Calculate insulation thickness and quantity to reach a target R-value.
Thickness follows from the R-value shortfall multiplied by the material conductivity, so a better insulator needs less depth for the same performance. Spray foam and PIR reach a given R-value in roughly 60 per cent of the thickness mineral wool needs. Where depth is constrained, such as between existing rafters, conductivity rather than price per square metre determines whether the target is achievable at all.
Insulation
Thickness = required R-value × thermal conductivity; U = 1 ÷ total R
Thickness = required R-value × thermal conductivity; U = 1 ÷ total R Thickness follows from the R-value shortfall multiplied by the material conductivity, so a better insulator needs less depth for the same performance. Spray foam and PIR reach a given R-value in roughly 60 per cent of the thickness mineral wool needs.
Where depth is constrained, such as between existing rafters, conductivity rather than price per square metre determines whether the target is achievable at all.
This calculator takes 5 inputs: Area to insulate, Target R-value, Existing R-value, Material, Coverage per pack or bag. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.