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High Humidity in a Spray Foam Attic, Part 2


Three weeks ago, I wrote about an attic with spray foam insulation and the interesting humidity pattern we observed in it. We placed data loggers near the ridge and floor of the attic as well as in the living space and outdoors. In that last article, I showed the dew point data for the four locations and said I’d come back with more explanation. So here’s the followup article I promised.

Humidity data

First, here’s the graph I showed last time. The really interesting part is the big difference in dew point between the highest and lowest points in the attic, shown by the red and green curves below.

The dew point temperature at four locations in a house with a spray foam insulated attic
The dew point temperature at four locations in a house with a spray foam insulated attic

Let’s look at the data for the ridge and floor in separate graphs now. The first one here shows the attic floor, and this time I’ve included the relative humidity. As you can see, the humidity, as measured both by dew point and relative humidity (RH), goes up during the day as it gets hotter outdoors and down at night when it cools off. But the humidity near the attic floor never gets out of control. The dew point and RH stay below 70° F and 70% at their peak (the peak of humidity, not the peak height in the attic).

Temperature, dew point, and relative humidity near the floor of a spray foam insulated attic
Temperature, dew point, and relative humidity near the floor of a spray foam insulated attic

In contrast, the humidity near the ridge in the attic gets very high. The dew point and RH get up close to 90° F and 90%. That’s way too high. On the bad days, when it’s sunny outdoors, the RH stays above 70% for about 8 hours per day and above 80% for more than 4 hours. Not good!

Temperature, dew point, and relative humidity near the ridge of a spray foam insulated attic
Temperature, dew point, and relative humidity near the ridge of a spray foam insulated attic

Why is this happening?

This has been a topic of conversation in Joe Lstiburek’s backyard at Building Science Summer Camp for the past three years. In 2014, Foster Lyons, Joe, and I talked a lot about this. Foster, in fact, kept pressing for better explanations and is the one responsible for Joe’s latest article, Ping Pong Water and The Chemical Engineer. There were also a lot of good comments on the first article in this series. Terry Brennan, a really smart guy with a physics background, nailed it with his explanation

Foster is the chemical engineer alluded to in the title of Lstiburek’s article. He loves to talk about chemical potential, which is part of his explanation for what’s going on. If you want the full, detailed explanation, including discussion of sorption isotherms and pressure swing adsorption, go read Joe’s article. I’ll give a more simplified version here.

What happens is that moisture makes its way into the attic. This could be from cooking, bathing, and breathing in the living space below. It could be from the very humid crawl space, down near the basement and coupled to the rest of the house. It could be from some mystery source. In the living space of this house, the air conditioner and dehumidifier handle the moisture load and keep the humidity under control. In the attic, however, there’s nothing to remove the water vapor that finds its way up there.

At night, the roof cools off. The cool OSB roof deck then offers refuge to the water vapor. Wood is hygroscopic, meaning it’s a porous material that can pull water vapor out of the air and store it on the surfaces and in the pores. The lower its temperature, the more it can store. The amount of moisture a material can store actually increases with relative humidity, and that increases as temperature drops.

So all night long, the roof deck is cooling off and storing moisture. When the sun comes out the next day and starts warming the outdoor air and radiating down onto the roof, the roof deck warms up and drives the moisture out.

When the water vapor exits the spray foam, it increases the relative humidity of the layer of air near the foam. Because water vapor is lighter than dry air (mostly nitrogen and oxygen), that layer of air can move upward in the attic. The result is higher humidity at the ridge than at the floor. Lstiburek calls this the ping-pong effect. The water vapor pings and pongs its way to the ridge through this daily cycling.

Another thing might be driving the upward migration as well. A couple of years ago, Bill Rose spoke about this at the Passive House conference, and I think it makes sense. The ridge of the roof has more exposure to the night sky than lower parts of the roof. That can cause it to cool down more than lower parts of the roof. The lower temperatures can help it pull more water vapor in.

The role of open cell spray foam

Open cell spray foam insulation doesn’t store the moisture. It’s just a pathway. It has a relatively high vapor permeance (~3 perms at a thickness of 6″), allowing water vapor to migrate to the roof decking at night and back into the attic air in the daytime. It’s the wood decking that stores the moisture, though.

