Cold climates are defined as regions where annual energy use for space heating vastly overshadows the energy used for cooling (air-conditioning). In such climates the risk of condensation in the wall cavities is predominantly via the movement of moisture from the warm, humid indoors to the colder regions of the wall. These cooler regions are reached as we near the exterior wall assembly adjacent to the outer portions of the batt insulation and the exterior sheathings.
Moisture movement occurs through two different and separate processes :
Diffusion:
The molecular movement of water through the wall assembly components. The movement is driven by a vapor pressure difference. From the fall to early spring, humid, warm indoor air has a higher vapor pressure than the colder exterior air. As a result moisture will traverse wall elements from the interior until it meets a zone in the wall where the vapor pressure conditions are not sufficient to have moisture continue its outward movement. At this location, moisture will condense and accummulate. The potential for moisture condensation by the process of diffusion is highly dependent on the Water Vapor Permeability of the wall assembly components, their location in the wall and the thermal regime across the wall assembly. Diffusion is a slow process and is a relatively minor potential contributor to moisture transport through walls.
Exfiltration:
This refers to the physical movement of indoor air (warm and humid) from the interior towards the exterior as a result of an air pressure difference. The pressure difference can be caused by wind effects on the building envelope, stack effects, etc…… While moisture diffusion occurs on a molecular level, moisture movement by exfiltration occurs when the indoor air physically moves through commonly occuring penetrations, unsealed openings and joints in the interior vapor/air barrier, electrical outlets, etc…. The potential for condensation by exfiltration is many times higher than diffusion due to the slowness of the diffusion process compared to the vast quantities of moisture moving through a wall even under low infiltration rates. The risks of condensation by exfiltration are therefore higher, especially if warm, high humidity interior conditions are combined with cold exterior climates. Indoor air can have a relatively high dewpoint temperature when high indoor relative humidities are maintained. This temperature is the one at which the interior air must be cooled to reach the point of saturation. If cooled below that dewpoint temperature condensation must occur.
Reducing the Potential for Condensation in Wall Cavities by Exfiltration:
In order to reduce the potential for condensation in the wall cavities by exfiltration of indoor air it is necessary to maintain the cavity temperatures above the dewpoint temperature. The most efficient way to maintain cavity temperatures above the interior air's dewpoint is to isolate the wall cavity from the cold exterior temperatures. This is achieved through the use of insulation on the exterior side of the cavity. As a greater portion of the total insulation is installed on the exterior of the cavity, the temperature of the cavity (as monitored by the temperature of the interface between the cavity and the exterior sheathing) will steadily increase, further reducing the likelihood of having the indoor air dewpoint temperature located in the wall cavity.
