How much insulation should you install in a ceiling or a roof? When the question comes up on GBA, I usually advise builders to install at least as much insulation as is required in the prescriptive table found in the International Residential Code (IRC) or the International Energy Conservation Code (IECC).
This prescriptive table is known as Table N1102.1.1 in the IRC (see Image #2 at the bottom of the page). In the IECC, the identical table is known as Table R402.1.2 (Image #3).
The minimum prescriptive requirements for ceiling (roof) R-value haven’t changed recently; the requirements in the 2018 code are the same as those in the 2015 and 2012 code. These requirements are:
Builders sometimes complain that these requirements are too stringent, and they ask, “Is there any way that I can get away with less insulation and still meet the code?” As it turns out, there are at least three ways to do that.
Trick #1: In a vented attic, make sure that your insulation covers the top plates of the perimeter walls
GBA has always pointed out that the insulation installed in a vented unconditioned attic must cover the top plates of perimeter walls. Although energy experts have been giving this advice consistently for at least 30 or 40 years, sloppy builders often screw this detail up.
As an incentive to do the right thing, the code allows builders who install insulation properly to get away with less insulation than required in the prescriptive table. The loophole I’m talking about is found in section N1102.2.1 of the various editions of the IRC, and in section R202.2.1 of the various editions of the IECC (see Image #4, below).
The relevant section reads: “Where Section R402.1.2 [or Section N1102.1.1] requires R-38 insulation in the ceiling, installing R-30 over 100 percent of…