The Effect of Moisture Accumulation in Below-Grade Insulation

As building codes continue to introduce more stringent performance requirements, the placement of insulation in the building enclosure has been shifted to the exterior to provide a continuous insulation layer and reduce thermal bridging. However, modern construction practices for foundation walls typically leave the below-grade insulation exposed to the exterior environment, resulting in the insulation layer absorbing moisture from the soil. This moisture absorption will decrease the long-term performance of insulation and is typically not accounted for in the design, resulting in buildings performing below the expected level. The magnitude of this reduction will vary depending on the type of insulation used. This study utilized hygrothermal modelling and laboratory testing to determine how significant this reduction in performance will be for various types of insulation typically used below-grade. The insulation types investigated in this study include EPS, graphite embedded EPS (GPS), XPS, mineral wool, and closed-cell spray foam. The results showed a minimal increase in thermal conductivity amongst the rigid polystyrene insulation products, while the thermal conductivities of the spray foam and mineral wool insulations increased by approximately 10% and 20% respectively. The minimal effect on the polystyrene insulation products is likely a consequence of the lower permeabilities of these products. Additional research should be conducted over a longer wetting period. In cold climates, the effect of freeze-thaw cycling on these insulations should also be studied.