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Use of John Mansville unfaced insulation and Dow Thermax

Behealthy | Posted in GBA Pro Help on

Question for Martin Holladay,

We are removing spray foam insulation from our attic that has started off-gassing after 5 years (installed when house built). The recommended replacement will be as follows:

change attic space from unvented to vented with install of ridge vent and removal of soffit blocking

install John Mansville unfaced insulation (will look into Willows, CA plant) with a 2 inch air space along roofline, followed by Dow Thermax isocyanurate foil faced (hence creating a “conditioned” space for the HVAC equipment in the attic). My question is that I am having trouble getting good info. on the chemical properties of the Thermax. We need to be careful with aldehydes and VOCs in general now due to this problem and we are worried that the Thermax will off-gas. Can’t find anything on Pharos. Any thoughts on how to research this? Also, what do you think about the construction solution we arrived at?

Thanks very much.

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  1. GBA Editor
    Martin Holladay | | #1

    If you intend to create a ventilation channel between fiberglass batts and the underside of your roof sheathing, you need to install rigid baffles (for example, AccuVent panels) or site-built baffles made from thin plywood or rigid foam. These baffles must be installed before you install the batts, and they must be installed in an airtight manner, so install them with caulk.

    Although I have never heard of anyone who suffered from the effects of off-gassing from polyisocyanurate insulation -- articles in Environmental Building News do not include any health warnings for polyiso -- it is impossible to predict whether chemically sensitive people will complain of odors or out-gassing. If you are worried, I recommend that you cover the underside of your polisocyanurate with airtight gypsum drywall (installed according to ADA principles).

    Of course, this recommendation comes with a caveat: there are always a few sensitive individuals who fall outside of the usual categories, and cannot tolerate materials that most people tolerate.

  2. user-869687 | | #2

    Foam insulation started off-gassing after five years? This is news to me. Anyone else hear about this happening?

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