Free Insulation R-Value Calculator – What R-Value Do I Need?

R-values and zone requirements verified from a real HOMEJECT insulation project. See the full build for context on assembly choices.

Related calculators: Insulating a new build? Start with the wall framing calculator to estimate your stud count, then use the concrete volume calculator for your foundation.

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What R-Value Do I Need for My Attic?

The Department of Energy recommends attic insulation R-values based on your climate zone. In hot Southern climates (Zones 1-2), aim for R-30 to R-49. In moderate Midwest climates (Zones 3-4), R-38 to R-60 is recommended. In cold Northern climates (Zones 5-8), the DOE recommends R-49 to R-60 for uninsulated attics, or adding R-25 to R-38 if you already have some insulation.

The single biggest return on investment for home insulation is almost always the attic. Hot air rises and escapes through the roof in winter; in summer, attic heat radiates down into living spaces. Proper attic insulation can reduce heating and cooling bills by 10 to 50 percent depending on current insulation levels.

R-Value by Climate Zone: DOE Recommendations

The United States is divided into eight IECC climate zones. Here are the Department of Energy recommended R-values for each zone for the three main insulation areas: attic, walls, and floors over unheated spaces.

Zone 1 (South Florida, Hawaii): Attic R-30 to R-49, Walls R-13, Floors R-13.
Zone 2 (Gulf Coast, Arizona): Attic R-30 to R-60, Walls R-13 to R-15, Floors R-13 to R-19.
Zone 3 (Southeast, Pacific Coast): Attic R-30 to R-60, Walls R-13 to R-15, Floors R-19 to R-25.
Zone 4 (Mid-Atlantic, Pacific Northwest): Attic R-38 to R-60, Walls R-13 to R-21, Floors R-19 to R-25.
Zone 5 (Great Lakes, New England): Attic R-49 to R-60, Walls R-13 to R-21 plus R-5 continuous, Floors R-30.
Zone 6 (Minnesota, Montana, Maine): Attic R-49 to R-60, Walls R-15 to R-21 plus R-5 continuous, Floors R-30.
Zone 7 (Northern Minnesota, Alaska interior): Attic R-49 to R-60, Walls R-15 to R-21 plus R-10 continuous, Floors R-38.
Zone 8 (Arctic Alaska): Attic R-49 to R-60, Walls R-15 to R-21 plus R-15 continuous, Floors R-38.

R-Value Per Inch: Comparing Insulation Types

Not all insulation delivers the same R-value per inch of thickness. Understanding R-value per inch helps you choose the right insulation type when space is limited — for example, in 2×4 wall cavities where you have only 3.5 inches to work with.

Fiberglass Batt: R-2.9 to R-3.8 per inch. The most common insulation. Fits standard 2×4 (R-13 to R-15) and 2×6 (R-19 to R-21) wall cavities. Widely available and DIY-friendly.
Mineral Wool (Rockwool): R-3.0 to R-3.3 per inch. Fire resistant, moisture resistant, excellent sound dampening. Good for party walls and areas near fireplaces.
Blown-In Fiberglass: R-2.2 to R-2.7 per inch. Ideal for attic floors. Settles over time — install to a slightly higher depth to compensate.
Blown-In Cellulose: R-3.2 to R-3.8 per inch. Made from recycled paper. Great for retrofitting existing walls and attics. Treated with fire retardant and borate for pest resistance.
Open-Cell Spray Foam: R-3.5 to R-3.6 per inch. Expands to fill gaps and cracks. Good air seal but allows some moisture vapor movement. Lower cost than closed-cell.
Closed-Cell Spray Foam: R-6.0 to R-7.0 per inch. Highest R-value per inch. Also acts as a vapor barrier and structural reinforcement. Best for rim joists, crawl spaces, and metal buildings.

What R-Value Do I Need for Attic Insulation?

Attic insulation delivers the highest return on investment of any insulation project. The DOE recommends R-49 to R-60 for attics in climate zones 5 through 8 (most of the northern United States). In warmer zones 1 through 3, R-30 to R-49 is the target. The most cost-effective material for attic floors is blown-in cellulose or blown-in fiberglass, both of which can be installed quickly and fill irregularly shaped spaces easily. Enter your zone in the calculator above to get the exact depth for your climate.

What R-Value Do I Need for Wall Insulation?

Standard 2×4 stud walls hold a 3.5-inch cavity, which fits R-13 to R-15 fiberglass or mineral wool batts. Upgrading to 2×6 framing gives you a 5.5-inch cavity for R-19 to R-21. In climate zones 5 and above, building codes increasingly require an additional continuous insulation layer on the exterior — typically R-5 rigid foam — to reduce thermal bridging through studs and raise the whole-wall R-value. Use the wall assembly selector in the calculator to compare your options.

What R-Value Do I Need for a Crawl Space or Floor?