The photo below shows a rusty OSB clip in the same house. They had the foam removed and replaced here (because of an off-ratio installation when it was first done) and I got a chance to get up there and inspect the roof deck before they sprayed new foam. I saw a lot of rusty nails and OSB clips. I also saw places where the nails and clips weren’t rusty.

This rusty OSB clip shows the effects of moisture at the roof deck
This rusty OSB clip shows the effects of moisture at the roof deck

The corrosion here indicates that moisture was indeed migrating through the foam and finding the roof deck. Well, at least most evidence points in that direction. In a 2014 article on spray foam and roof rot, I wrote about Joe Lstiburek’s research on different underlayments on top of the roof and how he showed that the moisture in the roof deck was not coming from above. The humidity data above show that there was plenty of moisture below. The conclusion is pretty simple. The moisture comes from below.

What’s the solution?

So, how do you fix this? Or better yet, how do you prevent it? Let’s look at prevention first. If the problem is moisture migrating through relatively permeable foam, you can make it less permeable. Put a vapor retarder on open cell spray foam. That’s actually required in climates 5 and higher. Or you can use closed cell spray foam. Those options would slow down the moisture migration and prevent the problem with the sheathing. It wouldn’t solve the problem of high humidity in the attic, though.

If you don’t do that, you’ve got to reduce the amount of water vapor in the attic. You can do that by directly conditioning the attic with supply air, by blowing conditioned air from the house into the attic, by exhausting air from the attic to outdoors, or by using a dehumidifier.

In his article, Lstiburek mentioned that he had proposed this for the 2018 International Residential Code (IRC). I just spoke with him and he told me his proposal passed, so this will be acceptable in the 2018 IRC. Note that it applies only to climate zones 1, 2, and 3, though.

The upshot of all this is that we need to start thinking of open cell spray foam installations differently. The days are over when you can spray open cell foam in an attic, close it up, and walk away. We know now that this can cause problems.


Related Articles

Humidity in a Spray Foam Attic

Will Open-Cell Spray Foam Insulation Really Rot Your Roof?

Dew Point — A More Meaningful Measure of Humidity?

3 Reasons to Remove Attic Floor Insulation in a Spray Foam Attic


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This Post Has 30 Comments

  1. Great topic as those of us
    Great topic as those of us concerned with structural issues, always use closed cell foam under roof decks.
    Also, still waiting for the humidity control article using supplemental dehumidifiers is tight houses with smaller HVAC systems.

  2. Great follow up article. We
    Great follow up article. We have done air seal testing on foamed homes for the past 6 years and I would say, in our climate, not all that moisture is coming from indoors. Poorly sealed soffits and gravely dangerous bypasses allow for a lot of outdoor air to enter and exacerbate the situation. Too many don’t know about these new ‘big holes’. This is both on new construction and retrofits. In retros, too many still leave ‘filterglass’ in place which creates a deceivingly unsealed attic in most conditions, not to mention the proliferation of non-vented bath fans in retros due to the fact many used to simply dump that exhaust air into the vented attic. We absolutely see more issues in open-cell foam jobs that are poorly sealed vs. closed-cell foam and these articles, mixed with Joe’s, put our thoughts and opinions onto paper!! Thanks again for bringing this up…

    One suggestion for the 3 approaches…exhausting air from attic to outside in our climate (zone 3) seems dangerous due to our high dewpoint air. We have done supply air and dehums which work well , but we have also used dual port bath fans and introduced air from the home into the attic (pulling drier indoor conditioned air from the home and dumping it into the attic with passive returns back to the conditioned space as to not create a pressure imbalance). The reason we like this approach is it provides constant air mixing and air movement in the attic since supply air isn’t always working and many don’t want to spend $ on a dehumidifier. It seems to be working well, but it’s also on homes that are Air Seal Verified and have proper ventilation systems (both exhaust and OA)…but we recommend have a supply duct up there too!! 🙂

    1. Jamie

      I have a home 14 years old home in Florida. Foam in the attic with really high humidity. Ducts sweating and having to keep my thermostat at 70-72 just to keep my humidity in the mid 60’s in my home.

      Should I add a small supply in the attic? Maybe a 8x4x4?

