Creating a retrofit conditioned attic when I cannot get to cathedral ceiling portion
Hey folks, I posted some of this in an article comment but it probably suits this forum better. I just bought a home that was built in 2021 and I want to condition the attic since it has two HVAC/furnace units in the attic plus ductwork. The contractor can get to all areas of the attic that have the ductwork and the HVAC units, but there is at least one cathedral ceiling that comes off the side of the house where the top of the cathedral connects inside the attic to the area that would get foam and sealed. Therefore, the cathedral would have open soffit venting at the bottom, but be closed off at the top. Is this OK or should I not create an unvented assembly in the rest of the attic since it would cut that venting of the cathedral off?
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Depending on how the cathedral area is connected to the open attic, I would either add roof vents at the upper part of the cathedral space, or I would “duct” the edge of the cathedral space to the open attic ridge using vertical vent baffles on the wall of the open attic space, which would then be sealed and insulated. It’s hard to give much more detail with some pictures, but you absolutely want to keep the cathedral part vented at the top and not rely only on soffits at the bottom if you seal things up.
Bill
Here is an exterior and interior picture of the area in question.
A row of roof vents is probably your best option here. You need one for each rafter bay, mounted as high on the roof as you can get it before the cathedral part transitions to the open attic space part.
Bill
So like 10 or so new perforations in the roof for that?
Maybe just more worthwhile to take off the drywall from the inside and fill the cavity with foam and seal it up. I'm also looking into exterior rigid foam if I'm going to tear this roof off anyway. I have another thread I started I'm trying to find a contractor that is experienced with that.
You could add enough exterior rigid foam for your climate zone, that would solve the problem too. You’d need to seal off the vent channels in that case, since you’d be converting to an unvented “hot roof” in that case. You could open up the interior and put in enough spray foam to get to code required minimum R values. Either way would work. If you use spray foam, use closed cell spray foam for most/all of the R value, then fill any remaining space in the rafter bays with either open cell spray foam or fiberglass batts. That will get you a bit more R value, and it’s best to not have an empty space in the assembly that would allow air to circulate.
If you use roof vents, you’ll need one per rafter pay, so ten vents if you have ten rafter bays. Every rafter bay needs its own vent in this case, since every vent channel needs an “intake” soffit vent and an “outlet” roof vent.
Bill