I’m looking for opinions on leaving a vent space when insulating right up to the roof. In the past, builders I’ve worked for have had me nail 1 x 2’s up against the roof and then nail in a piece of rigid foam and then fill the remaining space with fiberglass batts. I’ve also seen it done by just using fiberglass of proper thickness so that a space is left against the roof for venting. In the rigid foam option, is the rigid foam acting as a vapor barrier so that a vapor just under the drywall could cause that space (between the poly vapor barrier and the rigid foam) to hold moisture? Or would you consider the fiberglass only option to be less “tight”? Any thoughts would be appreciated.
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block the wind horizontally (blocking, sheathing or foam board) at the ends of the batts(outside edge of the top plates) and suppy ventilation with baffles against the roof sheathing.
Otherwise, the fiberglass isn't much more than a air filter.
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There are insulation baffles available that are used against the bottom of the roof sheathing to preserve the vent space. The ones available locally are made of cardboard similar to what egg cartons are made of. I assume they are highly vapor permeable.
I have heard of guys making baffles out of rigid foam. I suppose if you have a lot of vapor moving thru the drywall and into the insulation then it might be less able to dry out, as the foam would mostly eliminate drying potential to the exterior.
Rafter vents (Propavent is one brand) are commonly used. They are available for 16" or 24" rafter spacing. Formed from extruded polystryrene in 48" lengths and are stapled to the underside of the roof deck. They are generally "airy" enough to allow any vapor that gets into the rafterbays a quick ride out through the ridge vent.
I've seen the rafter vents used to keep a vent space where the roof passes over the outside wall to keep insulation from blocking the vent space between the soffit into the attic. In my situation, are you suggesting stapling those vents all the way up to the peak in each rafter space and then insulating up to them with fiberglass? That would be cheaper in materials and MUCH less labor than building the space as I mentioned earlier. Thanks much for your input!John
This method is standard operating procedure for ceilings with batts around these parts. Attached is a photo of a recent addition with cathedral ceilings. We installed OSB baffles between each rafter, leaving about an inch gap at the top where the baffle meets the underside of the roof deck. The foam vents get stapled to the underside of the deck all the way up to the ridge vent. Sorry the picture doesn't show the whole roof, but you get the idea. This job had 24" rafter spacing. You can get vents for 16" spacing as well. Your idea would work just fine as well, take your pick.