How to Insulate Basement Walls in MN

The short version

Foam against the concrete, not fiberglass. DOE says fiberglass or cellulose against basement walls can absorb moisture and stay chronically damp, made worse by plastic sheeting (DOE). Minnesota Commerce says studs with fiberglass against the foundation is "no longer recommended" (MN Commerce).
No plastic on the wall. DOE: don't put polyethylene, vinyl or foil wallpaper, or epoxy, oil or alkyd paint on interior basement walls, because they trap moisture (DOE). Building Science Corporation: no interior vapor barriers, so the wall can dry inward (BSC).
Cover the foam. Code requires foam to be covered by a thermal barrier like ½-inch drywall (MN Residential Code R316.4).
Don't skip the rim joist. Commerce calls it the best place to start: the simplest, least expensive and highest return (MN Commerce).

What Minnesota's energy code says

Minnesota's residential energy code is still the 2012 IECC with Minnesota amendments, in effect since 2015 (Minn. R. 1322.0010); an update to the 2024 code is in rulemaking with no adoption date yet (MN DLI). The Twin Cities counties are climate zone 6A (DOE).

R-15 for basement walls in zone 6, from the top of the wall down to 10 feet below grade or the top of the footing, whichever is less (Minn. R. 1322.0402).
Rigid foam goes directly against the foundation with joints, edges and penetrations sealed against air and water vapor, and the framing can't be fastened through it (R402.1.1.5).
Spray foam is closed-cell sprayed directly on the wall, with at least a 1-inch gap to the framing; open-cell needs a vapor retarder too (R402.1.1.6).
Fiberglass batts are allowed only where no more than 1.5 feet of foundation is exposed above grade, with sealed plates and a sealed vapor retarder on the warm side (R402.1.1.7).
Frame walls can't touch the foundation wall (R402.1.1.4).

St. Paul notes that existing foundations built before June 2009 are exempt from the energy code's basement requirements (St. Paul DSI). Good practice still applies.

Foam board or spray foam?

BSC says spray foam (closed- or open-cell) gives the least risky interior assemblies, and that the most cost-effective approach combines rigid insulation with an insulated frame wall (BSC). DOE says closed-cell spray foam gives the best moisture control of all interior foundation insulations, and that foam boards must adhere completely to the wall with no air gaps and tightly sealed seams (DOE). DOE also recommends leaving at least a ½-inch gap between the drywall and the floor (DOE).

Rim joists

Commerce recommends cut-to-fit rigid foam (extruded polystyrene or polyisocyanurate) caulked or foamed into place, or spray foam, and says fiberglass is not recommended there because moisture condenses on the cold rim joist and can lead to mold or rot (MN Commerce). BSC says the rim joist must be insulated with air-impermeable insulation (BSC).

Dry before you insulate

UMN Extension: "Walls must be dry before insulating" (UMN Extension). Commerce goes further: unless a basement has very low moisture risk, it's better to leave walls uninsulated on the inside so they can dry, and painted-on sealers won't stop water coming through (MN Commerce). If your basement takes on water, fix the grading, gutters and drainage before finishing. MDH also recommends testing for radon before and after adding insulation or finishing a basement (MDH).

Why it's worth it

DOE says basements can account for 10% to 30% of a home's total heat loss (DOE).

Rebates

CenterPoint Energy adds $200 for rim joist insulation when it's done with one of its air sealing or insulation measures, through an eligible installer (CenterPoint). Xcel Energy's rebates cover air sealing, wall and attic insulation for customers who heat with Xcel (Xcel Energy); ask before you start whether basement work qualifies.

What it costs in St. Paul

Homewyse's St. Paul calculator (ZIP 55101, September 2026) puts closed-cell spray foam on basement walls at $4.74–$7.00 per sq ft, and rigid foam board at $2.51–$4.23 per sq ft, before contractor overhead, permits and tax (Homewyse basement walls, Homewyse rigid insulation).

Frequently asked questions

What is the best way to insulate basement walls in Minnesota?

Foam against the concrete: rigid foam board or closed-cell spray foam, sealed at the edges and seams, then a frame wall that doesn't touch the foundation, covered with drywall. DOE and Minnesota Commerce advise against fiberglass directly against basement walls.

What R-value do basement walls need in Minnesota?

R-15 in climate zone 6, which includes the Twin Cities, from the top of the wall down to 10 feet below grade or the top of the footing (Minn. R. 1322.0402).

Should I put a plastic vapor barrier on basement walls?

No. DOE says polyethylene on interior basement walls traps moisture, and Building Science Corporation says no interior vapor barriers so the wall can dry inward.

Do I need to cover foam insulation in a basement?

Yes. Minnesota's residential code requires foam plastic to be covered by a thermal barrier such as ½-inch drywall.

How should I insulate rim joists?

Minnesota Commerce recommends cut-to-fit rigid foam caulked or foamed in place, or spray foam. Fiberglass isn't recommended there because moisture condenses on the cold rim joist.

How much does it cost to insulate basement walls in St. Paul?

Homewyse's St. Paul calculator puts closed-cell spray foam at $4.74–$7.00 per sq ft and rigid foam board at $2.51–$4.23 per sq ft, before contractor overhead, permits and tax.

How to Insulate Basement Walls in Minnesota