Supply ventilation systems work by pressurizing the building. They use a fan to force outside air into the building while air leaks out of the building through holes in the shell, bath and range fan ducts, and intentional vents (if any exist).
Exhaust ventilation systems work by depressurizing the building. By reducing the inside air pressure below the outdoor air pressure, they extract indoor air from a house while make-up air infiltrates through leaks in the building shell and through intentional, passive vents.
The decision to use whole-house ventilation is typically motivated by concerns that natural ventilation won't provide adequate air quality, even with source control by spot ventilation.
Spot ventilation improves the effectiveness of other ventilation strategies — natural and whole-house — by removing indoor air pollutants and/or moisture at their source.
Natural ventilation used to be the most common ventilation method of allowing fresh outdoor air to replace indoor air in a home. Today it's usually not the best ventilation strategy, especially for homes that are properly air sealed for energy efficiency. Natural ventilation also usually doesn't provide adequate moisture control.
When creating an energy-efficient, airtight home through air sealing techniques, it's very important to consider ventilation. Unless properly ventilated, an airtight home can seal in indoor air pollutants. Ventilation also helps control moisture—another important consideration for a healthy, energy-efficient home.
An air barrier/vapor diffusion retarder attempts to combine water vapor diffusion and air movement control with one material. This type of material is most appropriate for southern climates where keeping humid outdoor air from entering the building cavities is critical during the cooling season.
In most U.S. climates, vapor barriers or vapor diffusion retarders should be considered as part of a moisture control strategy for a home.
It is a myth that installing vapor barriers is the most important step for controlling moisture in walls. Vapor barriers only retard moisture due to diffusion, while most moisture enters walls either through fluid capillary action or as water vapor through air leaks.
To maximize your home's energy efficiency and to protect the foundation, you should use the following moisture and air leakage control techniques when installing slab-on-grade floors:
To effectively insulate your crawl space for energy efficiency and to create a comfortable home, you need to properly control moisture in your crawl space.
To effectively insulate your basement for energy efficiency and to create a comfortable space, you need to properly control moisture in your basement.
