There are many wood frame buildings in the Ottawa area with painted wood siding. As vinyl is a popular replacement, most historic buildings continue to use wood due to its heritage value. Painting the wood siding protects the underlying material from degradation from the surrounding exterior environment. For painted wood siding, periods of high moisture content can lead to premature decay of the wood or cause paint to blister. The role of the paint layer and its importance in protecting wood siding from moisture is the focus in this research. The permeability of the paint layer to vapour and liquid moisture sources is analyzed using WUFI Pro based on the Sd-value and Aw coefficient. Interior and exterior sources of moisture were found to impact the exterior siding. For Ottawa's climate, an exterior paint layer that is permeable (Sd<=0.5m) and with a lower Aw coefficient (<=0.0003 kg/m2h0.5) is recommended.