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Community and Q&A

High humidity and smell in spray-foamed attic

kaysmith | Posted in General Questions on

Hi – I am really hoping someone can help me out. I have seen similar topics posted, but no solution.

We had the roof deck of our attic (which is also our second floor – a cape cod style house) sprayed with Covestro (formerly Bayer) Bayseal Open Cell foam. The foam seemed fine for a few months until the summer hit. With the return of hot humid weather, we started to see very high humidity in the attic spaces behind the knee walls. On hot sunny days, there is also a distinct unpleasant smell on the second floor.

The attic spaces start out in the early morning at the same temp (72 degrees) and humidity (low 40s) as the house. There is almost no smell except for a typical attic smell. Around noon, when the sun hits the front of the house, the humidity shoots up into the 60s or 70s. This is when the smell starts. At first it only smells in the attic spaces behind the knee walls, but by 5 p.m. – it smells throughout the second floor. As the sun sets, the humidity and odor start to drop and by morning they are normal again. And then the cycle restarts. The smell and humidity are not as strong on cloudy or rainy days – so the sun must play a role.

I have read a bunch of these topics – so I will go ahead and answer as many questions upfront:
– We are in climate zone 4.
– The attic has 5.5 inches of Covestro (formerly Bayer) open cell, although some small areas look like they might have only 3 inches.
– The basement is sealed and dry. 40% RH
– We do not have plants, dry clothes inside, or do anything else that would create excessive moisture. All the fans (shower, stove, and dryer) are new as part of a renovation and do not vent to the attic. The house stays around 45% RH.
– We have had a Covestro rep out who said the foam looks good and on ratio. On a second visit, the rep took samples but we have not heard back yet.
– The HVAC units have all been checked and they are not short cycling and are not oversized. I have read many recommendations to add a supply and possibly a return to control the humidity – but there is not one big attic – it has several small attic areas behind knee walls which would make conditioning them challenging.

My questions are:
– Where is this humidity coming from? I have read different theories on whether vapor drive through the roof cause this situation of “ping-pong” humidity. Some say yes; others no. If the foam was less that 4-5 inches could this be part of the problem? But if the foam was sprayed too thin our problem would be high temperatures – not high humidity?
– What is the smell? The Covestro rep says cured spray foam doesn’t smell and doesn’t off-gas.
– Has anyone had any success in finding a solution to this problem?

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  1. GBA Editor
    Martin Holladay | | #1

    As you know, I answered a similar question recently -- and you commented on that page: Spray foam insulation post-install chemical smell (not rotten fish).

    I'll repeat what others have found: the smell returns with hot weather. Many homeowners have insisted that the spray foam contractor or the installer remove the smelly foam. I suggest that you document your case by sending registered letters to the spray foam manufacturer and installer.

    On the humidity question: Joe Lstiburek insists that the source of the humidity is interior moisture that rises to attics due to the buoyancy of water vapor. Others are skeptical of this explanation. Many others have reported the phenomenon you describe.

    In general, open-cell spray foam is associated with many more problems than closed-cell spray foam when the foam is used on the underside of roof sheathing.

    For more information, you may want to read Open-Cell Spray Foam and Damp Roof Sheathing.

  2. GBA Editor
  3. Expert Member
    Dana Dorsett | | #3

    Behind kneewalls the specualtive buoancy explanation is really a stretch.

    It's not a "vapor drive through the roof" situation, since most roofing materials are very very vapor retardent. But for the record, what's on the top side of the roof deck?

    I suspect the roof deck itself is behaving as the moisture reservoir. When the sun heats it up it drives the moisture off, and the high vapor permeance of the foam lets it into the crawlspace mini-attics, but without air exchanges with drier air to move that moisture out, when the deck cools of to roughly the outdoor dew point temperature overnight, the moisture in the mini-attic air is re-adsorbed into the roof deck.

    If this was all happening behind a class-II vapor retardency interior ceiling finish you wouldn't notice it- the entrained air in the foam would be very high RH when the sun was heating the roof deck, and become drier overnight, with very little of that moisture moving into or out of the attic air volume. One solution might be to install a smart vapor retarder over the interior face of the foam, or put up some air-tight wallboard and paint it with vapor barrier latex.

    Whatever the cause, some amount of active ventilation of those spaces with conditioned space air will probably fix it even without lowering the vapor retardency toward the interior.

  4. kaysmith | | #4

    Thanks for your help. I appreciate your answers.
    Martin, I have read those threads and they do sound very similar to what we are experiencing
    Dana - we have a dark gray 30 yr architectural shingles - the roof was replaced about 4-5 years ago.

  5. Expert Member
    Dana Dorsett | | #5

    Asphalt shingle lay-ups w/ #30 felt come in at about 0.1 perms (the edge of class-I vapor retardency) , and are waterproof to liquid moisture so it's definitely NOT an exterior vapor drive issue. If there is moisture cycling in/out of the attic air proportional to roof deck temperature, the moisture reservoir is the roof deck &/or foam.