      Maybe add an 80 CFM exhaust fan in the attic?

      Any help would be appreciated. Been dealing with this for many years

  3. Great article. Looking
    Great article. Looking forward to hearing more.

  4. Thanks for this further
    Thanks for this further analysis, Allison. As we have always said, insulating with open cell spray foam is like insulating with a sponge (quite literally). It can work, but only if you take critical precautions. And most everyone still is always looking for the quick/easy way out. We recommend using closed cell foam coupled with eliminating thermal bridging ENTIRELY – and using a non-permeable tape in addition with any foamed joint sealant on the interior, and a semi-permeable tape along with foamed joint sealant on the exterior. And of course condition any enclosed insulated spaces. And you will find that by ENTIRELY eliminating the thermal bridging you can significantly reduce the HVAC loading associated with cooling that space.

  5. I second JK’s motion – we
    I second JK’s motion – we pretty much insist on blower door testing every foam job with which we are associated, and the results are often daunting – 100s, sometimes 1000s of wayward CFMs at 50 Pa, particularly with retrofits.

    We recently upgraded our blower door capability to provide “twin engine” operation for particularly large and leaky homes. We’ll test at as high pressure as two blower door fans will go – 150+ Pa in order to magnify / amplify the leaks.

    Then it is a war of attrition between our leak seekers and the foam guys. We carry three different colors of fluorescent spray paint to distinguish leaks found during successive retests.

    Needless to say, results vary widely among foam companies depending on skill and commitment of the install crew.

    So, I’m with JK – I’m not at all convinced that attic humidity in a spray-foamed house originates entirely within the home.

  6. I inquire along w/ Dale and
    I inquire along w/ Dale and wonder if a vapor permeable ridge vent method could be the 4th way of eliminating attic moisture? I saw Dr Joe’s earlier post on this subject and wonder if it could be used instead of jumping up in cost by going to closed cell foam or an additional step using a sprayed on vapor barrier. I also wonder why conditioning the attic w/ 50cfm/1000sf of supply air is followed by dumping the same amount to the exterior? Couldn’t a return duct draw attic air back into the system? I’m sure many attics are too dirty to do this but in a newly built home that may never be the case if it’s sealed properly. If we are to dump to the exterior, then I’m inclined to use an HRV to re-capture energy before doing so, adding expense, maintenance and access to a normally closed off attic.

  7. Kevin wrote: “I also wonder
    Kevin wrote: “I also wonder why conditioning the attic w/ 50cfm/1000sf of supply air is followed by dumping the same amount to the exterior?”

    Some or all of that air will naturally exchange with the house depending on how tight the roof is. The adiabatic ceiling needn’t be air sealed (except over high moisture areas). In any case, I don’t see how a HRV solves the problem you describe since any loss to the outside (resulting from an attic supply) is unbalanced by definition.

    You can add a return if you prefer to do it that way, but code requires that the foam be covered.

    BTW, here’s a link to Joe’s report on vapor permeable ridge vent: As Dale noted, since the membrane is an air barrier, it avoids the need to use air impermeable insulation. Joe proposed this method as an option for 2018 IRC (only for CZ 1, 2 and 3). It was accepted by the committee but I haven’t heard if it passed at the ICC conference last week.

  8. So after the first article, I
    So after the first article, I moved a de-humidifier into my sprayfoamed attic, and I have had a full bucket (50 pint) every time I have checked it (every 2-3 day). Is that a lot? And is it typical that the sprayfoamed attic smells? We have a smell up there and it is not really strong but noticeable. So that makes me reluctant to exchange the air with the living space air, but maybe it smells because it is just stale? Or does it smell because the sprayfoam installer had incorrect settings when applying and it? And lastly it seems like it would be an idea to use the whole attic as a return plenum, that would take care of any humidity and stale air up there but I guess that is not allowed per the building code…..

  9. I have a similar situation,
    I have a similar situation, Terje. After reading some of Allison’s previous posts on this issue, I placed a cheap hygrometer in my spray-foamed attic (open-cell) and found that humidity was reaching 70% in the early afternoon. To combat, I added a couple of small supplies, which decreased humidity.

