Understanding material thermal conductivity is fundamental in high performance building design. This property is often advertised using a single value implied to be constant, though research shows that insulating materials have an effective conductivity that changes over a range of environmental parameters, including temperature and moisture levels. Various polyurethane and polyisocyanurate materials are analyzed in order to determine how the effective conductivity is altered by accelerated aging, obtained through exposure to high temperature, moisture, and freeze-thaw cycling. The measured results are used in hygrothermal simulations to determine the assumed and actual performance of insulating materials in the context of high performance wall and roof assemblies in cold climates. Results show that effects of aging and environmental temperature have higher impacts on the performance of polyisocyanurate materials than polyurethanes. Additionally, high moisture levels contribute to lower performance in all foam materials, with open cell materials experiencing the greatest performance reduction.