  6. kaysmith | | #6

    Dana - thanks again for your input. What you are saying makes a lot of sense.

    We have tried dehumidifying the attic spaces. Since they are small - they quickly drop down to 25%-30% RH - but as soon as the dehumidifier is removed - the humidity returns. Do you think if we ran the dehumidifiers long enough we could remove enough moisture from the sheathing/foam/attic to stop the daily humidity fluctuations? The house was at one point very humid - but it is now much better (40%-45% RH) and should not be adding any additional humidity to the attic spaces.

    We have been hesitant to add AC supply/return vents in the attic spaces because of the odor - we don't know what it is causing the smell - or if it is harmful - we don't want to add recirculate it through the HVAC system if it could impact our family's health.

  7. Expert Member
    Dana Dorsett | | #7

    An 8' x 100' sheet of MemBrain is about $100, and has sufficient fire ratings that it wouldn't need to be covered up. You can probably figure out a way to install it over the foam in a reasonably air-tight fashion.

    With the MemBrain in place if you kept running a dehumidifier when ever the RH in the space was elevated it would eventually purge the majority of the moisture being harbored in the foam & roof deck (assuming that's really the problem.)

    Without the vapor retarder the moisture in the roof deck will rise over the winter, and the whole moisture cycling would return again in the spring/summer season. MemBrain runs about 1 perm in the presence of 40%-45% RH air, lower if drier:

    At 5.5" of BaySeal ocSPF runs about 4 perms:

    So the total amount of moisture uptake over the winter would be less than 1/4 (and maybe less than 1/10) of what it takes on with just the ocSPF as the vapor retarder.

    The amount of ventilation needed to keep the kneewall spaces at the same humidity levels as the rest of the hosue would be tiny, not an HVAC register's worth. A 5 watt fan duty-cycled on a timer (or better yet, under dehumidistat control) would likely cover it.

  8. charlie_sullivan | | #8

    Martin provided an excellent answer on the smell, Dana an excellent answer on the humidity. The next question is whether they are related. They might be, if the water vapor is acting as a carrier to bring the smelly volatile compounds out of the foam. If you solve the humidity problem, it might or might not also solve the smell problem.

    The active ventilation, mixing the attic air with the interior conditioned air, would be a good way to address the humidity, but it would spread the smell through the house as well. So at least short term, it would be good to have a way to dehumidify separately. Given the many small spaces, buying a dehumidifier for each seems expensive. For a quick and cheap solution, an option would be the "damp-rid" calcium carbonate crystals you can buy at a hardware store or home center. The up-front investment needed there is much less than buying dehumidifiers, even though replenishing them on an ongoing basis would not be cost effective.

    [Edit--oops now I see that you have tried dehumidifiers. Yes, I think that running them for a while might be necessary to dry out the sheathing. You'd want a dehumidistat to control them so that they don't run unnecessarily after you get the humidity down. The damp rid might be useful as a way to keep the humidity low for longer between dehumidifier runs]

    If you get the moisture out, you might then install the smart membrane (MemBrain or Intello Plus) that Dana suggests to help prevent the humidity problem from returning. If getting the humidity out helps reduce the smell, then you could add vents to the conditioned space in order to maintain low humidity on an ongoing basis. If getting the humidity out doesn't help the smell, then you are back to the scenario on that that Martin describes.

  9. Rin4 | | #9

    Hi Kay,

    Did you ever find a solution to your problem? We just had spray foam applied a month ago with the promise of it being "odor free", etc, and it still smells so strongly of paint that I'm afraid to let my daughter sleep in her room upstairs. This is particularly strong during a sunny day as you mention. This is our dream home and it's been ruined by this horrible stuff. We also had Bayseal Open Cell and I just want to know if this is something that eventually corrected itself over time or if you found relief any other way. Like I said earlier, this is our dream home and I live in fear of the top floor being full of toxic off-gassing from the fumes. We are getting pretty close to having the roof and trusses ripped off and replaced. Did you ever try the MemBrain or an air exchange or anything?

  10. kaysmith | | #10

    Hi RIN4

    I am so sorry this happened to you - it has been a constant source of stress and problems and it has not been resolved. Here is what I can recommend:

    Follow up with the manufacturer. Call Accella (800-221-3626) and let them know that you are having unresolved odor problems and you are concerned about your child's health, When ours was sprayed it was bayer/covestro - now it is called Accella - but it is the same product. Document everything - send letters by certified mail documenting the conditions in your home. Let them know that you expect them to resolve this problem. Keep meticulous records of your correspondence with them. Take photos. Insist that they do air quality testing in your home immediately. Make sure they do the air quality testing in July or August (or a hot sunny month) and in the late afternoon (this is when ours smells the worst) Insist that they send you written reports of their findings each time they visit your home (even better - ask that they write down their findings before they leave). Make sure they measure the depth of the foam - watch where they measure and make sure they measure it in the shallowest areas. If it is found to be less than the recommended depth - make them document this! Insist they explain to you why your foam is causing a strong odor in the living spaces of your home when it does not in other homes. Escalate everything - find out who is in charge of the spray foam operations and contact them (it may still be Bill Brengel).

