Understanding the Difference Between Permeance vs Permeability

Water Vapour Permeance is the time it takes water vapour to transmit through a unit area of material that has a vapour pressure difference between the two exposed surfaces of the material. Solar Dry Insulation (EPS core) is moderately hygroscopic which means it will take on water vapour from its ambient environment and pass it on. So, a slab of Solar Dry subject to a vapor pressure gradient will exhibit the same permeability regardless of thickness. Making Solar Dry thicker does not change the rate of flow – it simply increases the distance water vapor can travel into the EPS core.