Crawlspace Insulation
Crawlspace insulation strategy depends on whether the crawlspace is vented (with foundation vents open to the outside) or unvented (sealed and sometimes conditioned). The traditional approach of venting crawlspaces and insulating the floor above has fallen out of favor in many climates because the vents allow hot, humid air to enter in summer, causing condensation on cool surfaces and promoting mold growth. The building science community and the DOE now generally recommend unvented (conditioned) crawlspaces where the walls are insulated and the space is sealed. In an unvented crawlspace, insulation is applied to the crawlspace walls rather than the floor above. Rigid foam or closed-cell spray foam is applied from the top of the wall to the footing, and a heavy-duty vapor barrier covers the ground floor. The crawlspace is then conditioned with supply air from the HVAC system or a dehumidifier. This approach provides better energy performance, eliminates moisture problems, protects plumbing from freezing, and creates a cleaner, drier space for mechanical equipment and storage.
Crawlspace R-Value Requirements by Climate Zone
The table below shows the DOE recommended R-value for crawlspace insulation in each climate zone. Click on any zone to see detailed insulation type comparisons with thickness and cost estimates specific to the crawlspace in that climate zone.
| Climate Zone | Zone Name | Required R-Value | Details |
|---|---|---|---|
| Zone 1 | Hot-Humid | None required | Compare insulation types → |
| Zone 2 | Hot | None required | Compare insulation types → |
| Zone 3 | Warm | R-5 | Compare insulation types → |
| Zone 4 | Mixed | R-10 | Compare insulation types → |
| Zone 5 | Cool | R-10 | Compare insulation types → |
| Zone 6 | Cold | R-10 | Compare insulation types → |
| Zone 7 | Very Cold | R-10 | Compare insulation types → |
| Zone 8 | Subarctic | R-10 | Compare insulation types → |
Recommended Insulation Types for Crawlspace
The following insulation types are specifically recommended for crawlspace 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 |
|---|---|---|---|---|
| Open-Cell Spray Foam | R-3.7 | $0.40 | Low | Pro |
| Closed-Cell Spray Foam | R-6.5 | $0.80 | High | Pro |
| Extruded Polystyrene (XPS) | R-5 | $0.18 | Very High | Yes |
Thickness Required by Insulation Type (Zone 5, R-10)
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 crawlspace has a typical cavity depth of 4 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 | 3.1" | Yes | $1,200 |
| Blown-In Fiberglass | R-2.5 | 4" | Yes | $800 |
| Blown-In Cellulose | R-3.5 | 2.9" | Yes | $700 |
| Open-Cell Spray Foam | R-3.7 | 2.7" | Yes | $4,000 |
| Closed-Cell Spray Foam | R-6.5 | 1.5" | Yes | $8,000 |
| Mineral Wool Batt | R-3.3 | 3" | Yes | $1,500 |
| Mineral Wool Board | R-4.3 | 2.3" | Yes | $2,500 |
| Extruded Polystyrene (XPS) | R-5 | 2" | Yes | $1,800 |
| Expanded Polystyrene (EPS) | R-3.9 | 2.6" | Yes | $1,200 |
| Polyisocyanurate (Polyiso) | R-6 | 1.7" | Yes | $2,200 |
| Cotton Denim (Recycled) | R-3.5 | 2.9" | Yes | $2,000 |
| Sheep Wool | R-3.5 | 2.9" | Yes | $3,500 |
| Aerogel Blanket | R-10 | 1" | Yes | $30,000 |
| Vermiculite (Loose Fill) | R-2.1 | 4.8" | No | $1,000 |
Construction Notes
Crawlspace insulation approaches depend on whether the crawlspace is vented or unvented (conditioned). In a vented crawlspace, insulation is installed in the floor above between the joists, similar to floor insulation. In an unvented crawlspace, insulation is applied to the crawlspace walls instead of the floor, and the crawlspace is sealed and sometimes conditioned. The unvented approach is generally more energy-efficient and eliminates moisture problems associated with vented crawlspaces in humid climates. Wall insulation in an unvented crawlspace should be moisture-resistant (rigid foam or closed-cell spray foam) and must extend from the top of the wall to the footing. A vapor barrier on the ground floor is essential in both vented and unvented crawlspaces.
Other Building Components
Browse insulation requirements and installation guidance for other building components.