What is the R-value of your wall, roof or floor?
Add each material layer and its thickness — the calculator sums them (with the inside and outside air films) into a total R-value, the SI RSI value and the U-value.
That is the whole assembly's resistance to heat flow — the layers added together, plus the air films. A higher R-value (lower U-value) means a better-insulated wall, roof or floor.
Total R = sum of each layer (R-per-inch × thickness) + air films. RSI = R ÷ 5.678; U-value = 1 ÷ R. The per-inch R-values are typical published material properties, not code minimums; check your product's data sheet and your local energy code for required values.
Want the full method, an R-per-inch chart for common materials and worked examples? Read: how to calculate R-value →
Common questions
- How do you calculate R-value?
- For a single layer, R-value = thickness × the material's R-per-inch. For a whole assembly you add the layers together and include the inside and outside air films: total R = layer 1 + layer 2 + … + films. For example a 3.5-inch fiberglass batt (about R-3.2 per inch) gives roughly R-11, and adding a half-inch of drywall plus the air films brings a simple stud wall to about R-12.5. Enter your layers above and the calculator sums them for you.
- What is the difference between R-value and U-value?
- They are reciprocals. R-value measures resistance to heat flow — higher is better. U-value (the U-factor) measures how much heat flows through — lower is better. U = 1 ÷ R. So an assembly of R-12.5 has a U-value of about 0.08 BTU/h·ft²·°F. Insulation is usually rated in R-value; windows, doors and whole-assembly energy codes are often rated in U-value.
- Is a higher R-value always better?
- For resisting heat flow, yes — a higher R-value means less heat lost in winter and less gained in summer. But R-value climbs with thickness, and there are diminishing returns: doubling insulation does not halve your heating bill, because the walls, windows and air leaks around it also lose heat. Match the R-value to what your climate and energy code call for rather than simply maximising it.
- How do you add up the R-values of a wall's layers?
- Thermal resistances in series simply add. Work out each layer's R (its R-per-inch × its thickness), add the still-air films on the inside and outside surfaces (about R-0.68 and R-0.17, so R-0.85 together), and total them. The calculator does this automatically — add a row for each layer and tick the air-films box.
- What R-value do I need for a wall or attic?
- It depends on your climate zone and local energy code, so treat published figures as guidance, not a rule. As a general guide the US Department of Energy suggests roughly R-30 to R-60 for attics and R-13 to R-21 for wood-frame walls depending on climate. Always check the recommended R-values for your climate zone and the energy code in force where you build — this tool computes the R-value you have, not the one you are required to hit.
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-07.