Floor (Over Unconditioned Space) Insulation in Zone 7 (Very Cold)
The DOE recommends R-25 for floor (over unconditioned space) insulation in Climate Zone 7 (Very Cold). This zone has 10,000 heating degree days and 300 cooling degree days annually. The typical floor (over unconditioned space) cavity depth is 9.25 inches. States in Zone 7 include Alaska, Colorado, Maine, Minnesota, Montana and 3 more.
Quick Facts
Insulation Comparison for Floor (Over Unconditioned Space) in Zone 7
The table below compares all insulation types for achieving R-25 in the floor (over unconditioned space) of a building in Zone 7 (Very Cold). Types marked "Recommended" are specifically suited for floor (over unconditioned space) applications. Material costs are shown per square foot and per 1,000 square feet for easy budgeting. Labor costs typically add 30 to 100 percent to material costs depending on the insulation type and project complexity.
| Insulation Type | R/Inch | Thickness | $/SqFt | $/1000 SqFt | Fits | Rec. |
|---|---|---|---|---|---|---|
| Fiberglass Batt | R-3.2 | 7.8" | $3.00 | $3,000 | Yes | Yes |
| Expanded Polystyrene (EPS) | R-3.9 | 6.4" | $3.00 | $3,000 | Yes | Yes |
| Mineral Wool Batt | R-3.3 | 7.6" | $3.75 | $3,750 | Yes | Yes |
| Extruded Polystyrene (XPS) | R-5 | 5" | $4.50 | $4,500 | Yes | Yes |
| Cotton Denim (Recycled) | R-3.5 | 7.1" | $5.00 | $5,000 | Yes | Yes |
| Blown-In Cellulose | R-3.5 | 7.1" | $1.75 | $1,750 | Yes | - |
| Blown-In Fiberglass | R-2.5 | 10" | $2.00 | $2,000 | No | - |
| Vermiculite (Loose Fill) | R-2.1 | 11.9" | $2.50 | $2,500 | No | - |
| Polyisocyanurate (Polyiso) | R-6 | 4.2" | $5.50 | $5,500 | Yes | - |
| Mineral Wool Board | R-4.3 | 5.8" | $6.25 | $6,250 | Yes | - |
| Sheep Wool | R-3.5 | 7.1" | $8.75 | $8,750 | Yes | - |
| Open-Cell Spray Foam | R-3.7 | 6.8" | $10.00 | $10,000 | Yes | - |
| Closed-Cell Spray Foam | R-6.5 | 3.8" | $20.00 | $20,000 | Yes | - |
| Aerogel Blanket | R-10 | 2.5" | $75.00 | $75,000 | Yes | - |
Installation Guidance
Floor insulation at R-25 in Zone 7 is installed between floor joists above unconditioned spaces. In cold climates, floor insulation is essential for comfort — uninsulated floors above crawlspaces or garages can feel uncomfortably cold even when the room air temperature is adequate. Use fiberglass or mineral wool batts sized to fit the joist spacing (typically 16 or 24 inches on center). The batts should be installed with the vapor retarder (if faced) facing up toward the heated living space. Support the insulation from below with wire rod stays, insulation netting, or rigid board. In Zone 7, add a continuous air barrier on the underside of the insulation to prevent cold air from washing through the fiberglass and reducing its effectiveness. This can be achieved with house wrap, rigid foam board, or a similar air-impermeable material stapled to the bottom of the joists. Ensure that plumbing pipes in the floor cavity are on the warm side of the insulation to prevent freezing. Pay special attention to rim joist areas where the floor framing meets the foundation wall — these areas are often uninsulated and represent a significant source of heat loss and air leakage.
States in Zone 7
The following states include areas within Climate Zone 7. Click any state to see its complete insulation requirements across all applicable zones.