting ready to put an addition onto my house. Strictly for looks, I want to have the ceiling rafters exposed with tongue and groove pine above them. Can you recommend a website or source that I can go to that will properly install this from a structural and insulation standpoint. Thanks

    Dave Arnold

    • I’m not sure if I can recommend a specific company, but from what you describe, I would suggest that you look into SIP panels for your roof. Those are insulated and structural so you can do whatever you want for aesthetics underneath that.

  6. Thank you, Ted. The information was useful and helpful as we are looking to “beef” up our attic insulation. We will check into having an emergy audit.

  7. I wouldn’t necessarily complain since every recessed can I’ve seen leaks. You might want to have them install “caps” over them if there’s access in the attic to them.

    I’ve had friends use these and swear by them. Easy to install with a can of spray foam to seal it up. And well worth the money for all the reasons I constantly write about.

  8. Question: I just had my attic air sealed and then insulated. They performed a blower door after all work was done. There is still some air coming through the can lights. I asked about this and they replied that “some air is normal”. But I imagine there has to be some way to make these seals air tight. And that air tight is what’s to be expected. Am I right about this? Am I justified in complaining because air is still coming through cans? Thanks!

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