Posted By wes on 05/27/2009 7:11 AM
So, in essence, you have an 18" deep attic to deal with.
Did the city give any directions as to quantity of vents or locations? Are your trusses open web in design?
If they gave no directions, and your trusses are open web, then a pair of decorative vents, one low on one end of the truss run, and the other near the peak, at the other end of the truss run, on each slope, should meet the definition of "ventilated".

Posted By Dana1 on 05/27/2009 9:14 AM
Are you sure there's a requirement that it be ventilated, and not just "air gap", between the gypsum & 2lb foam?

I've heard of ventilation being required between foam & roof deck, but never actual VENTIlATION between foam & gypsum. The theory behind the roof-deck/foam ventilation requirement is that the roof deck needs a means of drying from micro-leaks & exterior vapor-drives to keep the wood from saturating over time, but I can't imagine a valid reason for needing ventilation on the interior side, given the permeability of gypsum board.

Make the inspector read & explain the letter of the code to you- this sounds like a newbie misinterpreting things. Under no circumstances should you ventilate the interior-side forming a thermal bypass of the foam insulation. If ventilation is required between the foam & roof deck there are many ways of getting there.

If there's something in the code that requires air-communication through the interior side gap between foam & gypsum, see if they'll accept interior-side vents at the bottom & top of each cavity to allow interior convection loops behind the gypsum without introducing outside air. Seems like it would only be a "problem" if you used highly vapor retardent finishes on the interior forming a vapor trap between gypsum & foam, which would never be a problem as long as some minscule amount of interior air could loop through there. Condensation from that introduced convection would never be a problem, since it's all inside the thermal boundary, and the dew point temps will always occur well-inside the foam.

Posted By wes on 05/27/2009 8:19 PM
Mlennox,
You misunderstand the location of the 'vents' I mentioned. They would be located in the drywall ceiling. The 'ventilation' would be with conditioned air. Since the 'attic space' is inside the thermal envelope, it is part of the conditioned space.

Posted By Dana1 on 05/28/2009 9:09 AM
I've been admonished that it's really a "thermal barrier", not a "fire barrier" (which has a different definition in building-code-speak) that's required. There may be a requirement for a separate thermal barrier between the mechanical stuff and the foam if say, a failure in the air handler could become an ignition source (not likely , but if the inspector didn't call you on it, don't ask!)

Seems to me that if there are even tiny source & return vents in the air-handler space, and it that space is open to the rafter bays it's already vented from a drying-capacity point of view. There may be fire-code issues with potential spread of toxic smoke via the air handler should the foam ignite in that scenario, but the same would be true in the pressurized space with rafter-bay vents condition.