Insulation is one of the most important building components for energy efficiency, comfort, and code compliance. Getting the right R-value for your climate zone isn't optional — it's code-mandated. This guide breaks down what you need and where.
Understanding R-Value
R-value measures thermal resistance — the ability of a material to resist heat flow. Higher R-values mean better insulation performance. R-value is additive: two R-13 batts stacked together give you R-26.
R-value depends on three factors: the insulation material, its thickness, and its installed density. The same material installed at different densities has different R-values — this is especially important for cellulose and spray foam.
IECC Climate Zones
The International Energy Conservation Code (IECC) divides the US into 8 climate zones. Your zone determines the minimum R-values required by building code:
(S. Florida, Hawaii)
(Gulf Coast, S. Texas)
(SE States, S. California)
(Mid-Atlantic, Mid-South)
(Upper Midwest, NE)
(N. New England, Montana)
(N. Minnesota, N. Maine)
(Interior Alaska)
R-Value Requirements by Climate Zone (2021 IECC)
These are the minimum code requirements. Many builders exceed these values for better energy performance and comfort:
| Zone | Ceiling/Attic | Wood Frame Wall | Floor | Basement Wall | Slab Edge |
|---|---|---|---|---|---|
| 1 | R-30 | R-13 | R-13 | R-0 | R-0 |
| 2 | R-38 | R-13 | R-13 | R-0 | R-0 |
| 3 | R-38 | R-20 or R-13+5ci | R-19 | R-5ci | R-0 |
| 4 | R-49 | R-20 or R-13+5ci | R-19 | R-10ci | R-10, 2' depth |
| 5 | R-49 | R-20 or R-13+5ci | R-30 | R-15ci | R-10, 2' depth |
| 6 | R-49 | R-20+5ci or R-13+10ci | R-30 | R-15ci | R-10, 4' depth |
| 7–8 | R-49 | R-20+5ci or R-13+10ci | R-38 | R-15ci | R-10, 4' depth |
"ci" = continuous insulation (installed on the exterior of the framing without thermal bridging through studs). R-13+5ci means R-13 cavity insulation plus R-5 continuous exterior insulation.
These are code minimums, not targets. In zones 5–8, going beyond code R-values (especially in the ceiling) is one of the highest-ROI energy investments. Adding R-19 to an attic already at R-38 to reach R-49+ costs $0.50–$1.00/sq ft but saves significant heating costs for decades.
Insulation Types Compared
Fiberglass Batts
| Property | Value |
|---|---|
| R-value per inch | R-3.1 to R-3.7 |
| Cost (installed) | $0.50–$1.50/sq ft |
| Common sizes | R-13 (3.5"), R-19 (6.25"), R-30 (9.5"), R-38 (12") |
| Best for | Standard stud cavities, attic floors, budgets |
| Moisture | Not moisture resistant — retains water if wetted |
| Air sealing | Does NOT air-seal — air can pass through |
The most affordable and widely used insulation. Available in faced (with kraft paper vapor retarder) and unfaced versions. Performance depends entirely on installation quality — compressed batts, gaps, and voids dramatically reduce effective R-value. A poorly installed R-19 batt may perform like R-11.
Blown Cellulose
| Property | Value |
|---|---|
| R-value per inch | R-3.2 to R-3.8 |
| Cost (installed) | $0.75–$2.00/sq ft |
| Application | Blown into attics (loose-fill) or wall cavities (dense-pack) |
| Best for | Attic insulation, retrofit wall insulation |
| Fire resistance | Treated with borate — Class 1 fire rating |
| Air sealing | Better than batts when dense-packed, but not a true air barrier |
Made from recycled newspaper treated with fire-retardant borate compounds. Excellent for attic insulation because it fills irregular spaces and covers over framing members, reducing thermal bridging. Dense-pack cellulose (3.5 lbs/cu ft density) in wall cavities provides good air leakage reduction.
Spray Foam — Open Cell
| Property | Value |
|---|---|
| R-value per inch | R-3.5 to R-3.7 |
| Cost (installed) | $1.00–$2.50/sq ft (for 3.5" wall cavity) |
| Air sealing | Excellent — acts as air barrier at 3.5"+ thickness |
| Moisture permeability | Vapor permeable — allows drying |
| Best for | Wall cavities, unvented attic assemblies (with conditions) |
Open-cell foam expands to ~100× its liquid volume, filling every gap and crack in the cavity. It's an air barrier but remains vapor-permeable, which can be an advantage in mixed climates where drying in both directions is desirable. Must be covered with a thermal barrier (1/2" drywall) per code in all occupied spaces.
