Spray foam insulation and furring strips for drywall?

thisoldbungalowSeptember 30, 2010

My husband and I recently bought a 1920s bungalow with a finished attic. Since it was a cook box in the summer, we tore down the drywall in order to pull out the deteriorated Kimsul insulation and get the whole roof and exterior walls insulated with open-cell spray foam.

Over the 2x6 rafters (24" o.c.), previous owners had nailed furring strips to which the drywall was fastened. We are planning to have the spaces between the rafters filled with 5.5" of foam and attach new drywall to the existing furring strips. This would leave about a 3/4" gap between the surface of the foam and the drywall.

In my research on the web, I've come across occasional comments about the risk of moisture condensation and mold in this airspace between the drywall and the insulation. Our insulation contractor, however, says he has never heard of this problem. I haven't been able to find any official information about it either, all I've found are forum comments.

Does anybody have any information if there is indeed an issue with having a gap between the insulation and the drywall? My husband thinks the furring strips might actually be beneficial, because they would minimize contact and thus heat transmission through the rafters to the drywall. It seems like since both the gap and the drywall are in a conditioned space, there should not be an issue, but the comments I have come across make me wonder. The possibility of mold is obviously something we would like to rule out. (We live in a climate with hot, humid summers and relatively cold winters.)

Thanks in advance.

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Unless you have some further insulation over top of the o.c. foam, I'd say there is the possibility of condensation because R 19.8 is about half the minimum R Value you need in an attic in your climate. I assume you're north of the Mason-Dixon Line.

    Bookmark   September 30, 2010 at 8:50PM
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I should add it has nothing to do with the gap, but the fact that the bottom of the o.c. foam may be a condensing surface in the winter. When the warm air of the conditioned space touches the foam, water could condense out. On the plus side, the foam should be little affected and much of the water vapour will pass through and, hopefully, be vented out through adequate roof vents.

    Bookmark   September 30, 2010 at 9:02PM
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