Creating unconditioned attic (and ducting/furnace question)
I’ve been reading the excellent guidance on attics here: https://www.greenbuildingadvisor.com/blogs/dept/musings/creating-conditioned-attic
I’m in Austin TX, so that’s zone 2. We almost never get snow, but we do have humid periods. The current attic is a very complex shape, with multiple gables, valleys etc. More, when the house was extended (at least twice) they left part of the old roof in place (asphalt shingles and all), so it’s hard to move around in most of the roof (and about 1/3 third of it has almost no crawl space under it, making sealing the underside of the roof almost impossible there). The attic currently has about 3 inches of a combination of old fiberglass, cellulose and rock wool. We have one room at the back stripped to the roof for renovation. There are two large solar powered attic fans. There are no soffit vents (venting is at the top of the gables, currently covered with a very fine mesh screen.) We’ve had lots of rodents in the roof in the past (droppings in the insulation we removed in the room we’ve stripped).
Our HVAC ducting runs around the place and is all flexible tubing with fiberglass wrap at about R2 lying on the attic floor. It’s failing in multiple places and is poorly sealed. Our Furnace and blower for the AC is a vertical Ruud Shiloutte II, about an 80% efficiency model. It’s not a sealed combustion unit. It’s in a closet on the ground, and the top of the closet is open to the attic. Our building inspection mentioned that it should have combustion air brought to it from outside.
So I’ve been considering sealing the sloping roof but it seems that I’d have to upgrade to a sealed combustion furnace, which given the ~1200 heating degree days and my budget doesn’t seem worth it. Especially as the 2″ spray foam on the underside would come to about $2000, with rigid foam under that to R-38 being another $2000. And that leaves out the section with no crawl space.
Short of replacing the entire roof which is beyond budget (but something we’d like to do long term, perhaps adding clerestory windows)
So I’m thinking that the best option is to leave this an unconditioned attic, based on blown-in insulation (maybe Green Fiber to about 12 inches, approx $600 all up). That means at least four things.
The first is new ducting. I’m thinking that fabbing them from duct board makes the most cost/effort sense, something like CeertainTeed Duct Board. But I’d like the ducts to benefit from the R-38. So I was thinking about how to do that and wondered if it made sense to build a box around the ducts (with plywood or OSB) such that the sides were covered by the blown-in insulation. On top I’d continue the sides up a foot so that the insulation could build up there. This then means that the ducts are in a semi-conditioned space. I think the major concern here is condensation on the ducts, which I hope would be ameliorated by the duct itself being about R6 (polyiso or fiberglass board).
We’d also like the dreaded recessed lights, very likely using LEDs. I think the move here is to build sealed boxes around them, out of rigid foam with spray foam sealing.
Third is sealing against rodents, for which we’re thinking metal mesh around the roof corners (where the walls meet the roof), sprayed with foam.
Fourth is dealing with combustion air supply for the furnace. Here I’m thinking that above the furnace closet we build a foam box rising to the roof, then a vent in the roof, so that the furnace has access to combustion air from outside, but air from the unconditioned part of the attic can’t find its way down into the house. Maybe the vent needs to be piped down to the combustion air intake somehow (is that even possible or do these just take air from the register for combustion)?
I’m not sure about how to go about air sealing the older parts of the house before blowing in the insulation. The older roofs are metal lath and some very hard plaster. Even assuming I seal all that well, I’m concerned that it doesn’t function as a vapor barrier between the conditioned house and the cellulose insulation.
I’m also wondering if, by the time I box the ducts and the can lights, it might not be easier to just build and seal a whole false floor for the attic, about 15 inches above the current rafters, sealing the sloping sides up to 15 inches with spray foam (hey, I can afford that much ;), with blown-in insulation on top, running the ducts and cans through that space. I don’t hear that approach discussed much, although it seems to make sense to me.
Any comments appreciated. (sorry for the length, but I wanted to convey the whole situation).
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