Floor (Over Unconditioned Space) Insulation
Floor insulation is installed between floor joists above unconditioned spaces — garages, open crawlspaces, cantilevers, and other areas where the underside of the floor is exposed to outdoor or unconditioned air. The primary challenge with floor insulation is maintaining contact with the subfloor above while preventing the insulation from sagging away over time. Fiberglass and mineral wool batts are the most common materials, installed with the vapor retarder facing up toward the heated space. The batts must be supported from below using mechanical supports — wire rod insulation supports (tiger claws) are the most common, but stapled netting and rigid board underlayment are also effective. In cold climates, air washing through floor insulation can significantly reduce its effectiveness. Adding a continuous air barrier on the bottom of the insulation assembly prevents cold air from circulating through the fiberglass and carrying heat away by convection. For floors over garages, 5/8-inch Type X fire-rated drywall is required on the garage ceiling in addition to the insulation.
Floor (Over Unconditioned Space) R-Value Requirements by Climate Zone
The table below shows the DOE recommended R-value for floor (over unconditioned space) insulation in each climate zone. Click on any zone to see detailed insulation type comparisons with thickness and cost estimates specific to the floor (over unconditioned space) in that climate zone.
| Climate Zone | Zone Name | Required R-Value | Details |
|---|---|---|---|
| Zone 1 | Hot-Humid | R-13 | Compare insulation types → |
| Zone 2 | Hot | R-13 | Compare insulation types → |
| Zone 3 | Warm | R-19 | Compare insulation types → |
| Zone 4 | Mixed | R-25 | Compare insulation types → |
| Zone 5 | Cool | R-25 | Compare insulation types → |
| Zone 6 | Cold | R-25 | Compare insulation types → |
| Zone 7 | Very Cold | R-25 | Compare insulation types → |
| Zone 8 | Subarctic | R-25 | Compare insulation types → |
Recommended Insulation Types for Floor (Over Unconditioned Space)
The following insulation types are specifically recommended for floor (over unconditioned space) applications based on their material properties, moisture resistance, fire rating, and installation characteristics. While other insulation types can technically be used, these options provide the best combination of performance and practicality for this specific building component.
| Insulation Type | R/Inch | Cost/SqFt/R | Moisture | DIY |
|---|---|---|---|---|
| Fiberglass Batt | R-3.2 | $0.12 | Low | Yes |
| Mineral Wool Batt | R-3.3 | $0.15 | High | Yes |
| Extruded Polystyrene (XPS) | R-5 | $0.18 | Very High | Yes |
| Expanded Polystyrene (EPS) | R-3.9 | $0.12 | Moderate | Yes |
| Cotton Denim (Recycled) | R-3.5 | $0.20 | Low | Yes |
Thickness Required by Insulation Type (Zone 5, R-25)
Using Zone 5 as a representative example, the table below shows how much of each insulation type is needed to achieve the DOE recommendation. The floor (over unconditioned space) has a typical cavity depth of 9.25 inches, so insulation types that require more than this thickness will need deeper framing, additional layers, or a combination of insulation approaches.
| Insulation Type | R/Inch | Thickness Needed | Fits Cavity | Cost/1000 SqFt |
|---|---|---|---|---|
| Fiberglass Batt | R-3.2 | 7.8" | Yes | $3,000 |
| Blown-In Fiberglass | R-2.5 | 10" | No | $2,000 |
| Blown-In Cellulose | R-3.5 | 7.1" | Yes | $1,750 |
| Open-Cell Spray Foam | R-3.7 | 6.8" | Yes | $10,000 |
| Closed-Cell Spray Foam | R-6.5 | 3.8" | Yes | $20,000 |
| Mineral Wool Batt | R-3.3 | 7.6" | Yes | $3,750 |
| Mineral Wool Board | R-4.3 | 5.8" | Yes | $6,250 |
| Extruded Polystyrene (XPS) | R-5 | 5" | Yes | $4,500 |
| Expanded Polystyrene (EPS) | R-3.9 | 6.4" | Yes | $3,000 |
| Polyisocyanurate (Polyiso) | R-6 | 4.2" | Yes | $5,500 |
| Cotton Denim (Recycled) | R-3.5 | 7.1" | Yes | $5,000 |
| Sheep Wool | R-3.5 | 7.1" | Yes | $8,750 |
| Aerogel Blanket | R-10 | 2.5" | Yes | $75,000 |
| Vermiculite (Loose Fill) | R-2.1 | 11.9" | No | $2,500 |
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.
Other Building Components
Browse insulation requirements and installation guidance for other building components.