Windows and exterior doors account for roughly 25 percent of residential heat loss in a typical Canadian home. That figure climbs sharply in houses built before triple-pane glazing became standard, and in any building where the original weatherstripping has degraded or where settling has pulled frames slightly out of square. The good news is that the most significant leakage points can be located without a blower door test or thermal camera — with the right method and a cold day.
How Heat Escapes Through Window Assemblies
Heat moves through and around windows through three distinct mechanisms, and understanding which is dominant in a particular window changes the fix.
Conduction Through the Frame and Glass
Heat conducted through solid materials — glass, aluminium frames, older wood frames — moves from the warm interior to the cold exterior regardless of whether the window is sealed. This is measured by U-factor (the inverse of R-value). A single-pane window with an aluminium frame may have a U-factor of 1.3 or higher. Current Energy Star requirements for Canadian Climate Zone 2 (southern BC) specify U-0.30 or lower. The gap between those numbers represents a sixfold difference in conductive heat loss.
Conductive loss through the frame cannot be addressed by weatherstripping or caulking. It requires replacing the unit with a thermally broken frame and appropriate glazing for the climate zone.
Radiation from the Interior Surface
Cold glass surfaces absorb radiant heat from interior objects — furniture, walls, occupants — and re-radiate it outward. Low-emissivity (low-e) coatings reduce this by reflecting long-wave radiation back into the room. In older windows without this coating, the radiant loss component can be significant, particularly in rooms with large unobstructed glass areas. This also explains why occupants near uncoated windows feel cold even when the room air temperature is adequate.
Air Infiltration at Seals and Frames
Air infiltration — the movement of cold outdoor air through gaps in the assembly — is the mechanism most affected by age, settling, and weatherstripping condition. Unlike conductive loss, infiltration is localized. A single failed seal, a compressed gasket, or a gap at the rough opening can account for a significant fraction of the total window heat loss. This is also the component most easily addressed without replacing the window.
Finding Leaks Without Specialized Equipment
Professional blower door tests depressurize the house to amplify infiltration and are performed with diagnostic instruments. Useful results are also achievable without that equipment on a day when outdoor temperatures are at least 10°C below interior temperatures.
The Hand Test
Moving a bare hand slowly along the perimeter of a closed window — sash edge, frame-to-wall junction, and along the bottom sill — will identify moving air with a temperature difference of a few degrees. The outer edges of the frame, where it meets the rough opening, are often the highest-infiltration areas and are frequently overlooked in the focus on glass.
Smoke Pencil or Incense
Holding a stick of incense or a lit match at arm's distance from suspected gaps makes air movement visible. The thread of smoke deflects toward or away from the gap depending on interior-to-exterior pressure difference. This method works well around door frames, sliding window tracks, and where pipes or wires penetrate through an exterior wall near a window.
Plastic Sheet Test
A thin piece of plastic film taped loosely over the interior window frame will flutter or press against the glass in the presence of infiltration. This is a low-precision method but useful for identifying which windows in a multi-room house have the most active leakage before deciding where to invest in repairs.
Sealing Products and Their Applications
| Product Type | Best Use | Cold-Weather Performance | Longevity |
|---|---|---|---|
| Silicone caulk | Frame-to-wall junction, fixed gaps | Excellent (rated to –40°C) | 20+ years |
| Acrylic latex caulk | Interior gaps, paintable surfaces | Moderate (avoid below –5°C) | 5–10 years |
| EPDM foam weatherstrip | Operable sash compression seals | Good (retains memory to –30°C) | 8–15 years |
| V-strip (tension seal) | Sliding sash side channels | Excellent | 15–20 years |
| Door sweep (aluminium + brush) | Door bottom threshold gap | Good | 10–15 years |
| Expanding foam (low-expansion) | Rough opening cavities | Excellent once cured | 20+ years |
One consistent error in DIY sealing is applying low-expansion foam to the gap between an operable window sash and its frame. Foam compresses against moving parts, preventing the window from closing fully and compounding the original problem. Foam belongs at fixed rough openings and structural penetrations only.
When Sealing Is Insufficient
Air sealing is cost-effective when infiltration is the primary loss mechanism. When conductive loss through the frame or glass dominates — which is typically the case with single-pane or older double-pane windows in climate Zones 6, 7, and 8 — the aggregate energy saving from sealing may not justify the time and material cost relative to replacing the unit.
A rough guide: if the inner surface of the glass is visibly frosted, condensation-streaked, or noticeably cold to the touch at –10°C outdoors, the window's thermal performance is poor enough that sealing alone will not materially reduce heat loss. Energy Star-rated windows for Canadian Zone 3 or higher are required to meet U-0.28 or lower and include appropriate glazing specification (triple-pane for Zones 4 and above in most provincial programs).
The Canada Mortgage and Housing Corporation estimates that air sealing and weatherstripping, when applied systematically throughout a house, can reduce heating energy consumption by 5 to 15 percent depending on the existing leakage rate and climate zone.
Door Frame and Threshold Leakage
Exterior doors share the same failure modes as windows — frame-to-rough-opening gaps, degraded compression weatherstripping at the stop — but add a threshold component that windows don't have. The gap between the door bottom and the floor covering increases as thresholds settle or as doors are adjusted to close more easily over time. A gap of 3mm along a 90cm door opening is equivalent to a hole with the area of a golf ball; at –25°C outdoors, cold air infiltration through that gap is substantial.
Door sweeps should brush the floor surface across the full width with even contact. Threshold seals that rely on compression with a raised ridge are more effective but require the door to close against the ridge with consistent pressure — which requires the door to be true in its frame. A door that has dropped or twisted in its hinges needs the mechanical issue corrected before a new seal can function properly.