UPDATED on August 13, 2017 with new concluding paragraphs
Millions of Americans live in states where residential HVAC contractors routinely install ductwork in unconditioned attics. In many cases, these attics also contain a variety of appliances, including air handlers, furnaces, or water heaters.
Because the disadvantages of this arrangement are fairly well known, I’ll mention them only briefly:
The bottom line: running ducts through an attic saves money for the builder, but costs the homeowners dearly in increased energy costs.
Ideally, HVAC appliances and ductwork should be located inside a home’s conditioned envelope. In the northern half of the country, appliances and ductwork are routinely located in basements or crawl spaces. If your house has a slab foundation, HVAC appliances can be located in an equipment room and ductwork can be located in interior soffits.
Another solution is to move the insulation from the attic floor to the sloped roof, thereby creating a conditioned attic.
Assuming you want to create a conditioned attic — either during new construction or as a retrofit project — how do you go about doing it? Before getting down to the nitty-gritty details, you’ll need to answer at least five overlapping questions:
There are at least three ways to insulate a sloped roof:
A fourth approach combines between-the-rafters insulation with rigid foam insulation (either above the roof sheathing or below the rafters).
Installing rigid foam insulation above the roof sheathing makes a lot of sense. The foam keeps the roof sheathing warm, and therefore dry. Rigid foam also interrupts thermal bridging through the rafters — a big benefit.
Of course, there is a limit to how much foam you can install above roof sheathing; the main problem is the difficulty of hitting the underlying rafters when attaching furring strips or plywood above the foam with long screws.…