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Smell in attic from spray foam or fire retardant paint

John DeMotte | Posted in General Questions on

I’m an architect & designed a 2 story addition to a house 2 years ago. I just got a call from the homeowner about some trouble he’s been experiencing since it was built. I’ve never heard of this problem before & would appreciate any advice.
   Rafters were insulated with open cell spray foam to create a conditioned space since an air handler & gas fired furnace are in the attic. A fire retardant paint was then applied over the spray foam. The paint odor was very bad at first & the owner was told the smell would subside….but it hasn’t in 2 years & is especially strong in summer. The odor is getting into the HVAC system & into the bedrooms below, which is a problem.
   The HVAC installer says the mechanical equipment is sweating in the attic since there’s no air circulation & recommends ventilation, but I disagree….kind of defeats the concept of a conditioned attic.
   I don’t know what was used for the fire retardant paint, but that seems to be the culprit. Has anyone else experienced anything similar?
  Thanks in advance…..

Brad DeMotte

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  1. Steve Knapp CZ 3A Georgia | | #1


    What client's climate zone (location)? Open cell foam in an attic is problematic in cooler parts of the country. (See So there could be a moisture/rot issue.


    You are using a combustion appliance in a conditioned space? Without a fresh air source!?

  2. John DeMotte | | #2

    Project is in Southern NY, climate zone 4. I don't think the issue is moisture/rot.......I think the problem is improper installation of the foam as it didn't cure right.

    There is a gas fired furnace in the attic, but I haven't seen how it was installed. I'd assume it has a fresh air source, but will verify.


  3. Steve Knapp CZ 3A Georgia | | #3


    Reports suggest bad foam jobs usually smell like rotten fish. This does not sound like that sort of situation. It probably would be worthwhile to hire a RESNET professional to inspect the project and determine what is going on.

  4. Michael Johnson | | #4

    We just completed a new renovation with similar setup: open cell insulation with fire retardant paint in attic with air handler. The smell is very strong and is coming through the HVAC into the bedrooms. Insulation was installed ~6 months ago and this is in Massachusetts in the South Shore area. We think the odor is coming from the fire retardant paint but we are not sure.

    We are considering adding an exhaust fan in the attic, our HVAC contractor did not have any other suggestions. Have you had any luck on this issue?

  5. Expert Member
    Kohta Ueno | | #5

    If you were interested in more information on high humidity issues in conditioned/unvented attics with spray foam, see the following:

    BSI-077: Cool Hand Luke Meets Attics

    Why do we have to worry about low-density open-cell SPF? It is very vapor open—around 30 perms per inch of thickness—and will allow moisture to pass through it and migrate to the underside of the roof deck (Photograph 5). This is not typically a problem as solar radiation drives this moisture back down out of the foam and back into the attic space air where it is usually removed by air change created by leaky ducts.

    So why not just use high-density closed-cell foam or apply a vapor retarder? Well, there are all sorts of other desirable properties associated with low-density open-cell SPF such as its fire performance and its “drying” properties and its “green” properties if “green” is your thing. For some folks blowing agents matter especially if they are “green”. And, I am not entirely convinced that in some climates that even high-density closed-cell foam with be without issues if there is no “communication”.

    Also, further information:

    BSI-016: Ping Pong Water and The Chemical Engineer

    Lots of attics insulated with open cell low density spray foam (Photograph 1, Photograph 2 and Photograph 3) are having problems – in hot humid climates, mixed humid climates, and cold climates. The problems are moisture related. The attics are “unvented” – open cell low density spray foam is installed directly on the underside of roof sheathing. The attics are humid. Very humid. Unacceptably humid. And the humidity collects at the upper portion of the attics.

    In terms of solutions:

    The HVAC installer says the mechanical equipment is sweating in the attic since there’s no air circulation & recommends ventilation, but I disagree….kind of defeats the concept of a conditioned attic.

    To clarify: by 'ventilation' is the intent 'cut openings in the roof to passively let air flow in and out,' or 'add a fan that moves a fixed amount of air'? I am hoping the latter was the intent.

    Two options that might be worth considering in isolation or together:

    - Per a previous commenter, add an exhaust fan, sending air from the conditioned attic to outdoors. This will increase air change in the attic (diluting odors), and draw conditioned air (assuming air conditioner running) from the main house into the attic (see the Cool Hand Luke 'communication' comments). Lastly, if the exhaust fan is large enough, airflow will generally go from the house into the attic, thus 'containing' the odors.

    - Add a dehumidifier. This will directly address the condensation problems occurring. Also, it is *possible* (not sure how likely) that this might reduce the odor issue. Odors are typically a function of temperature and humidity--we can't do much about the temperature, but reducing humidity might make chemicals 'less active' (less likely to react or evaporate).

  6. brad_glazier | | #6

    The sweating of the ductwork is the key to the mystery. There is a moisture issue in the space caused by insufficient air exchange. I would suspect that the old insulation/vapor retarder may still be in the floor of the attic space. They have in intent created a conditioned space but left out the means by which to condition it. Be sure to remove any insulation from the floor, tie the attic into the air handler of the main house to circulate/exchange air from the attic and/or install an air exchanger to control the humidity.

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