Basement, Crawl Space & Slab Insulation In Depth

Deep-ground temperature in Maine hovers around the low 50s°F year-round (roughly the annual mean air temperature — the ASHRAE Handbook of Fundamentals' ground heat-transfer chapter is the design source), while the top few feet of soil track the winter down toward freezing. Bare concrete has almost no thermal resistance — around R-1 for a full 8-inch wall — so an uninsulated foundation conducts heat outward all heating season, hardest at the above-grade and shallow sections where ΔT is biggest. A full basement can account for a fifth or more of a leaky home's heat loss, and it makes itself felt upstairs: the basement ceiling becomes a cold radiant surface under the first floor, and cold floors are the #1 comfort complaint in Maine houses (ASHRAE Standard 55 — the thermal comfort standard — treats cold surfaces and vertical air-temperature stratification as measurable comfort defects, not just perception; more on the standards page).

The bottom of the house is also the intake side of the stack effect: below the neutral pressure plane, the basement runs at negative pressure and pulls in outdoor air — plus soil gases and whatever the basement air carries — and sends it up through the living space. Sealing and insulating the foundation improves the air you breathe on the first floor, not just the fuel bill.

Basement walls — inside or outside the envelope?

You have a choice of where the thermal boundary runs: insulate the foundation walls (basement becomes conditioned-ish space — best when mechanicals, ducts, or laundry live down there, which is most Maine homes) or insulate the basement ceiling to R-30 and treat the basement as outdoors (then every duct and pipe below needs its own protection). Wall insulation usually wins on cost, freeze protection, and comfort. The code numbers for Zone 6 (2021 IECC §R402.2.8 / Table R402.1.3): R-15 continuous, R-19 cavity, or R-13+R-5ci, from the top of the wall down 10 feet or to the floor. Finishing a basement into living space makes this mandatory — it's a change of space conditioning triggering full compliance (details here).

The rim joist — the leakiest twelve inches in the house

Where the floor framing sits on the foundation, wood meets concrete in a long, cracked, uninsulated band that is both a major conductive loss and a premier air leak at maximum stack suction. The fix — closed-cell spray foam or cut-and-sealed rigid board in every bay, sealed at sill and subfloor — is inexpensive, invisible, and included in virtually every professional weatherization scope.

Crawl spaces — seal them, don't vent them

The modern, code-recognized approach in our climate is the unvented, conditioned crawl space: heavy ground vapor barrier sealed up the walls, wall insulation at the basement-wall levels (IECC §R402.2.10), vents closed, and the space either supplied with a little conditioned air or exhausted per the IRC (§R408.3). Open crawl vents in Maine import humid summer air onto cold surfaces and arctic winter air under your floors — the old advice made both problems worse.

Slabs

For new work, slab-edge insulation is code: R-10 extending 4 feet down/under/out, plus R-5 under the full slab when it's heated (IECC §R402.2.9). The slab edge is the detail to fight for in any addition or garage-conversion project — it's unreachable later, and an uninsulated edge in Zone 6 is a permanent cold ring around the room.