Community and Q&A

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 https://www.buildingscience.com/documents/insights/bsi-077-cool-hand-luke-meets-attics

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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”.

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Also, further information:

BSI-016: Ping Pong Water and The Chemical Engineer https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-016-ping-pong-water-and-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:

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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).

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