How this calculator works
For a homogeneous layer, thermal resistance is thickness divided by conductivity. Resistances add in series; the reciprocal gives U-value. Add surface films or evaluated air-space resistance separately. Framing and thermal bridges create parallel heat-flow paths that this series model does not represent.
Formula
RSI_i = thickness_i / conductivity_i; RSI_total = sum(RSI_i)+additional RSI; U=1/RSI_total; US R-value = 5.678263337 x RSI; target thickness_1 = conductivity_1 x (target RSI-other RSI).
Worked example
A 100 mm layer at 0.04 W/(m K) has RSI 2.5, US R-value 14.1957 and U-value 0.4 W/(m^2 K), before surface films.
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Frequently asked questions
Why are there no fixed material presets?
Conductivity depends on product, density, moisture, ageing and test conditions. Enter documented data for the product being assessed.
Can I model an air gap as a solid layer?
Use an evaluated air-space resistance in the additional RSI field. Radiation and convection make thickness divided by air conductivity inappropriate.
Does this give the whole-wall R-value?
Only for a uniform series path. Studs, fasteners, junctions, compression and gaps can lower whole-assembly performance.