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.