A good insulation is measured by its ability to restrict heat transfer. This is expressed by a number of different ‘values’.
Lambda values
‘Lambda’ [lam-duh] is the 11th letter of the Greek alphabet (λ). However, we’re not here to talk semantics.
Also known as ‘thermal conductivity’ or portrayed as ‘λ–value’, a material’s lambda value shows how well it can conduct heat. It’s measured in units of W/m·K (Watts per metre Kelvin).
A good insulation will have a low lambda value as possible to reduce heat loss. It’s worth noting that lambda values can vary widely depending on the type of insulation, so be sure to do some research into which of your insulation options achieves the lowest.
Lambda values are general measurements. If you want to be more specific and know how a certain thickness of insulation affects heat transfer, you need to know the R-value.
R-values
Also known as ‘thermal resistance’, R-values tell you how well an insulation resists heat transfer at a specific thickness. It is calculated by dividing a material’s thickness (in metres) by its lambda value and is measured in units of m²·K/W (metres squared Kelvin over Watts).