    Currently my “return” is just a hole in the ceiling at one end of my house that formerly had a supply duct in it. Unfortunately, my attic also smells a bit musty and unpleasant. I think it is a combination of the spray foam smell with an old attic smell.

    Consequently, when the AC runs for a bit, I can smell the air coming out of this “return” while it travels to the central return/closet. It seems like it would be a better strategy to add a small return grill in the attic above the central return/closet so that the return air goes directly to the air handler/filter, but this would still mean I am distributing this musty (dirty?) attic air throughout the house.

    Allison (or anyone!): can you comment on the issues that Terje and I are experiencing, as well as my proposed return (grill connecting attic to central return closet)?


    1. What you smell is the product
      What you smell is the product of a hydrocarbon-based chemical with amine catalysts breaking down. All foam products break down over time regardless of what any manufacturer tells you. The higher the water vapor (RH), the more off-gas chemical gets transferred in the air you breath. The fix…get rid of all of it. Trust me!

      1. We have been having air
        We have been having air quality issues with our 13 year old spray foam house. We are fairly certain that are only option may be to get rid of it. We would greatly appreciate more of your insights before proceeding with removal.
        Thank You

  10. @Ken, you’re correct, a
    @Ken, you’re correct, a ducted return would only make it worse. I would try closing the hole. There’s no requirement to have a transfer grille between house and attic.

    As Kevin noted previously, having a supply in the attic without a low-static return path can tip zonal pressure balances (depends on relative leakiness of roof vs. ceiling). This could cause a small increase in outside air leakage. However, your only other option – a dehumidifier – would likely consume far more energy than whatever you’re losing due to the induced pressure imbalance.

    You should also try reducing the supply volume. Keep in mind that it’s not necessary to maintain low RH levels in your attic. For example, I wouldn’t consider 65% (peak) to be a problem.

  11. Ken wrote: “I placed a cheap
    Ken wrote: “I placed a cheap hygrometer in my spray-foamed attic…”

    Given the repercussions of the your RH measurements, you should be using a quality meter. RH is notoriously difficult to measure with better than about 5% accuracy. Most consumer grade meters don’t even list an accuracy spec for RH. In your situation, a 5% error could mean the difference between acceptable and not acceptable.

    The better PRO meters have an accuracy spec of +/-3% or +/-2% RH and can be field calibrated (all RH sensors drift over time). The Extech RH305 is the least expensive model I’m aware of that can be field calibrated, but you won’t like the price (

  12. So if one had an ERV or HRV
    So if one had an ERV or HRV one should have a supply and return branch in the attic?
    Maybe a small 4 inch duct for each. I’m contemplating installing one for my house and this seems like a option to consider. I could use a HRV core for winter time and ERV for summertime as I live in the Atlanta area.

  13. @Terje, ventilation can be
    @Terje, ventilation can be used to manage moisture in a sealed attic, but only if you exhaust air from the attic while pulling conditioned air from the house.

    A balanced ventilation system would only reduce attic moisture when outdoor dew point is lower than in the attic. An enthalpy core reduces moisture in the air stream by some percentage of the difference between the outside and inside air, but does nothing to dry the ventilated space if outside dew point is higher.

  14. @David, thank you for your
    @David, thank you for your comments/answers to my questions. I keep pondering the best fix for my home.
    So a big question here then becomes, do the humidity comes from the inside or outside? Well we had bad condensation problems our first winter in our house (newly built Energy Star rated home), to the point that we had mold growth in the window frames. So we certainly do not seem able to get rid of the stuff we have inside in the winter. I do not want humidity in the attic. Open cell spray foam seems like a massive “sponge” that could potentially hold a lot of it.
    we are running two floor standing de-humidifiers all the time and that is really energy inefficient. Having passed the 1 year mark in the house we finally got the Energy Star yardstick score and it was 3.8. This seems to be a really low score for an ES house…?
    So my thinking was that for winter time, the outside air is in general dryer so if I installed a ERV/HRV and popped a HRV core in the machine it would help on the drying of the house. For summer time I could use a ERV core and the HVAC system would do most of the heavy lifting in getting rid of the humidity.
    This would also allow me to get some fresh air into the home as the system we currently have does not work (a ill commissioned CFIS, on a single stage air handler….and oh all the issues with that system….).

