If you’re looking at insulation for your home or building, you have likely heard product comparisons in terms of R-value – the conductive heat flow resistance of the material. This measurement is made using the guarded hotbox apparatus (see photos), a test procedure that measures the thermal conductivity of insulation material. One side of the material is heated to a specific temperature and after steady state heat flow has been reached, the temperature on the other side is measured. This measurement solely defines the conductive heat flow resistance of the material.
Once the R-value is determined, the heat flow through it can be calculated using Fourier’s Law, an equation that calculates the benefit of increasing the thickness of any type of insulation as long as there is no air movement through it.