The owner of a new home under construction in southern Utah (zone 3B), kermit47 (we’ll call him Kermit), would like advice on the best way to insulate his partially finished walkout basement. He says the single-story main house will be well air-sealed and insulated to make the home comfortable and energy-efficient. The basement, whose walls are half concrete and half 2×6 framed, will be used for storage only, although it does contain a heat-pump water heater, and it has one exterior door and no windows.

Kermit needs to decide whether to insulate the basement walls or the room’s ceiling trusses. “My understanding is that code requires one or the other. No one says it can’t be both. Although I like the idea of both, it can get expensive.” He says he’s given this a lot of thought, but is still not sure of the best approach.

Should Kermit insulate his basement walls or ceiling? What should he use? This is the topic for today’s Spotlight.

Insulating the walls makes sense

DCContrarian suggests insulating the walls, because it’s less expensive than the ceiling and Kermit shouldn’t have to worry about the mechanical equipment freezing. He adds that there’s no reason to do both.

It’s true wall insulation would be cheaper and easier to install, and freezing mechanical equipment is an unlikely problem in Kermit’s climate, but he reports that the average ambient outdoor winter temperature is 21°F, according to Manual J calculations, so an R-value between 13 and 19 for walls is ideal.

Martin Holladay says he’s not sure of the applicable building code in that region, but according to the 2018 International Residential Code, basement wall insulation is required in climate zone 3, which means it’s not optional.

Kermit says that he will…