Roof Insulation U-Value Calculator
Enter the insulation thickness, conductivity, and the resistance of the remaining layers to find the roof U-value.
Result
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 Rother 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; Rother 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.
Worked example
Take 200 mm of insulation with λ = 0.022 W/m·K and 0.3 m²K/W for all other layers. Convert the thickness: 200 ÷ 1000 = 0.2 m. Insulation resistance: 0.2 ÷ 0.022 = 9.09 m²K/W. Total resistance: 9.0909 + 0.3 = 9.39 m²K/W. U-value: 1 ÷ 9.3909 = 0.106 W/m²K. Setting Rother to 0 would give 0.110, so the other layers change the answer only slightly when the insulation is thick.
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). That makes the insulation look about a thousand times worse.
- 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.
About this calculator
A roof's U-value measures its overall rate of heat loss, expressed in watts per square meter per degree Kelvin (W/m²K), and is calculated as the reciprocal of the total thermal resistance (R-value) of all the roof's layers combined: R equals the insulation thickness divided by its thermal conductivity (lambda, in W/mK), plus the resistance contributed by every other layer — plasterboard, roof deck, air gaps, internal and external surface resistances — added together, then U equals 1 divided by that total R.
Architects, builders and energy assessors calculate roof U-value to confirm compliance with building regulations, which typically cap roof U-value at a maximum figure (commonly around 0.15-0.18 W/m²K for new builds in much of Europe), and to model expected heating energy use in an EPC (Energy Performance Certificate) or SAP calculation — a roof is usually the single biggest source of heat loss in a home because hot air naturally rises, so getting this number right matters more here than for almost any other building element.
This calculator takes your insulation thickness, its thermal conductivity, and the resistance of the roof's other layers and returns the resulting roof U-value.
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