    In our situation, the foam still smells strongly every spring/summer in our children's bedrooms. We do not have problems during the winter months. The company has documented that they believe our foam to be properly installed, safe, and will not cause health issues. In retrospect, I wish we had insisted that they remove the foam immediately and not tried to remediate the problem with attic fans, or other proposed solutions (do not allow the foam to be scraped off and re-sprayed) as we are still living with the problem. Try and get the problem documented as much as possible - have your contractor, spray foam installer, and a certified home inspector document the conditions (smell, foam depth, appearance of foam) inside your home and send these to Accella.

    I wish I had more helpful advice or a solution for you - but in truth - I still don't know why this happens in some homes and not others. Our foam was tested and we were told it was properly sprayed (although sprayed too thin in some areas)

    Please send update on whether you find a way to resolve the odor problem as we are still looking for a solution.

  11. Rin4 | | #11

    Hi Kay,

    Is there an email or some way I can contact you privately? We've taken further steps and I wanted to give you an update, but not in a public forum.

  12. GBA Editor
    Martin Holladay | | #12

    It seems that Rin4 has revived this 2105 thread. And GBA readers with similar problems may want to read these two articles:

    "High Humidity in Unvented Conditioned Attics"

    "Spray Foam Jobs With Lingering Odor Problems"

  13. Chicago_IL | | #13

    Hello, Rin4 and Kay. I've been experiencing similar issues since our foam was installed in 2016 and I haven't been able to completely resolve the issue. Have you had any success?


    1. Rin4 | | #16

      Hi Chicago,

      We went over 100k in debt and had to remortgage the house to pay for foam removal. Most companies wouldn't touch it with a ten foot pole. After replacing the roof and OSB, we were finally able to find "the spot" that didn't fully cure tucked away in the back of the rafters. once the spot was cut out, the smell disappeared. We unfortunately couldn't find the uncured spot by smell alone and it wasn't until the end of the project that we reached the uncured spot. Foam insulation is a grievance against humanity. All foam companies are toxic and careless, and the company who did ours- BaySeal- sent out a rep and did nothing to help with the problem. They do claim to have "trained" the tech who applied it so I don't see how they felt it is not their problem. If I could go back, I wouldn't spend the 100k on completely redoing the roof. Rent a dumpster, grab a paint scraper, and hire a bunch of laborers to start scraping and bagging until you find the uncured spot and cut out the smell. I am so sorry that you fell victim to this trash they produce. It ruins homes and ruins lives.

  14. walta100 | | #14

    First let me say I did not reread all the responses today.

    Hello Chicago_IL you may want to tell us your name.

    But it sounds like you sealed your attic away from the outdoors with spray foam and failed to make the attic part of the conditioned space inside your house. So you have done half the job.


  15. Chicago_IL | | #15

    Pete here; thanks for the note. They foamed the roofdeck and above the ceiling in our finished attic space. I'm not sure if that's what you mean, but I hope it helps clarify.

    Any thoughts on the residual smell would be great. It gets a chemical odor on hot days, especially if the sun is beating on the roof.

    Thanks again

  16. Chicago_IL | | #17

    Wow. Thanks for the note. Were you experiencing any health issues from the odor? I think I'm going to have our air quality tested. Did you do that, and were there specific chemicals that you tested for?

    Thanks again for sharing your thoughts. It sounds like it was an awful experience.

    1. Expert Member
      BILL WICHERS | | #19

      The smell isn’t known to be a health issue, but it is some of the speayfoam chemicals floating around in the air that is what you’re actually smelling (“smell” is your brain’s interpretation of chemical compounds in the air that are being detected by your nose), so you could potentially build up a sensitivity over time. I’m not aware of that happening, but I supposed it’s possible. You’d want to test for isocyanates if you hire a testing company.

      I suspect you have some uncured foam somewhere. The application process is pretty critical, and the required mix varies with temperature and other things. It’s very important to have an experienced crew to get a good install. You do NOT want to be anyone’s first job.

      The good thing is that any uncured foam encapsulated within cured foam should be sealed and safe. You need to check the surface and remove any gooey uncured foam if there is any. Once that’s done, you should find the smell goes away, but you may need to provide a few days of forced air ventilation.


  17. walta100 | | #18

    My wild guess is odor problem are from uncured foam for whatever reason.

    I have noticed the people with bad odor problems suddenly stop posting. I suspect they do so after getting a lawyer and end up with a non disclosure agreement that keeps them quiet.

    The moisture problem is often about failure to make the attic part of the conditioned space.


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