For decades, designers and builders of wood-framed homes didn’t spend much time thinking about insulation. The usual approach — still followed in much of the U.S. — was to fill the stud bays with fiberglass batts, and, once the ceiling drywall was installed, to unroll some fiberglass insulation in the attic.
Because of this decades-long legacy, it’s not unusual for a designer, builder, or homeowner to post the following question on Green Building Advisor: “We just finished framing, installing windows, and roofing. Now we have a few questions about the best way to insulate.”
My usual reaction is, “Really? You’re asking now?”
These decisions need to be made early
In many cases, these insulation questions are posted a few weeks too late. Why?
Why would builders leave these decisions to the last minute? Well, some builders think, “We’ll either fill the stud bays with fiberglass batts — or, if the homeowner wants to pay for an upgrade, we’ll just use spray foam.” The trouble with that approach is that a wall with insulation between the studs has a lot of thermal bridging through the framing. If you care about this thermal bridging, you really need to include exterior rigid foam insulation or exterior mineral wool insulation — or frame a double-stud wall.
So this important decision — about how to address thermal bridging — has to be made at the design stage, before the walls are framed.
You have to plan ahead for roof insulation, too
In most climates, cathedral ceilings are also best insulated with exterior rigid foam. But the decision to install rigid foam has to be made before the roofing is installed.
True, it’s possible to thoroughly insulate a cathedral…