Insulation R-value & U-value
Total R-value and U-value of a wall, roof or floor from its layers — thickness ÷ conductivity or a declared R — plus surface films and heat flow.
Each layer’s R and its share of the total, the total R and U in metric and imperial units, and the heat flux and heat flow for a temperature difference and area you enter.
Example: 12.5 mm plasterboard (λ 0.25) + 100 mm mineral wool (0.035) + 102 mm brick (0.77) with 0.13 and 0.04 films: R = 3.21 m²·K/W, U = 0.31 W/(m²·K), 6.2 W/m² at 20 K.
Resistances in series,
like resistors.
How layers add up, what the films are, and what the simple sum leaves out.
R and U
Heat crossing a layer meets a resistance R = thickness ÷ conductivity λ (a metre of thickness per unit of λ). Layers in series add: R_total = Σ R + inside film + outside film, and the U-value is its reciprocal — the heat flow per square metre per kelvin. Heat flux = U × ΔT, heat flow = flux × area. A product’s declared R (an insulation batt, a reflective cavity) can be entered directly.
Surface films
Air next to a surface adds resistance; standards give values that depend on the surface, the direction of heat flow and exposure (ISO 6946, ASHRAE Fundamentals) — enter yours. With no films the U is the construction’s own, which is what a product test reports.
What the sum ignores
Studs, rails, fixings and mortar joints bridge the insulation; the combined method in ISO 6946 or a 2-D calculation handles that. Moisture, ageing of foam blowing agents and installation gaps lower real performance. λ values are the data sheet’s at the declared temperature — none are embedded here. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- R = Σ d/λ (+ declared R layers + surface films), U = 1/R; 1 m²·K/W = 5.678 ft²·°F·h/BTU; no material conductivities embedded — λ from the product data sheet
Last reviewed 21 September 2026. How results are checked: How we verify.