Insulation R-Value Calculator
Free insulation R-value calculator. Total R-value from layers, plus U-factor and rough heat loss. No signup.
An R-value calculator totals the thermal resistance of an insulation assembly, layer by layer, then derives the U-factor and a rough heat-loss figure for a given area and temperature difference.
How total R is calculated
Each line of input is one layer in the format “R-value per inch, thickness in inches”. The contribution of a layer is its R per inch multiplied by its thickness, and the layers add together:
layer R = R per inch × thicknesstotal R = sum of all layer R valuesU-factor = 1 ÷ total Rheat loss (BTU/hr) = area × temperature difference × U-factor
The heat-loss line is a rough steady-state estimate. It ignores sun, air leakage, thermal bridging, and humidity, so treat it as a comparison tool rather than an energy bill prediction.
Worked example: a single 3.5-inch fiberglass layer
Defaults: one layer entered as 3.5,3.5, area 1,000 ft², temperature difference 40 °F.
| Step | Calculation | Result |
|---|---|---|
| Layer R | 3.5 × 3.5 | 12.25 |
| Total R | 12.25 | R-12.3 |
| U-factor | 1 ÷ 12.25 | 0.08 |
| Heat loss | 1,000 × 40 × 0.0816 | 3,265 BTU/hr |
Add a second line for 2 inches of polyiso board at R-5 per inch, entered as 5,2. That adds 10 to the total, lifting R to 22.25, dropping the U-factor to 0.04, and cutting the heat loss to about 1,798 BTU/hr — a useful way to see what one extra layer buys.
Reference: R-value per inch by material
| Material | Approx. R per inch |
|---|---|
| Fiberglass batt | 3.1–3.5 |
| Mineral wool batt | 3.1–3.5 |
| Blown cellulose | 3.1–3.8 |
| Open-cell spray foam | 3.5–3.7 |
| EPS rigid board | 3.6–4.0 |
| XPS rigid board | 5.0 |
| Polyisocyanurate board | 5.6–6.5 |
| Closed-cell spray foam | 6.0–7.0 |
To use the table, multiply R per inch by the installed thickness. A 3.5-inch fiberglass batt is about R-13; a 5.5-inch batt about R-19; 1 inch of XPS is R-5.
| Assembly | Common code target |
|---|---|
| Attic / ceiling | R-38 to R-60 |
| Above-grade walls | R-13 to R-21 |
| Floors over unconditioned space | R-19 to R-25 |
| Crawlspace walls | R-10 to R-15 |
Targets depend on climate zone, so check your local code. Cold climates sit at the top of each range.
R-value is not the whole story
Insulation slows heat through the material, but air leaks move heat far faster. Sealing gaps around pipes, wiring, and framing before insulating usually delivers a bigger improvement than adding another inch of R. The heat-loss figure here assumes a sealed assembly, so real buildings lose more than it predicts.
Moisture matters too. A vapour barrier on the warm side of the wall, and ventilation above the insulation in an attic, protect the R-value you paid for. Compressed or damp insulation performs well below its label.
Common mistakes
- Confusing R-value with U-factor. R measures resistance and bigger is better; U measures heat transfer and smaller is better. U = 1 ÷ R.
- Adding parallel paths as if they were layers. Layers in series add, but studs and joists sit in parallel and conduct heat around the insulation. This tool sums insulation layers only.
- Compressing batts. Squeezing insulation into a shallower cavity lowers its R-value rather than raising it.
- Reading R per inch as a total. A package labelled R-3.5 per inch is not R-3.5; multiply by thickness.
What R-value do I need?
It depends on climate, but common targets are R-38 to R-60 in attics, R-13 to R-21 in walls, and R-19 to R-25 in floors. Colder climates need the higher end.
How do I add more than one layer?
Put each layer on its own line in the format “R per inch, thickness”. For example, 3.5,3.5 on the first line and 5,2 on the second gives R-22.25.
Does the heat-loss number include framing?
No. It uses the insulation R-value you enter and ignores thermal bridging through studs, which reduces real-world performance.
Does this calculator include the R-value of drywall or siding?
No. It sums only the insulation layers you enter. Drywall, sheathing, and cladding each add a little R, but the insulation dominates the total.
Insulation is an efficiency purchase, so the payback period calculator is a natural next step. For the surfaces it sits behind, the drywall calculator and paint calculator use the same room measurements.