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Fire Code and Off-Gassing With Above-Deck Insulation

jsbb | Posted in Building Code Questions on

I’m converting my old roof to one with above-deck rigid insulation. I’m in climate zone 5 and, and I’ll be insulating with 4″ EPS atop 5.5″ ISO. Below this new roof, I’ll have a cathedral ceiling with the original rafters and roof deck exposed. The house is 150 years-old and the wood is yellow pine (and an occasional piece of fur, perhaps).

The problem is that each row of decking boards has a gap separating it from the rows above and below. The gap is usually less than 1/4″, but sometime closer to 1/2″. You can see the gap in the accompanying photographs.

1) Is this a problem for fire code. The decking itself is thick enough to serve as a fire barrier. Do the gaps undermine its ability to qualify as a thermal barrier between my living space and my roof insulation?

2) Are these gaps a problem for off-gassing from my forthcoming above-deck insulation, and from the stick-and-peel air and water control membrane?* If so, is there a control membrane less likely to off-gas, and/or should I consider adding OSB or some other decking material atop the current decking? Cheers!

* I’m still searching for roofers to tackle the job, but at least one has suggested that I use Owens Corning Rhino Ice and Water Shield for my air and water control membrane.

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  1. ssnellings | | #1

    I don't see attached photos.

    Question 1 is something you should ask your inspector about, since they will be making the determination regarding whether the gaps are acceptable or not.

    If you layer on above the current decking, paint the side visible from below black before installing. If the gaps are really 1/2-inch wide, people will be able to see through the gaps, and painting the layer black will make it look like a design choice instead of a design omission.

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