The connection between air sealing and ventilation in your home
Over the past several decades there has been a trend in the residential building industry to make houses more energy efficient during construction. In the early nineties, the national building code was changed to include the requirement for a continuous air barrier (essentially a large sheet of plastic) to be installed around a building’s thermal envelope. A thermal envelope is the surface between the heated and unheated ceiling of a home.
Creating continuous air barriers makes buildings more airtight, but as a result, lack of natural ventilation often occurs, causing poor air quality (polluted, stale, moisture laden air). The solution has been to install mechanical ventilation equipment in houses in conjunction with the continuous air barrier, to simultaneously keep indoor pollutant levels down and maintain high energy efficiency.
Different types of mechanical ventilation equipment are available:
There are three primary types of ventilation equipment on the market, including exhaust-only, supply-only and balanced ventilation (HRVs/ERVs) systems. Each of these has pros and cons, depending on the situation and conditions present in a particular home.
Exhaust-only equipment includes bathroom and kitchen exhaust fans. This type of equipment is commonly installed in homes that have a forced air heating system and natural ventilation rates of 0.2 to 0.3 air changes per hour. The installation and proper use of a principal exhaust fan, which usually doubles as a bathroom exhaust fan, is often sufficient to meet the 0.3 air changes per hour that is recommended.
Supply-only equipment includes a direct-connected outdoor air duct to the return air plenum of the furnace. Every time the furnace fan turns on, outside air is brought into the home. These systems tend to slightly pressurize the home and can reduce infiltration of soil gases, such as radon, into the home.
Balanced ventilation equipment, such as a heat recovery ventilation system (HRVS), draws in fresh air from the exterior and discharges stale air from the interior in equal amounts. These systems are installed in order to achieve the goal of 0.3 air changes per hour. In cases where humidity control is also a concern, energy recovery ventilation systems (ERVS) are used. These systems are also recommended when blower door tests completed on new houses with extremely well sealed thermal envelopes show a result of less than 0.2 air changes per hour, or in cases where there is no duct work installed in association with the heating system (i.e. electric baseboard or hydronic heating systems).
To ensure the efficient supply and return of ventilated air, ductwork must be well designed and installed. Ducts should be airtight yet accessible for cleaning. Using a water-based sealer (mastic) is the best way to seal ducts to ensure efficient distribution of heating, cooling and ventilation air.