liminfoInsulation R-Value Calculator
Free web tool: Insulation R-Value Calculator
Insulation Layers
R-11.2
Total R-Value
R-11.2
Recommended R-Values by Climate Zone
| Zone | Attic | Wall | Floor |
|---|---|---|---|
| 1-2 (Hot) | R-30 to R-49 | R-13 to R-15 | R-13 |
| 3 (Warm) | R-30 to R-60 | R-13 to R-15 | R-25 |
| 4 (Mixed) | R-38 to R-60 | R-13 to R-21 | R-25 to R-30 |
| 5-6 (Cold) | R-49 to R-60 | R-13 to R-21 | R-25 to R-30 |
| 7-8 (Very Cold) | R-49 to R-60 | R-21+ | R-25 to R-30 |
About Insulation R-Value Calculator
The Insulation R-Value Calculator helps homeowners, builders, contractors, and energy auditors determine the total thermal resistance (R-value) of a multi-layer insulation assembly in walls, attics, floors, or any building component. R-value measures resistance to heat flow — higher values mean better insulation and lower heating/cooling energy costs. The calculator supports layered assemblies because real-world insulation systems often combine multiple materials (e.g., fiberglass batt in wall cavities plus rigid foam on the exterior).
The tool supports ten common insulation materials with their R-value per inch: Fiberglass Batt (R-3.2/in), Blown Fiberglass (R-2.5/in), Cellulose blown (R-3.5/in), Open-Cell Spray Foam (R-3.7/in), Closed-Cell Spray Foam (R-6.5/in), Rigid XPS Foam (R-5.0/in), Rigid EPS Foam (R-4.0/in), Rigid Polyisocyanurate (R-6.5/in), Mineral Wool (R-3.3/in), and Recycled Denim (R-3.5/in). For each layer, the total R-value = R/inch × thickness in inches, and all layers are summed to give the assembly total.
The calculator also displays a reference table of DOE/IECC-recommended R-values by US climate zone for attic, wall, and floor assemblies. This lets you immediately compare your calculated total against the code-required minimum for your climate. Climate zones range from Zone 1-2 (hot climates like Florida) requiring R-30 to R-49 in attics up to Zone 7-8 (very cold climates like northern Minnesota or Alaska) requiring R-49 to R-60 in attics and R-21+ in walls.
Key Features
- Ten insulation materials with pre-loaded R-value per inch constants from published data
- Multi-layer assembly support — add as many layers as your wall or roof assembly contains
- Per-layer R-value displayed alongside total for each material type and thickness combination
- Dynamic add/remove layer buttons for easy assembly configuration
- Total R-value prominently displayed with real-time update as layers change
- Built-in DOE/IECC climate zone reference table for attic, wall, and floor R-value targets
- Supports both cavity insulation (batts, blown) and continuous insulation (rigid boards) scenarios
- 100% client-side — no building data uploaded, works offline for job site use
Frequently Asked Questions
What is R-value in insulation?
R-value (thermal resistance) measures how well an insulation material resists heat flow. Higher R-values mean better insulating performance. It is calculated as the temperature difference across the material divided by the heat flux (BTU per hour per square foot). R-values are additive: if you stack two R-13 batts, you get R-26. The unit is (°F·ft²·hr)/BTU in imperial units.
What is the R-value per inch for common insulation types?
Approximate R-values per inch: Fiberglass batt R-3.2, blown fiberglass R-2.5, cellulose (blown) R-3.5, open-cell spray foam R-3.7, closed-cell spray foam R-6.5, rigid XPS (extruded polystyrene) R-5.0, rigid EPS (expanded polystyrene) R-4.0, polyisocyanurate board R-6.5, mineral wool R-3.3, recycled denim R-3.5. Closed-cell spray foam and polyiso have the highest R/inch, making them valuable where space is limited.
How do I add multiple insulation layers?
Click the "+ Add Layer" button to add additional material layers to your assembly. Each layer has a material selector and a thickness (inches) input. The per-layer R-value is displayed automatically, and the total R-value updates in real time. This is useful for assemblies like "R-13 fiberglass batt in stud cavity + 2 inches of rigid XPS on the exterior" which together give R-13 + R-10 = R-23.
What R-value do I need for my climate?
The calculator includes a DOE/IECC climate zone reference table. Zone 1-2 (hot climates, e.g. Florida): attic R-30 to R-49, wall R-13 to R-15. Zone 3 (warm, e.g. Texas): attic R-30 to R-60, wall R-13 to R-15. Zone 4 (mixed, e.g. Virginia): attic R-38 to R-60, wall R-13 to R-21. Zone 5-6 (cold, e.g. Chicago, Minneapolis): attic R-49 to R-60, wall R-13 to R-21. Zone 7-8 (very cold, e.g. Alaska): attic R-49 to R-60, wall R-21+.
What is the difference between open-cell and closed-cell spray foam?
Open-cell spray foam (R-3.7/in) has a soft, spongy texture and permeable cells. It is vapor-permeable, less expensive, and expands to fill irregular cavities well. Closed-cell spray foam (R-6.5/in) has dense, rigid cells filled with a blowing agent, giving it nearly double the R-value per inch. It is also a vapor barrier, adds structural strength to walls, and is more moisture-resistant — but costs more per square foot.
Can I use this calculator for walls, attics, and floors?
Yes. The calculator computes total R-value for any layered insulation assembly regardless of location. The climate zone reference table at the bottom provides separate recommended R-values for attic, wall, and floor applications. Simply configure the layers matching your actual wall assembly (or attic or floor assembly) and compare the total R-value against the appropriate recommendation for your climate zone.
Does R-value account for thermal bridging through studs?
No. This calculator computes the clear-field R-value (insulation only, in the cavity between studs). In real wall assemblies, wood studs (R-1.25/in) create thermal bridges that reduce the effective whole-wall R-value by 10–20% compared to the clear-field value. For whole-wall R-value including thermal bridging, use a calculation method per ASHRAE 90.1 or the parallel path method, which weights cavity and framing areas by fraction.
What is the most cost-effective insulation upgrade for an existing home?
Air sealing followed by attic insulation is typically the highest ROI upgrade for existing homes. Attics are easiest to add insulation to (blown cellulose or fiberglass on top of existing) and have the largest temperature differentials. Going from an inadequate R-19 to code-recommended R-49 in an attic can reduce heating and cooling costs by 15–25%. After attics, rim joists and crawl space floors are typically next in cost-effectiveness.