    I do find building science fascinating…..maybe its time for a career change….:)
    Thanks for all your insight!

  15. Thanks for your thoughts,
    Thanks for your thoughts, David. Very helpful!

    On your advice, I think I will acquire a pro-quality hygrometer to accurately determine the RH in the attic.

    You stated that you wouldn’t worry about 65% RH in the attic. But based on these blog posts by Allison, it seems like 65% RH in the attic might mean somewhat higher RH at the roofline, leading to rot in the roof sheathing. This is what I’m really concerned with.

    On a related note: for retrofit homes such as mine, a ridge vent exists. The ridge vent has been “sealed” with open-cell foam. What problems is this existing ridge vent causing or solving? It is not clear to me whether I should be removing the ridge vent and sealing it now that the attic has been foamed.

    Regarding a transfer grill: if I don’t use a transfer grill (and thus have an unbalanced zone in the attic), won’t at least some (and possibly most) of that air end up in the living area anyway due to the leaky ceiling? If so, then why is a dedicated return path a bad idea? e.g., a return grill in the air handler closet. My thinking is that by using a dedicated return grill in the air handler closet, then at least the attic air will be filtered and treated before being returned to the living area as supply air. Without a dedicated return grill in the air handler closet, the attic air “pushed” into the living area via the leaky ceiling is unfiltered and untreated.

    Another potential reason for supply air in the attic: in my limited experience at my home, having supply air in the attic lowers the temperature in the attic, leading to a more comfortable home in the summer – particularly at night, when I am lying prone in bed and am especially subject to the radiant heat being released from above.

    Thanks again for your thoughts.

  16. @Ken, I was talking about
    @Ken, I was talking about peak RH readings in the context of roller coaster swings. I should have qualified that I was referring to RH at the ridge, for obvious reasons.

  17. Sorry, I hit the submit
    Sorry, I hit the submit button before completing what turned out to be a lengthy reply…

    Since open cell is relatively vapor open, I’d leave the ridge vent alone since it will allow some moisture to escape without passing any air.

    Regarding transfer grille… what I said was to /try/ closing the hole and see what happens. If may reduce or eliminate attic odors in the house. Otherwise, you have several options:

    a) do a better job sealing up the ceiling plane

    b) go the other direction and make the xfer grille permanent and live with localized odor (but keep in mind, even a high-MERV filter won’t remove odors!)

    c) clean the attic best you can and cover the foam with drywall or other ignition barrier so you can add a code-compliant return

    Regarding radiant heat from the ceiling on the bed…
    I’ve written about that previously, explaining how a ceiling fan, in addition to evaporatively cooling the skin, serves to break up the boundary layer (air film) at the ceiling. This lowers the ceiling temperature (and thus the MRT) when attic is warmer than house.

    Most folks don’t realize that encapsulating may actually raise the MRT, depending on a number of factors. If you can get to it, some foam board pieced in above your bed will help.

  18. @Terje, your reference to
    @Terje, your reference to winter condensation and mold suggests the moisture source is internal. That can be solved with ventilation.

    First, make sure spot ventilation is being used effectively and that the fans are moving enough air due to restrictive ducting.

    If you get winter RH under control in the living space, you shouldn’t need to duct the ventilator to the attic, since you’ve eliminated the moisture source.

    An enthalpy core can be used effectively in winter as long as you don’t use it as primary exhaust for high moisture areas like a bathroom. Anything over about 40% in cold weather is a problem (even lower for homes with older windows). Also make sure you don’t have any bypass paths from high moisture areas to the attic.

    If you’re experiencing high summer RH in the living space (above 60% for any length of time), that’s an indication of an oversized A/C and/or leaky envelope. Based on your comments, it sounds like the house is relatively tight. In that case, direct ventilation to the attic will only make things worse. Even with an enthalpy core, ventilation always ADDS moisture when the outside dew point is higher outside than inside. You have to rely on the A/C, and keep ventilation to a minimum (which is probably why CFIS didn’t work for you).

  19. @David Butler: Thanks for the
    @David Butler: Thanks for the detailed reply!

