Walk into any hardware store and you'll find three main categories of insulation staring back at you. They all promise to keep your home warm — but they don't perform the same way when it's -20°F outside, and choosing the wrong type for the wrong location can quietly cost you money on heating bills for years.
If you're in the Midwest, Wisconsin, the Dakotas, or anywhere else that sees sustained subzero temps, this is the comparison you actually need. We're not going to cover what works fine in Georgia. We're going to talk about what works here.
Spray Foam: The Best Air Sealer, at a Price
Spray foam comes in two types — open-cell and closed-cell. In cold climates, closed-cell is the one that matters.
Closed-cell spray foam expands to fill every gap, creating both an insulation layer and an air barrier in one shot. R-value runs R-6 to R-7 per inch — the highest of any common insulation. In a 2x6 wall cavity it can hit R-21 on its own, and it won't lose performance when temperatures drop.
The air-sealing piece is what makes it worth the cost up here. When it's -20°F outside, even tiny air leaks are serious heat loss points. Fiberglass batts insulate, but cold air moves right through them. Spray foam stops both heat transfer and air movement.
Where to use it in a cold climate home:
- Rim joists — the most common air infiltration point in basements
- Around window and door rough openings
- Crawl space walls
- Cathedral ceiling assemblies
The honest downside: closed-cell spray foam runs $1–$2 per board foot installed. That's 2–4x the cost of fiberglass. Don't use it everywhere — use it strategically where it earns its keep. For DIY sealing at smaller penetrations, Great Stuff Pro Gaps & Cracks on Amazon → is the pro-grade version of the orange can and handles rim joist gaps and pipe penetrations well.
Fiberglass: Reliable When Installed Right
Fiberglass batts are the most common insulation in American homes for good reason — affordable, widely available, and solid performance when installed properly. R-values run from R-11 (2x4 walls) to R-21 (2x6 walls) to R-49+ for attics using blown-in.
The key phrase is "when installed properly." Fiberglass has two weaknesses that really show up in cold climates:
Compression kills performance. A batt stuffed into a cavity slightly too small drops R-value fast. An R-21 batt compressed to 4 inches might only perform at R-15. This sounds obvious, but it happens constantly on job sites.
Air moves through it. Fiberglass is not an air barrier. In a cold climate, wind-washing — cold air infiltrating the wall cavity and flowing through the insulation — can gut effective R-value dramatically. Air sealing before insulation goes in isn't optional up here; it's the whole game.
For blown-in attic work, Owens Corning L77 Blown-In Fiberglass at Home Depot → is a workhorse product — the store will rent you the blower, and it's a solid DIY weekend project for an attic floor.
Best uses in cold climates:
- Wall cavities with 2x6 framing (R-21 batts)
- Attic floors — blown-in at R-49 to R-60 is the most cost-effective attic approach
- Floor cavities over unconditioned spaces
Rigid Foam Board: The Underused Upgrade
Rigid foam doesn't get enough attention in residential construction, but it solves a problem batts and spray foam can't: thermal bridging through studs.
Wood studs conduct heat significantly better than insulation. In a standard 2x6 wall with R-21 fiberglass batts, the studs (which make up 15–25% of the wall area) are only performing at about R-6.5. The effective R-value of the whole wall assembly drops to R-14 to R-16 as a result.
Adding even 1 inch of continuous rigid foam over exterior sheathing — before siding goes on — bumps the assembly's effective R-value to R-20+ and eliminates most of the thermal bridging. On a re-siding project, this is one of the highest-value upgrades you can make.
Rigid foam comes in three types:
| Type | R-Value per Inch | Notes |
|---|---|---|
| EPS (expanded polystyrene) | R-3.8 | Most affordable, vapor-permeable |
| XPS (extruded polystyrene) | R-5 | Moisture-resistant — good for basements |
| Polyiso | R-6 to R-6.5 | Highest R/inch, but loses performance in extreme cold — less ideal as exterior sheathing in MN |
For basement walls, Owens Corning FOAMULAR 150 XPS at Home Depot → (the pink board) is the go-to — handles moisture well and cuts cleanly with a utility knife. For exterior wall sheathing on a budget, 2-inch EPS is excellent value.
The Right Combination for a Cold Climate Home
No single product wins. The best-performing cold climate homes layer these materials based on where each one earns its keep:
| Location | What to Use |
|---|---|
| Exterior walls | R-21 fiberglass batts + 1" rigid foam on exterior sheathing |
| Attic floor | Blown-in fiberglass at R-49 to R-60, spray foam sealing penetrations first |
| Rim joists | Closed-cell spray foam, 2–3 inches thick |
| Basement walls | XPS rigid foam against concrete + fiberglass batts in framed wall |
| Crawl space walls | Closed-cell spray foam or XPS rigid foam |
This combination hits cold climate performance targets without blowing the budget on spray foam everywhere.
The Bottom Line
The mistake most homeowners make is treating insulation as a single product decision. It's not — it's a system. Spray foam where air sealing is critical. Fiberglass for bulk fill in walls and attics. Rigid foam to knock out thermal bridging on exterior assemblies. Use each one where it's strongest and they work together beautifully.