Spray Foam — Closed Cell
| Property | Value |
|---|---|
| R-value per inch | R-6.0 to R-7.0 |
| Cost (installed) | $2.00–$4.00/sq ft (for 2" thickness) |
| Air sealing | Excellent — air and vapor barrier |
| Structural | Adds racking strength to walls |
| Moisture | Vapor barrier at 1.5"+ thickness — also resists bulk water |
| Best for | Rim joists, crawlspaces, exterior wall cavities in cold climates |
The highest R-value per inch of any common insulation. Acts as an air barrier, vapor barrier, and moisture barrier. Also adds structural rigidity. The most expensive option but delivers the most R-value in the least thickness — ideal where cavity depth is limited.
Rigid Foam Board
| Type | R-Value/Inch | Cost/Sq Ft (1") | Best Use |
|---|---|---|---|
| EPS (Expanded Polystyrene) | R-3.8 to R-4.4 | $0.50–$1.00 | Below-grade, under slab, EIFS |
| XPS (Extruded Polystyrene) | R-5.0 | $0.75–$1.50 | Below-grade, foundation exterior |
| Polyiso (Polyisocyanurate) | R-5.7 to R-6.5 | $0.80–$1.75 | Above-grade exterior walls, roofs |
Polyiso note: Performance drops significantly in cold temperatures. At 25°F, polyiso may perform at only R-4.5/inch. For cold-climate below-grade applications, XPS or EPS is a more reliable choice.
Where to Insulate
- Attic floor/ceiling: The single most impactful location. Heat rises — an uninsulated attic is like leaving a giant vent open.
- Exterior walls: Both cavity insulation and continuous exterior insulation in cold climates.
- Floors over unconditioned spaces: Over crawlspaces, garages, and cantilevered sections.
- Basement walls: Interior rigid foam or spray foam on foundation walls in conditioned basements.
- Rim/band joists: One of the leakiest spots in a house. 2" closed-cell spray foam is the gold standard here.
- Slab edges: Rigid foam on the exterior of slab-on-grade foundations in zones 4+.
- Around ductwork: Ducts in unconditioned spaces should be insulated to R-8 minimum.
Vapor Barrier Placement
Vapor barriers (or more accurately, vapor retarders) prevent moisture from diffusing through wall and ceiling assemblies. Placement depends on climate:
- Cold climates (zones 5–8): Vapor retarder on the warm side (interior) of the wall assembly. Kraft-faced batts or polyethylene sheeting.
- Hot-humid climates (zones 1–2): Vapor retarder on the exterior — or use no interior vapor barrier to allow inward drying.
- Mixed climates (zones 3–4): Use a "smart" vapor retarder (like MemBrain) that adjusts permeability based on humidity, or use unfaced insulation with latex-painted drywall as a Class III retarder.
Never install two vapor barriers on opposite sides of a wall assembly. Moisture that enters the wall from either side will be trapped and can cause mold, rot, and structural damage. In mixed climates, the safest approach is often a smart vapor retarder on the interior and a vapor-permeable WRB (weather-resistant barrier) on the exterior.
Cost per R-Value Comparison
To compare insulation value across types, look at cost per R-value per square foot:
| Insulation Type | R-Value/Inch | Installed $/Sq Ft/R-Value |
|---|---|---|
| Fiberglass batts | R-3.2 | $0.04–$0.08 |
| Blown cellulose | R-3.5 | $0.05–$0.09 |
| Open-cell spray foam | R-3.6 | $0.08–$0.15 |
| EPS rigid foam | R-4.0 | $0.10–$0.18 |
| XPS rigid foam | R-5.0 | $0.12–$0.22 |
| Closed-cell spray foam | R-6.5 | $0.15–$0.30 |
| Polyiso rigid foam | R-6.0 | $0.10–$0.20 |
Fiberglass batts and blown cellulose deliver the most R-value per dollar. Spray foam costs more per R-value but provides air sealing that batts and cellulose cannot — often eliminating the need for separate air sealing work.
Estimate Your Insulation Needs
Use our framing calculator to determine wall cavity dimensions, then select the insulation type and R-value that matches your climate zone requirements: