Zone 8 Floor (Over Unconditioned Space) Insulation — R-25
The DOE recommends R-25 insulation for the floor (over unconditioned space) in Climate Zone 8 (Subarctic). The typical floor (over unconditioned space) cavity depth is 9.25 inches. This page compares all insulation types showing the thickness needed, material cost per square foot, and whether each type fits within the standard cavity depth.
Quick Summary
Insulation Type Comparison for Zone 8 Floor (Over Unconditioned Space)
The following table compares all insulation types for achieving R-25 in the floor (over unconditioned space) of a building in Climate Zone 8. Types marked as "recommended" are specifically suited for this building component. The cost estimates represent material costs only and do not include labor, which can add 30-100% depending on the insulation type and project complexity.
| Insulation Type | R/Inch | Thickness | Cost/SqFt | Cost/1000 SqFt | Fits Cavity | Recommended |
|---|---|---|---|---|---|---|
| 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 8 is installed between floor joists above unconditioned spaces such as garages, crawlspaces, and cantilevers. Standard 2x10 joists provide 9.25 inches of cavity depth, and 2x12 joists provide 11.25 inches. Fiberglass or mineral wool batts are the most common materials, installed with the vapor retarder (if faced) facing upward toward the heated space. The insulation must be in firm contact with the subfloor above — sagging insulation creates an air gap that significantly reduces thermal performance. Support the batts from below using wire stays (tiger claws), netting, or rigid board insulation. In cold climates, supplement batt insulation with a continuous air barrier on the underside to prevent cold air from washing through the insulation. For crawlspace applications in Zone 8, consider converting to an unvented conditioned crawlspace with wall insulation instead of floor insulation for better overall performance.
Construction Notes
Floor insulation is installed between floor joists above unconditioned spaces such as garages, crawlspaces, and cantilevers. Standard floor joists are 2x10 (9.25-inch depth) or 2x12 (11.25-inch depth), providing ample space for batt or blown insulation. The insulation must be in contact with the subfloor above and supported from below, typically with wire stays, netting, or rigid boards. In cold climates, a continuous air barrier on the bottom of the insulation helps prevent cold air washing through the insulation. For floors over unconditioned garages, fire-rated drywall is also required on the garage ceiling per building code, which provides incidental support for the insulation.