Basement, crawlspace & rim-joist thermal insulation
Below-grade is where houses quietly bleed heat — and where the rim joist is the single leakiest strip most people miss. This is thermal insulation only; water, drainage and radon are a separate trade.
First, the scope line
This guide covers thermal insulation of basement walls, crawlspace floors/walls and rim joists — area, R and cost. Waterproofing, drainage, sump and radon are out of scope and belong to a pro (or basementcalcs). Insulate a wet basement and you'll trap moisture; fix the water first, then insulate.
Basement walls: cost & R
area = length × height; total = area × your $/ft² × (1 + contingency); R = rigid R + cavity R if you're doing foam board plus a framed batt wall. Example: 120 lf of wall × 8 ft = 960 ft²; at $2.20/ft² with 10% contingency = 960 × 2.20 × 1.10 = $2,323. With 2" XPS (R-10) plus an R-13 batt, the assembly is R-23. Price it in the basement-wall insulation cost tool. Rigid foam against the concrete first is what keeps the assembly warm and dry-side.
Floors over crawlspaces / unconditioned space
total = area × your $/ft² × (1 + contingency). 800 ft² at $1.60/ft² with 10%: 800 × 1.60 = 1,280; × 1.10 = $1,408. Use the floor / crawlspace cost tool. Note: a sealed, conditioned crawlspace is often insulated at the walls instead of the floor — and the vapor/moisture detailing there is set by code, not by this calculator.
The rim joist: small strip, big leak
The rim (band) joist is the strip where the floor framing sits on the foundation — thin, cold and notoriously leaky. rim area = perimeter × rim height. Then either rigid pieces = ceil(rim area ÷ piece area) or foam board-feet = rim area × thickness. Example: 140 lf perimeter × 0.92 ft (11") rim = 128.8 ft². At 2" closed-cell: board-feet = 128.8 × 2 = 257.6 bd-ft → sets = ceil(257.6 ÷ 200) = 2 kits; or rigid 4×8 sheets cut to the bay = ceil(128.8 ÷ 32) = 5 sheets. Count it in the rim-joist insulation calculator.
| Area | Formula | Example result |
|---|---|---|
| Basement wall | len × h × $/ft² × (1+cont) | ~$2,323 (960 ft²) |
| Floor / crawlspace | area × $/ft² × (1+cont) | ~$1,408 (800 ft²) |
| Rim joist (foam) | perimeter × rim h × thickness | 257.6 bd-ft / 2 kits |
Why cut rigid foam or spray the rim — not batts
Fiberglass batts stuffed into rim-joist bays are air-permeable: warm indoor air reaches the cold rim, hits the dew point and condenses behind the batt — hidden mold and rot. Cut rigid foam sealed at the edges, or closed-cell spray, blocks that air path. It's the one spot where material choice is really a moisture decision, so follow the data sheet and local code.
What to check first
- Is the basement/crawlspace dry? Fix water before insulating.
- Perimeter and rim height for the rim-joist count.
- Wall length × height for the wall cost.
- Whether code wants a fire/ignition cover over exposed foam.
- Your installed $/ft² from a quote.
Two ways to insulate a crawlspace — pick one
There's a fork here, and mixing the two causes moisture problems. Vented crawlspace: insulate the floor above (batts between the joists, faced up toward the heated room) and leave the vents working. Sealed / conditioned crawlspace: close the vents, insulate the perimeter walls with rigid foam, and lay a sealed ground vapor barrier — now the crawlspace is inside the thermal envelope. The wall-insulation route usually uses less material (perimeter × height is smaller than the whole floor) and keeps pipes and ducts in conditioned space, but the moisture detailing is stricter. Either way, price the surface you chose in the floor / crawlspace cost tool.
Worked example: rigid vs. spray on the rim
Same 128.8 ft² of rim (140 lf × 11"), two ways. Cut rigid: 2" XPS gives R-10; sheets = ceil(128.8 ÷ 32) = 5 sheets of 4×8, cut into bay-width blocks and sealed at every edge with canned foam. Closed-cell spray: board-feet = 128.8 × 2 = 257.6 → ceil(257.6 ÷ 200) = 2 small kits, and it air-seals in the same pass. Rigid is cheaper in material and DIY-friendly but slow (every block cut and sealed); spray is faster and seals better but costs more and needs the kit warm. Count either in the rim-joist calculator.
Basement wall R: how the layers stack
A basement wall usually gets rigid foam against the concrete (for the warm-side vapor control and to keep the framing dry) plus an optional framed batt wall inside: R = rigid R + cavity R. 2" XPS (R-10) + an R-13 batt = R-23, which suits most cold zones. Keep the foam continuous and sealed at the top where the wall meets the rim, and don't put a poly vapor barrier on the interior face of a below-grade wall — it can trap ground moisture. Cost is area × your $/ft² × (1 + contingency) in the basement-wall cost tool.
Quick numbers to leave with
- Fix water first. Insulating a wet crawlspace or basement traps moisture.
- Basement wall: area × $/ft² × (1 + cont); R = rigid + cavity (2" XPS + R-13 batt = R-23).
- Floor / crawlspace: area × $/ft² × (1 + cont). 800 ft² at $1.60 + 10% = ~$1,408.
- Rim joist: perimeter × rim height; foam board-feet = area × thickness, or rigid pieces cut to the bay.
- Rim = rigid foam or closed-cell, never plain fiberglass batts (condensation risk).
Do the rim joist even if you do nothing else down there — it's cheap, small, and stops a real draft.