About this tool
R-Value / U-Value Insulation Calculator is a free, in-browser tool that adds up the thermal resistance of every layer in a wall, roof or floor assembly and converts the total into a U-value and heat-loss rate. Pick materials from the built-in layer library, type a thickness, and the numbers update instantly — nothing is uploaded.
Each layer's resistance is its per-inch R-value times its thickness (fiberglass 3.14/in, XPS 5/in, EPS 3.8/in, closed-cell spray foam 6.5/in, drywall 0.9/in, wood 1.25/in, and more). The total is R_total = Σ(R_layer) plus the interior and exterior air films (a combined R-0.85), and the assembly U-value is simply U = 1 / R_total. Results are shown in both imperial (ft²·°F·h/BTU) and metric RSI (m²·K/W) so the tool works anywhere.
Enter an area and a temperature difference and it also returns the conductive heat loss Q = U · area · ΔT in BTU/hr and watts — useful for sizing heating or spotting a weak spot in the building envelope. Add or remove layers to compare a stud wall against a foam-sheathed upgrade.
Frequently asked questions
What is the difference between R-value and U-value?
R-value measures thermal resistance — how well a material stops heat — so higher is better and layers add up. U-value is the reciprocal, U = 1 / R_total, and measures how much heat passes through, so lower is better. This tool reports both, plus the metric RSI equivalent (RSI = R × 0.176).
Why does the total include air films?
Still air on the inside surface and moving air on the outside each add real thermal resistance (about R-0.68 and R-0.17, so R-0.85 combined). Standard assembly R-values include these interior and exterior air films. Untick the box if you want the material-only resistance.
How is heat loss calculated?
When you enter an area and a temperature difference (ΔT) between inside and outside, the tool computes conductive heat loss with Q = U · area · ΔT. In imperial that gives BTU/hr from ft² and °F; in metric it gives watts from m² and °C. It does not add infiltration — use the Home Heat Loss calculator for a whole-house load.
Are the per-inch R-values exact?
They are widely used nominal values for typical products; real R-values vary with density, temperature and moisture, and foam boards can differ by brand. Treat the result as a solid design estimate and check the manufacturer's rated R-value for critical work.
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