Floors over unheated crawl spaces, garages, and cantilevered sections lose significant heat. The DOE recommends R-19 to R-38 for floors depending on climate zone. Standard R-19 or R-30 unfaced fiberglass batts fit between floor joists and are the most common choice. For sealed, conditioned crawl spaces, insulate the walls instead of the floor above, targeting the same R-value for your zone. Select the Floor assembly type in the calculator to compare thickness for each insulation material.

How Much Insulation Do I Need for Basement Walls?

Basement insulation requirements depend on whether the basement is conditioned (heated or cooled) and your climate zone. For conditioned basements in climate zones 3 and above, the DOE recommends insulating basement walls to at least R-5 to R-15. In zones 5 and above, aim for R-15 or higher.

Rigid foam board (XPS or EPS) is the most popular basement wall insulation choice because it resists moisture. Apply it directly to the concrete wall using construction adhesive and mechanical fasteners. A 2-inch layer of XPS provides approximately R-10. Follow with a fire-rated drywall cover per code requirements.

Insulation R-Value for Floors Over Crawl Spaces and Garages

Floors above unheated spaces — like an unheated crawl space, garage, or cantilever — lose significant heat in winter. The DOE recommends R-19 to R-38 for floors depending on climate zone. Fiberglass batts are the standard choice: unfaced R-19 or R-30 batts fit between floor joists and are held in place with wire supports or insulation hangers.

For crawl spaces, you have two main strategies: insulate the floor above the crawl space (vented crawl space) or insulate the crawl space walls and seal it (conditioned crawl space). A conditioned, sealed crawl space is more energy-efficient and keeps pipes from freezing, but requires additional work to properly seal and condition the space.

Vapor Barriers and Insulation: What You Need to Know

In cold climates (zones 5 and above), a vapor retarder on the warm-in-winter side of the insulation helps prevent moisture from migrating into wall cavities and causing mold or rot. Kraft-faced fiberglass batts have a built-in vapor retarder. In hot, humid climates (zones 1-2), the vapor drive reverses — moisture tries to move from outside in, so the vapor retarder should be on the exterior side.

Closed-cell spray foam acts as its own vapor barrier. Open-cell foam does not. When in doubt, consult local building codes — some jurisdictions have specific vapor retarder requirements for your climate zone.

Insulation R-Value FAQs — Common Questions Answered

What is a good R-value for exterior walls? For 2×4 stud walls (3.5-inch cavity), R-13 to R-15 is standard. For 2×6 walls (5.5-inch cavity), R-19 to R-21 is typical. Adding continuous exterior rigid foam improves whole-wall R-value significantly by eliminating thermal bridging through studs.

Is R-30 enough for an attic? R-30 meets code minimums in warmer climates (zones 1-3) but is insufficient for cold climates. In zones 5 and above, the DOE recommends at least R-49 for attics. Upgrading from R-30 to R-49 in a cold climate typically saves $100 to $300 per year in heating costs.

Can you have too much attic insulation? More insulation is generally better up to a point of diminishing returns (typically R-60 for most climates). Beyond R-60, the energy savings do not justify the additional cost. Critically, make sure your attic is properly air-sealed before adding insulation — air leaks reduce effective R-value far more than adding more insulation.

What is the highest R-value insulation for walls? Closed-cell spray foam has the highest R-value per inch at R-6 to R-7. A 5.5-inch 2×6 wall cavity filled with closed-cell foam achieves R-33 to R-39, far exceeding standard batt insulation. However, spray foam is significantly more expensive and requires professional installation.

Does higher R-value always mean better insulation? R-value measures thermal resistance to conductive heat flow, but it does not account for air infiltration or radiant heat. A perfectly installed R-13 batt in an air-sealed wall performs better than R-21 batts with air gaps around them. Always air-seal first, then insulate.

What is the R-value of a 2×4 wall? An uninsulated 2×4 stud wall has an effective R-value of about R-4 to R-5 from the wood framing and sheathing alone. Adding R-13 fiberglass batts brings the cavity value to R-13, but the whole-wall effective R-value — accounting for thermal bridging through studs, which make up about 25% of the wall area — is closer to R-10 to R-11. Adding continuous exterior rigid foam greatly reduces this bridging effect.

How thick is R-38 insulation? The thickness required for R-38 depends on the material: fiberglass batts need about 12 inches, blown cellulose about 10.3 inches, blown fiberglass about 15.2 inches, and rigid foam (polyiso) about 6.3 inches. Closed-cell spray foam achieves R-38 in just 5.8 inches — the thinnest option when space is limited. Enter R-38 as a custom target in the calculator above to see thickness for each material in your assembly.

What climate zone am I in? The United States uses the International Energy Conservation Code (IECC) climate zone map. As a quick guide: Florida and Hawaii are Zone 1, Texas and the Gulf Coast are Zone 2, the Southeast and Pacific Coast are Zone 3, Virginia and the Mid-Atlantic are Zone 4, the Midwest and New England are Zone 5, Minnesota and Montana are Zone 6, northern Minnesota and Alaska interior are Zone 7, and Arctic Alaska is Zone 8. The calculator includes a zone reference for each selection to help you confirm your zone.