calcs.homes
calcs.homes
Add up your wall or ceiling assembly's layers — insulation, sheathing, drywall, and air films — to see the total R-value against your target. Uses the US/Imperial R-value scale printed on North American insulation products.
Estimate only — includes a standard air film allowance (R-0.85) for interior and exterior surfaces.
We multiply your chosen material's R-value per inch — R-3.1 for fiberglass batt, R-6.5 for closed-cell spray foam, R-3.7 for open-cell spray foam, or R-5 for rigid XPS foam — by thickness, then add R-1.0 for sheathing and R-0.45 for drywall if either box is checked, plus a fixed R-0.85 allowance for the air films that cling to interior and exterior surfaces. That sum is compared against your target R-value as a percentage.
This total only reflects the R-value of materials stacked straight through the wall — it doesn't account for thermal bridging, the faster heat loss through wood studs compared to insulation, which typically drops a wall's real-world "effective" R-value 10-20% below this material-only sum. Building codes set minimum R-values (commonly R-13 to R-21 for framed walls, varying by climate zone) because insulation is the main defense against heat loss, so undersizing it raises energy bills and can invite condensation inside the wall cavity.
It depends on your climate zone — many US codes call for R-13 to R-21 in 2x4 or 2x6 walls. Check your local energy code for the exact requirement.
Wood studs conduct heat faster than insulation (thermal bridging), so a wall's "effective" R-value is typically 10-20% lower than the simple sum of its layers — continuous exterior rigid foam is a common way to reduce this effect.