Exterior roof sheathing ventilation
I am designing a 2,000 sf [pretty good passivehouse] house in Colorado. The site is in Zone 7B, 10,000 HDD; -16 Design Temperature @ 8500′ elevation, in other words, damn cold.
We are implementing most, best building practices where practical for this budget [current energy model at 21,000 BTU/hr]. We will have interior and exterior air barriers.
My question/observation is concerning our wood roof construction. We are designing a cold roof [to reduce or eliminate ice damning] with a standing seam metal roofing, in a cathedral ceiling condition with 16″ TJI’s. It is a 24’x48′ shed roof. We will dense pack this with cellulose insulation with a polyethylene interior air barrier enclosure.
I would like to put the roof sheathing ventilation above the exterior air barrier roof sheathing because:
1: maximize and protect the roof insulation;
2: protect this insulation from cold air washing which reduces the effectiveness of the insulation;
3: we will have an above exterior roof batten system on top of the sheathing because of the metal roof application [needs ventilation] and applied roof battens for the roof overhangs that are not part of the exterior air barrier. So I am already need the exterior roof batten system to cool the metal roof and extend my roof overhangs.
Why would I not install as roof sheathing a permeable Zip roof sheathing system or a permeable roofing wrap on OSB and let the above sheathing ventilation do the job of the under sheathing ventilation? I cannot find good literature on this approach.
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Replies
Tim,
It's perfectly acceptable to install ventilation channels above your primary (lower) roof sheathing. For more information, see How to Build an Insulated Cathedral Ceiling.
In that article, I wrote, "If you prefer, you can locate your ventilation channels on top of the roof sheathing rather than under the roof sheathing. You can create 1 1/2-inch-high ventilation channels above the roof sheathing with 2x4s installed on the flat, with the 2x4s located above the rafters, 16 inches or 24 inches on center."
Ventilating on the exterior is not the moral-equivalent of venting under the roof deck, but as long as the interior poly is air tight it'll work.
The major reason it differs is the OSB roof deck itself only has a vapor permeance of about 1 perm, which is too vapor-tight to work in flow-through stackups with a semi-permeable interior. If the inteirior side leaks air or has too much vapor diffusion, with the venting under the roof deck the moisture burden is diluted and moved out via convection. before it can get into the OSB in a big way. But with the very low-permeance of the poly it doesn't matter- exterior venting through permeable wraps is just fine, since the moisture drive from the interior is effectively nil.
On this above roof venting, it would seem that the purlins (2x4 or other) for metal roofing would have to run horizonally on the roof decking in order to provide appropriate fastening surface. This would not allow for the typical soffit and ridge venting since the purlins would block the air flow and the (warm) air rising dynamic would not be in play. Even if vented at the sides, -the gable ends, I don't think there would be much air movement, though perhaps I misunderstand the plan here. This interests me because I have something similar on my house, although it was never intended for venting. I put metal roofing over a temp asphalt roof (rolled roofing) by using 1" rough cut furring strips layed horizontally and nailed to rafters (for a thicker fastening surface), -hence that cold, albeit somewhat dead air space. I've always felt it was helpful; I never have icicles save for sun generated, and have never had a roof leak (save for professionally done chimney flashing). If there were minor leaks they would run down the rolled roofing or evaporate. Oh, but this is above vented gable attic space, so different.
Howard,
In my experience, venting with purlins installed parallel to the ridge works fine. The air is exchanged regularly due to pressure changes resulting from temperature changes. At night, the air cools; during the day, it warms up. This causes a pumping action which keeps the air moving.
Martin:
Are there actual vent strips installed at the gable ends on each side? Good to know this can work, but I don't think I have ever noticed this type of side vent. If the system below the decking here is an unvented one, is this over decking ventilation sort of an "extra mile" situation, -can only help and be extra protection?
Howard,
The ends of the purlins (along the rakes) can be trimmed with insect screen if you want, or can be covered with a trim board. In either case, the pumping action I described will still occur.