Retrofitting a conditioned attic where there is an attached garage (Phoenix, AZ)
A few years ago, we purchased a 20-year old, single story production home in Phoenix, AZ, where the both air handlers, induced combustion furnaces and ductwork are located in the attic. The ventilated roof is covered with low profile cement tiles and O-Hagin roof vents. We recently had an energy audit where the CFM50 = 2100 and estimated R-23 in the attic the auditor recommended more blown in insulation and upgrading our HVAC and ductwork. But we’d rather create a conditioned attic.
The home’s underlayment is original, curling and crumbly, so it’s a great time to re-roof (called a remove and replace) and create a conditioned attic. We are planning to remove and seal the O’Hagin vents, replace the original underlayment with Titanium UDL50 membrane, and add 4.5″ foil faced polyiso above the roof deck. With the foil facing an air gap, we calculate we might achieve an R-30. Next, we’ll remove the blown in fiberglass from the attic floor and air seal the exterior top plates around the perimeter of the house from inside the attic using polyiso board and closed cell spray foam. We’d also planned to seal around the boundary between the house and the attached garage. Next steps include replacing the original HVAC and duct system after we remove the blown insulation as well as adding more insulation to the underside of the roof deck.
We have an attached garage (see the yellow area in the roof framing plan). The highlighted area indicates where the OSB roof and walls of the attached garage are actually enclosed within the home’s larger attic. We were walking through our plans when we realized we appear to be bringing the entire garage into our conditioned space. Oops!
Is there some way we can effectively separate and seal off the attached garage attic from our conditioned attic? We’re thinking of applying a roofing membrane over the garage attic roof and then closed cell spray foam over that. Would appreciate any suggestions you might have.
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Replies
Roof framing plan
It's hard for me to visualize the three-dimensional spaces from the information you have provided, but the concept if simple: if you want to exclude the garage from the conditioned space of the home, you need the insulation barrier (and its adjacent air barrier) to follow the boundary you need to establish.
This usually means that you need framed walls that extend into the attic; these framed walls delineate the boundary between the unconditioned garage and the conditioned house. (The partitions extend from the attic "floor" or the top plates of the garage walls up to the underside of the roof sheathing.)
Once these attic partitions are framed, insulate the partitions. Make sure that the air barrier is continuous and the insulation is continuous.
-- Martin Holladay