Identifying and Fixing Drafts in Residential Buildings

A draft is a perceptible movement of outdoor air into a conditioned space through unintended gaps in the building envelope. In most Canadian homes built before the mid-1980s, air infiltration accounts for a substantial portion of annual heating costs. More importantly, uncontrolled air movement carries moisture from warm interior air into cold wall and ceiling assemblies, where it can condense and cause long-term structural damage.

The distinction between conductive heat loss (addressed by insulation) and convective heat loss (addressed by air sealing) is important. Adding insulation to a leaky assembly reduces conductive losses but does little for air infiltration. Effective thermal upgrades typically address both in sequence — air sealing first, then insulation — because sealing from inside an insulated cavity is not feasible after the fact.

What Constitutes a Draft

From a building science perspective, air infiltration occurs wherever there is a pressure difference between inside and outside combined with a gap in the building envelope. Pressure differences arise from three sources: the stack effect (warm air rising and escaping through upper openings while cold air is drawn in below), wind pressure on the building exterior, and mechanical systems (exhaust fans, combustion appliances) that depressurize interior spaces.

The stack effect is particularly significant in cold Canadian climates because the temperature differential between inside and outside is large and sustained. A two-storey home in Ontario during January can have a significant pressure difference between its basement floor and its attic, which drives air exchange through any available path. This is why attic bypasses — air pathways from the conditioned space into the attic through gaps around light fixtures, partition walls, and plumbing chases — are a priority target in northern retrofits.

Common Leakage Locations

Air leakage in residential construction is not evenly distributed. Studies conducted by Natural Resources Canada and various university building science programs consistently identify the following as the most significant contributors to total infiltration in wood-frame Canadian homes:

Detection Methods

Several methods exist for locating air leakage in a residential building, ranging from low-cost qualitative approaches to quantitative diagnostic testing. The appropriate method depends on the goal: a general sense of problem areas versus a measured baseline for before-and-after comparison.

Visual inspection

In attics with accessible insulation, visual inspection can reveal obvious bypasses: insulation disturbed by air movement (often shown by dust deposition on the surface of batts directly above top-plate gaps), frost accumulation in cold weather, or visible gaps around light fixtures and exhaust fan housings. Basement rim joists can be inspected from below for gaps in framing and missing vapour barriers.

Smoke pencil and incense stick

On a cold, windy day with exhaust fans running to depressurize the building slightly, a slow-burning smoke source held near suspected gaps will show air movement. This is a qualitative method useful for confirming suspected locations but not for measuring total leakage. The limitation is that this approach only identifies leaks large enough to produce a visible smoke disturbance.

Thermal imaging

An infrared camera used during cold weather, with the building under slight negative pressure from exhaust fans, shows temperature differences at wall and ceiling surfaces that indicate cold air infiltration. Thermal imaging can identify leakage areas invisible to the eye, including those behind finished walls where cold air is tracking along framing before entering the room. It requires a trained operator and a sufficient temperature differential — generally at least 10°C difference between inside and outside.

Blower Door Testing

The blower door test is the standard quantitative method for measuring the total air leakage of a building. A calibrated fan is temporarily sealed into an exterior door opening, and the building is pressurized or depressurized to a standard reference pressure of 50 Pascals (Pa). The flow rate required to maintain that pressure, measured in air changes per hour at 50 Pa (ACH50), is the reported metric.

The National Building Code of Canada 2020 sets an air tightness target for new construction in Part 9 (houses). Many older Canadian homes, particularly those built before widespread awareness of air sealing in the 1990s, significantly exceed this target. Retrofit air sealing programs typically aim to reduce ACH50 by 40–60% through targeted intervention at the locations described above.

A blower door test measures total leakage but does not tell you where the leaks are. Combining the blower door with a smoke pencil or thermal camera while the fan is operating is the standard diagnostic approach for identifying specific locations.

Energy auditors certified through Natural Resources Canada's EnerGuide program use blower door testing as part of the standard home evaluation. The test produces a label showing the home's air tightness relative to code requirements and provides a baseline for measuring the effect of retrofit work.

Sealing Approaches by Location

Attic bypasses

The most effective and durable seal for attic bypasses is rigid blocking combined with spray foam. Partition wall top plates are sealed by placing a rigid cover (plywood, rigid foam, or rigid mineral wool) over the wall cavity opening at the attic floor and sealing the perimeter with canned polyurethane foam. Plumbing and electrical penetrations are sealed with backer rod and acoustical sealant, or with two-component spray foam for larger gaps.

Rim joist

In accessible unfinished basements, the rim joist is typically sealed with closed-cell spray foam applied to the interior face. A 2-inch application provides both the air seal and meaningful additional R-value. The foam must contact the top of the foundation wall and the underside of the subfloor without gaps. Fire blocking requirements may apply in some jurisdictions.

Window and door frames

The gap between the window frame and rough framing is sealed from the interior with low-expansion polyurethane foam during initial installation, or with acoustic sealant where the gap is narrow. The exterior is typically addressed with flashing tape integrated with the housewrap or WRB. Weatherstripping on operable components — door sweep, compression seals on casements — degrades with use and requires periodic replacement.

Electrical outlets on exterior walls

Foam gaskets installed behind the outlet cover plate reduce but do not eliminate leakage at these locations. Sealing the box itself to the vapour barrier or air barrier behind the wall is more effective but requires opening the wall finish. In accessible unfinished basement ceiling applications, electrical penetrations are often sealed from below with spray foam.

Vapour Control Considerations

In cold climates, air sealing changes the moisture dynamics of the assembly. A leaky wall allows water vapour to move through it by both diffusion and air movement; sealing it changes the dominant mode of moisture transport. When a house is tightened significantly, it is important to ensure adequate mechanical ventilation — either through a heat recovery ventilator (HRV) or energy recovery ventilator (ERV) — to maintain acceptable indoor air quality and control humidity levels.

Natural Resources Canada recommends installing or upgrading mechanical ventilation in houses that undergo significant air sealing retrofits. The relationship between air tightness and indoor air quality is discussed in detail in the Keeping the Heat In guide.

Last updated: June 8, 2026. Source references: Natural Resources Canada — Keeping the Heat In; National Building Code of Canada 2020.