The degradation of the polymer building materials polypropylene (PP) and high density polyethylene (HDPE) intended for application as water barriers/repellants around building foundation walls have been studied. The PP and HDPE samples have been subjected to various accelerated climate ageing methods for comparison, including exposure to ultraviolet and infrared radiation, heated air, water spray and freezing. The climate ageing processes have been qualitatively and quantitatively investigated by attenuated total reflectance (ATR) Fourier transform infrared (FTIR) radiation spectroscopy.