    Regarding the ridge vent: am I correct that the relatively vapor open (foamed) ridge vent will only allow moisture to escape when the RH in the attic exceeds the RH outside? I live in the hot & humid South, and I worry about moisture entering the attic more than exiting. I also wonder if some of the humidity in the attic that I am observing is entering from the “open” ridge vent. What do you think?

    Great suggestions on the radiant heat issue.

  20. @Ken, good question and one I
    @Ken, good question and one I can’t answer. My recommendation (and presumably the source of your posit) was Joe’s research and testing, and his proposed code change to IRC R806.5 to eliminate the impermeable insulation requirement in CZ1-3 when combined with an air impermeable vapor diffusion port. I’d have to go back and re-read the papers but I think the rationale is that thermal and hygric buoyancy (see ping-pong paper) tend to trump vapor drive, which is the force that would cause vapor to move through air impermeable materials.

    The only way I know to confirm the source of your moisture would be to monitor indoor, attic and outdoor temperature and absolute humidity (e.g., dew point) over some period of days, ideally covering a period of changing weather.

  21. hi allison/all,I live in
    hi allison/all,I live in chicago in a 100 year old house i turned my unused attic into a bedroom for my 2 young daughters and used 6 inches of open cell foam directly to the old wood on the underside of my roof i didnt use a vapor retarder before the drywall was installed i have a separate A/C and furnace installed to take care of the attic space i was reading all the horror stories and didnt do proper research before having it installed and now am just curious to how bad im screwed with no vapor retarder and my furnace dosent have fresh air intake on it but the rest of my house is ballon framing so its not an air tight house just the attic roof. any comments or suggestions would be much appreciated loosing a lot of sleep over this thinking about how bad i could of messed up my house thanks all

    1. Cory, did you ever get an
      Cory, did you ever get an answer to your question? We live in Mi and have a similar problem and would love to know what you may have discovered.

  22. What about zone 4 in
    What about zone 4 in northeast Georgia? You say Joe’s recommendations were approved for zones 1,2,3. Zone 5 is known for issues. Constructing a new house here and have 3 1/2” of open cell in 2×4 walls and 7” average on the roof decking. After installation, I see these articles of problems. Haven’t covered with my interior paneling yet and still have time to make changes of neccasary and can even remove it if needed (pain in the butt and several thousand wasted). Help, I need answers quick. It’s been installed for a week and I still have a few smell issues and I get a slight headache when I’m the house also which scares me about mixing my house air and attic air through a supply and return to the HVAC. And In the spring and fall we may go a few weeks at time with the doors and windows open enjoying the outdoors. In those times Could you just run the fan on your air handler to circulate the air in the attic given you have a supply and return up there a few hours a day? I’m nervous and considering having someone come get this out of my house and go back to a vented attic which will require some modifications to my soffit, siding and ridge.

  23. We used open cell spray foam
    We used open cell spray foam in our large attic to combat issues we has with condensation (ice) in the underside of the roof decking and gables.

    Build and building code inspector thought spray foam was the best option. We created an invented system, closing the ridge and soffit vents. Helped some, but we had issues in the walls too. So we retrofitted the walls with spray foam. No more leaking into the house and down the exterior.

    The only lingering problem we have now is on warm days. The attic is very warm and humid. I am concerned about roof rot.

    What are some options that will not break the bank? Run a supply and return vent? Add rigid insulation panels over the spray foam?

    Not quite time for roof replacement- it is 14 years old. We live in southeast Michigan.

    Well researched suggestions appreciated. Would like to avoid damage or collapse of the roof structure down the road.

  24. I have a large attic space.
    I have a large attic space. On each end of the peak, I have an exhaust fan that activates by temperature control. I recently added a ridge vent. Now I am concerned if adding the ridge vent was a wise idea since when the exhaust fans are running, are they also pulling in moist air through the ridge vents thus defeating the effectiveness of the fans?
    Please help me understand whether adding the ridge vent helped or hindered the attic moisture situation. Thank you.

  25. 3850 sf new closed cell foam
    3850 sf new closed cell foam hoist with high attic humidity. Ac rheem variable speed 18 seer house 72 humidity 53-55%, attic 75,humidity 70+% how do I get humidity out of attic separate portable 70 pt units or one whole house dehumidifier

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