The exterior walls of your house lose more energy than the windows in most homes — typically 15-25% of total heating and cooling costs leak through under-insulated stud cavities. Picking the best insulation for exterior walls depends on whether you are building new (open stud cavities, easy access), retrofitting an existing house (cavity is closed, drilling required), or adding continuous insulation to the outside of the sheathing. The right choice also varies dramatically by climate zone — what works in Atlanta is wrong for Minneapolis.
This guide covers the four insulation types that dominate exterior wall installations, their R-values, costs, and the situations where each one wins.
Understanding R-Value and Code Requirements
R-value measures resistance to heat flow per inch of material. Higher R-value means better insulation. The DOE recommends the following minimum R-values for exterior walls by climate zone (US):
- Zones 1-2 (Florida, Gulf Coast) — R-13 cavity insulation
- Zone 3 (Southern states) — R-13 to R-15 cavity
- Zone 4 (Mid-Atlantic, Pacific NW) — R-15 to R-20 cavity, +R-5 continuous
- Zone 5 (Northeast, upper Midwest) — R-20 cavity, +R-5 continuous
- Zone 6 (northern tier) — R-20 cavity, +R-7.5 continuous
- Zone 7-8 (Alaska, far north) — R-20 cavity, +R-15 continuous
The “continuous” insulation refers to a continuous layer of rigid foam or mineral wool board on the outside of the structural sheathing, which eliminates thermal bridging through the wood studs. Modern energy codes increasingly require this in colder zones.
Fiberglass Batts: The Budget Standard
Fiberglass batts are pre-cut to fit between standard 16-inch or 24-inch on-center studs. Owens Corning, CertainTeed, and Johns Manville dominate the market.
R-values by thickness:
- R-13 — 3.5 inch (standard 2×4 wall)
- R-15 — 3.5 inch high-density (better in same cavity)
- R-19 — 6 inch (2×6 wall)
- R-21 — 5.5 inch high-density (2×6 wall)
Cost runs $0.40-$0.80 per sq ft installed. Pros: cheapest option, easy DIY install, widely available. Cons: only works as well as it is installed — air gaps, compression, and missed corners cut effective R-value by 30-50%. Fiberglass also lets air pass through, so it must be paired with an air barrier (housewrap or rigid foam) for full performance.
Best fit: new construction in moderate climates where budget rules and the contractor knows how to install fiberglass without compression or gaps.
Mineral Wool: The Quality Upgrade
Mineral wool (also called rock wool) is made from spun basalt and recycled steel slag. Rockwool ComfortBatt is the dominant brand. The product looks similar to fiberglass but performs significantly better in three ways: higher R-value per inch (R-15 in a 3.5-inch cavity vs R-13 for standard fiberglass), fire resistance to 2150°F, and superior sound absorption.
Mineral wool batts are also stiff enough to friction-fit precisely between studs without compression — installers consistently get full rated R-value, unlike fiberglass which often underperforms its rating in real installs.
Cost: $1.00-$1.80 per sq ft installed, roughly 50-100% more than fiberglass. Worth the premium in zones 4 and colder. Fire-rated wall assemblies (multifamily, apartment-over-garage) often require mineral wool by code anyway.
Closed-Cell Spray Foam: The Performance Winner
Closed-cell polyurethane spray foam delivers R-6 to R-7 per inch — nearly double fiberglass. It also air-seals the wall cavity completely, eliminates thermal bridging at gaps, and adds structural rigidity to the wall assembly.
For a 2×6 wall, 5 inches of closed-cell spray foam gives R-30 in the same cavity that fiberglass would deliver R-21. The performance gap is enormous in cold climates.
Cost: $4-$7 per sq ft installed. That is 4-8x the cost of fiberglass. Spray foam also requires a professional installer with the right equipment — DIY kits exist (Tiger Foam, Touch’n Foam) but produce inferior results. Off-gassing during cure is a concern; some installers require occupants to leave the house for 24 hours.
Best fit: high-performance new builds in cold climates, problem walls with persistent air infiltration, and renovations where you want to upgrade a single room without re-insulating the whole house.
Open-Cell Spray Foam: The Middle Path
Open-cell spray foam is lower density and lower cost than closed-cell. R-value runs about R-3.7 per inch — less than closed-cell but still air-sealing. Cost: $1.50-$3 per sq ft installed.
Open-cell expands more than closed-cell during application, which fills cavities with greater consistency. The downside is permeability — open-cell allows water vapor transmission, which is helpful in some wall assemblies and harmful in others. Talk to a building science consultant before specifying open-cell in cold climates with poly vapor barriers.
Rigid Foam Continuous Insulation
Adding rigid foam board on the outside of the sheathing creates “continuous insulation” that breaks the thermal bridge through wood studs. A typical 2×4 wall with R-13 fiberglass actually performs at about R-9 effective because of the studs. Add R-5 continuous foam outside and the same wall performs at R-14 effective — a 55% improvement.
Three rigid foam types:
- EPS (expanded polystyrene) — R-3.6 to R-4.2 per inch. Cheapest, most permeable, white beadboard. Brands: ATAS, Plasti-Fab.
- XPS (extruded polystyrene) — R-5 per inch. Better moisture resistance, blue/pink/green colored. Brands: Owens Corning Foamular, Dow Styrofoam.
- Polyiso — R-6 per inch. Highest R-value, foil-faced. Brands: Hunter, Atlas, Carlisle.
Cost runs $0.80-$1.50 per sq ft installed for 1-inch boards, more for thicker. Most modern code requirements in cold climates demand 1.5-2 inches of continuous insulation outside the sheathing.
Retrofit Insulation for Existing Homes
Existing homes with closed walls (drywall on inside, siding outside) cannot use batts without major demolition. Three retrofit options:
- Blown-in cellulose (dense-pack) — drilled holes from inside or outside, dense-packed to 3.5 lbs per cubic foot. Achieves R-13 to R-15 in a 2×4 wall. Cost: $1.50-$3 per sq ft.
- Blown-in fiberglass loose-fill — similar process, slightly lower R-value but cheaper material. Cost: $1.20-$2.50 per sq ft.
- Injection foam (Tripolymer or Core Foam) — chemical foam injected into wall cavity, expands to fill. Cost: $2.50-$5 per sq ft.
Done by a quality contractor with thermal imaging verification, retrofit insulation cuts heating bills 15-30% in older homes. The single best ROI energy upgrade for most pre-1980 houses.
Quick Decision Framework
Picking the best insulation for exterior walls:
- New construction, mild climate, budget priority — fiberglass batts with quality install
- New construction, cold climate, performance priority — closed-cell spray foam in cavity, polyiso outside
- New construction, balanced — mineral wool batts with continuous rigid foam exterior
- Existing house, moderate budget — dense-pack cellulose retrofit
- Existing house, gut renovation — full rebuild with mineral wool or closed-cell foam
- Single problem wall (cold spot, drafts) — spot retrofit with injection foam
The cheapest insulation done well outperforms the most expensive insulation done poorly. Whatever product you specify, install quality matters more than the spec sheet.
