Question

Why is a vapor barrier placed on the warm side of insulation?

Answer

Most building codes require the placement of vapor barriers on the inside of a building envelope, since the strongest vapor flows are during cold weather from the indoors (typically the warm side of the insulation) to the outdoors. Only in a hot and humid climate, such as southern Florida, would the vapor barrier be placed on the exterior side of a wall or roof assembly.

A vapor barrier restricts the transmission of water vapor through a wall or roof cavity. When located on the warm side of the insulation, the vapor barrier is positioned to prevent the moisture held in warm air from penetrating the wall assembly and reaching the dew point somewhere within the wall assembly.

Along the gradient from warm to cool, the air temperature can cross the dew point, at which moisture in the air condenses on cooler surfaces. Air from the warm side is capable of carrying sufficient moisture to condense inside the assembly, while air from the cool side is not. As you can imagine, condensation can lead to significant problems with mold and rot inside wall assemblies.