Roof Insulation U-Value Calculator

How it is calculated

The tool models the roof as one insulation layer plus a lump of everything else. Insulation resistance is Rins = (thickness ÷ 1000) ÷ λ, where thickness is in millimetres and λ (conductivity) is in W/m·K. Total resistance is R = Rins + Rother, with R_other in m²K/W covering plasterboard, deck, air gaps and surface films, which you must work out yourself. The U-value is U = 1 ÷ R in W/m²K, shown to 3 decimals, and R values are shown to 2. Thickness and conductivity must be above zero; R_other must be entered (0 is allowed, blank is not) and cannot be negative. The method assumes heat flows straight through the layers, so rafters, joists and fixings that bridge the insulation are not accounted for.

Common mistakes

• Entering the thickness in metres or centimetres. The field is millimetres, so 200 is correct and 0.2 gives a U-value above 3 W/m²K.
• Typing the R-value of the product in place of its conductivity. λ is a small number such as 0.022; an R-value such as 4.5 belongs in a different field.
• Entering λ in mW/m·K (22 instead of 0.022). The insulation then adds almost no resistance, so in the example below the U-value comes out about 30 times too high (about 3.2 instead of 0.106 W/m²K).
• Leaving out the other layers and surface resistances. Using 0 is allowed but gives a U-value slightly too low, because internal and external films and the deck do add resistance.
• Ignoring thermal bridging. Timber between the insulation lets more heat through than this single-path calculation shows, so a real roof can perform worse than the figure.