Myths of the Airtight Home
There is a misconception that airtight homes are mouldy, have poor indoor air quality, and are susceptible to rot. These misconceptions stem from a misunderstanding in what causes moisture accumulation in the building envelope.
Airtight skeptics are typically concerned that inadequate ventilation will increase the indoor relative humidity in a home, making it more likely for moisture to condense on cool surfaces. This is a legitimate concern if adequate ventilation is not supplied to a space. In British Columbia, section 9.32 of the Building Code states minimum continuous ventilation rates for new homes.
A well-ventilated air-tight home is less susceptible to moisture accumulation. This is because tin airtight construction, less moisture-laden air travels through building structure, which reduces the frequency and quantity of moisture accumulation via condensation that can form on an interior surface
Additionally, there is a popular concern that airtight homes will not dry as quickly as non-airtight homes. In cold climates however, drying time is largely controlled by vapour diffusion), not air transfer. Vapour diffusion is directly correlated to water-vapour transmission coefficients, which is a property of the building materials used rather than the assembly’s airtightness.
*Source: 2018 BCBC, Section 9.32
Indoor Air Quality
Before homes were built to be airtight, homes relied on air infiltration through the building envelope to supply “fresh” air. This air would travel through open orifices through walls, floors, and ceiling potentially collecting mould spores, formaldehyde, radon gas, or other unhealthy contaminants., So, although this strategy may be acceptable at lowering the carbon dioxide concentration of the interior air, it does not supply “fresh” air in the sense occupants would expect.
To resolve this problem, the British Columbia Building Code specifies minimum fresh air requirements that must be supplied by mechanical ventilation. By controlling the amount of fresh air through a mechanical system, occupants are able to control the amount of fresh air being introduced. This ensures every room is supplied with adequate ventilation. Moreover, the air introduced to the home through a mechanical nature may be filtered to reduce airborne contaminants such as smoke from forest fires that is sometimes experienced during our Kootenay summers.
If your home is designed to have an equal volume of intake and exhaust air, the balanced ventilation will reduce pressure differences, further minimizing air transfer through the building envelope.
Additional benefits of Air tight houses
An overlooked benefit of an airtight home is pest mitigation. An air-tight envelope is generally a pest-tight envelope. Less openings between the interior and exterior environment make it more difficult for pests to travel through your walls. Airtight homes also have improved Noise Control and accumulate less dust compared to homes that are not airtight.
With required continuous ventilation already in the BC Building Code, and all of the benefits airtightness offers, it’s easy to see why air leakage is one of the criteria to meet the British Columbia Step Code.