Air leakage is one of the primary mechanisms by which heat escapes Canadian homes in winter and enters in summer. Unlike conductive heat loss — which insulation directly addresses — air infiltration bypasses insulation entirely, carrying conditioned air out and drawing unconditioned air in through gaps, cracks, and penetrations in the building envelope. Effective air sealing reduces heating and cooling loads and is required by the NBC 2020 as part of a continuous air barrier system.

A blower door fan installed in a residential doorway for air leakage testing

Measuring Air Leakage: The Blower Door Test

Blower door testing is the standard diagnostic method for quantifying whole-house air leakage. A calibrated fan is installed in an exterior doorway and used to depressurize the house to 50 pascals below outdoor pressure. The airflow required to maintain that pressure difference is measured and expressed as air changes per hour at 50 pascals (ACH50) or as normalized leakage area (NLA).

The NBC 2020 sets a maximum of 2.5 ACH50 for new Part 9 houses (houses and small buildings). The EnerGuide Rating System and the Net Zero Ready specifications target 1.5 ACH50 or lower. Older Canadian housing stock — particularly pre-1980 balloon-frame and platform-frame construction — commonly measures between 6 and 15 ACH50 when tested.

A house measuring 8 ACH50 is exchanging its entire air volume with outdoor air roughly 0.5–0.8 times per hour under typical winter pressure differentials — a significant source of heat loss and moisture transport.

Priority Air Leakage Locations

Research by Natural Resources Canada and the Building Science Corporation has identified consistent patterns in where air leakage concentrates in Canadian housing archetypes. Targeting these locations produces the greatest reduction in ACH50 per dollar spent on materials and labour.

Attic Bypasses

The junction between the top plate of exterior walls and the attic floor is a major air leakage pathway, particularly in platform-frame construction where the wall cavity is open to the attic. Interior partition walls that run up into the attic space create additional bypasses. Sealing these with acoustical sealant and polyethylene sheeting before adding attic insulation is standard practice in energy retrofit programs.

Plumbing stacks, electrical wiring, and HVAC ducts that penetrate the top-plate level all require sealing. In houses with pot lights recessed into the ceiling below an attic, each fixture represents a significant uncontrolled opening. Replacing recessed lights with surface-mounted fixtures or installing rated air-tight (ICAT) housings and sealing the box perimeter addresses this pathway.

Rim Joists and Sill Plates

At the foundation level, the rim joist (band joist) — the framing member at the perimeter of each floor — is exposed to basement conditions on one side and often to exterior air on the other. Gaps between the sill plate and foundation also admit air. Closed-cell spray polyurethane foam applied to the interior face of rim joists is effective because it simultaneously insulates and air-seals in a single application without requiring a separate vapour barrier in most Canadian climate zones.

Electrical and Plumbing Penetrations

Outlet boxes on exterior walls and ceiling-mounted boxes are common infiltration points. Foam gaskets installed behind outlet and switch cover plates on exterior walls reduce air flow through the box. Plumbing supply and drain penetrations through the floor and wall should be sealed with fire-rated intumescent caulk where they pass through fire separations, and with acoustical sealant or backer rod plus caulk elsewhere.

Window and Door Rough Openings

The gap between a window or door frame and the rough opening framing is typically filled with low-expansion spray foam. Using the correct foam product matters: standard one-component foam can exert enough pressure during curing to bow window frames. Low-expansion or "window and door" foam is specified for these locations. The flashing and weather-resistive barrier (WRB) integration at the rough opening also determines whether bulk water is managed correctly alongside air.

HVAC Penetrations

Ductwork that passes through unconditioned spaces — attics, crawlspaces, garages — represents both a leakage pathway and an energy penalty when the ducts themselves are not sealed. Sheet-metal ducts at joints and fittings should be sealed with UL181-listed mastic, not standard duct tape. Flexible duct connections require mechanical fastening plus mastic. Combustion appliance vents and HRV/ERV supply and exhaust penetrations through the envelope require fire-rated collar assemblies and sealant.

Air Barrier Materials

Canadian building science distinguishes between the air barrier system — the assembly of materials and components that collectively resist air movement — and individual air barrier materials. The NBC 2020 requires that the air barrier be continuous, durable, rigid or supported, and capable of withstanding pressure differentials encountered during construction and occupancy.

Material Form Typical Location Notes
6-mil polyethylene Sheet membrane Interior of wall/ceiling assembly Also serves as vapour barrier in most Canadian zones; must be supported
Rigid air barrier (OSB, plywood) Structural sheathing Exterior of wall framing Joints taped with compatible tape; integrates with WRB
Fluid-applied membrane Liquid-applied coating Exterior sheathing, transitions Good for complex geometry; covers fastener holes
Closed-cell SPF Spray-applied foam Rim joists, difficult transitions Combined insulation and air barrier; vapour retarder at sufficient thickness
Acoustical sealant Non-hardening caulk Polyethylene laps, box penetrations Remains flexible; compatible with poly-to-framing and poly-to-poly joints
One-component foam (low-expansion) Spray can Rough openings, small penetrations Use window-and-door type at frames to avoid bowing
Self-adhered membrane Peel-and-stick tape/flashing Sheathing joints, penetrations Acrylic-based products maintain adhesion in cold installation temperatures

Ventilation After Air Sealing

Tighter envelopes require controlled mechanical ventilation to maintain indoor air quality. The NBC 2020 requires heat recovery ventilators (HRVs) or energy recovery ventilators (ERVs) in new houses below 3.5 ACH50. In houses being retrofit to lower air-change rates, ventilation strategy should be reassessed alongside the air-sealing work to avoid indoor air quality problems from combustion appliances, radon, or elevated humidity.

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