Why windows and doors lose so much heat

Windows and exterior doors account for a disproportionate share of heat loss in Canadian homes — not primarily because glass is a poor insulator (though it is), but because the interface between the frame and the rough opening is a chronic source of air leakage. The wood framing around a window or door moves seasonally with changes in moisture content and temperature. Over years, the gaps between the frame, the rough opening, and the surrounding trim can open enough to allow substantial air movement.

Air leakage through these gaps is a more significant heat loss mechanism than conductive heat transfer through the glass itself. A single unaddressed gap around a door frame can cause the equivalent heat loss of leaving a window open by a centimetre for the entire heating season.

Freeze-thaw cycling — which occurs repeatedly throughout autumn and spring in most of Canada, and intermittently even during winter — accelerates the deterioration of caulking and compressible weatherstripping. Products that were functional after installation may be cracked, hardened, or displaced by the time they are most needed.

Assessing the current condition before buying products

Before purchasing any weatherproofing materials, a straightforward inspection on a cold, windy day reveals where the actual problems are. Running a hand slowly around the perimeter of each exterior door and at the interior trim of operable windows will identify noticeable draughts. A stick of incense or a thin piece of tissue held near suspect locations makes air movement visible.

Common locations for air leakage at windows:

  • Between the window frame and the rough opening, behind interior casing trim
  • Around the perimeter of the sash where it meets the frame (weatherstripping failure)
  • At the meeting rail of double-hung windows when the sash locks are worn
  • Through the glazing compound or silicone bead between the glass and the sash frame

At exterior doors, the most frequent leakage points are:

  • The threshold seal at the bottom of the door
  • The compression strip on the door stop around the top and sides
  • The corner joints of the door frame where the head jamb meets the side jambs
  • Behind the exterior brick mould or casing where it contacts the siding or brick veneer

Weatherstripping types and their applications

Weatherstripping products fall into several categories based on their mechanism and the type of gap they are designed to seal.

Compression strips (foam and EPDM bulb)

Foam tape and extruded EPDM (ethylene propylene diene monomer) rubber bulb strips work by being compressed when the door or window closes. They are the standard product used on door stops — the narrow strip of wood that the door closes against. EPDM bulb strips are more durable than foam tape in freeze-thaw conditions; foam tape can harden and lose compressibility after a few seasons of cycling between −30 °C and +10 °C. EPDM retains flexibility at temperatures as low as −50 °C, making it well-suited to Canadian conditions.

The limitation of compression weatherstripping is that it requires the door to compress the strip fully when closed. If the door has settled or the frame has racked slightly, compression may be uneven, leaving a gap on one side.

V-strip (tension seal)

V-strip weatherstripping — a thin folded piece of metal or durable plastic in a V profile — is installed in the channels along the sides of double-hung and single-hung window sashes. As the sash slides past the V-strip, the strip flexes and maintains contact with the sash surface. It does not rely on compression, which makes it effective even when the sash has shifted slightly from its original position. Metal V-strip is more durable and more difficult to install than plastic versions; both types are available at most Canadian building supply retailers.

Door sweeps and automatic door bottoms

The gap at the bottom of an exterior door is often the largest single air leakage path. Standard door sweeps attach to the interior face of the door at the bottom and drag across the threshold as the door opens and closes. A dragging sweep does seal effectively but wears faster on thresholds with surface irregularities.

Automatic door bottoms are mechanically more complex — a plunger mechanism retracts the sweep when the door opens and extends it to contact the threshold when the door closes. This prevents the sweep from dragging across the threshold, extending its service life substantially. The additional cost of an automatic bottom is generally justified for frequently used exterior doors.

Whatever sweep type is used, the threshold itself should be inspected. Many door thresholds have a vinyl gasket that wears flat over time. Replacement gaskets are available for most common threshold profiles and cost far less than replacing the threshold assembly.

Caulking: materials and application

Caulking seals static gaps — those that do not move as doors and windows operate. The primary caulking locations at windows are the exterior joint between the window frame and the surrounding casing or siding, and the interior joint between the casing trim and the wall surface. The interior caulk joint is relevant for air sealing; the exterior joint is relevant for both air and water control.

Silicone caulk

100% silicone caulk is the most durable option for exterior applications around windows and doors. It remains flexible at low temperatures, adheres well to non-porous surfaces (glass, aluminum, vinyl), and has a service life measured in decades rather than years. Its limitation is that it cannot be painted and is difficult to tool neatly without practice. It also requires a thoroughly clean, dry surface for adhesion — applying it over old caulk or on a damp surface compromises the bond.

Siliconized acrylic latex caulk

Acrylic latex caulks with silicone added are the standard choice for interior joints and for exterior joints where paintability is required. They are easier to apply and tool than pure silicone and can be painted after a short curing period. Their flexibility in cold temperatures is inferior to pure silicone, and they tend to crack at exterior joints after several freeze-thaw cycles in climates with significant temperature swings.

Polyurethane foam backer rod and sealant

For gaps wider than approximately 6 mm behind window and door casing, backer rod — a flexible foam rope — should be inserted before applying caulk. Backer rod limits the depth of the caulk joint to the optimum 2:1 width-to-depth ratio, which improves the elasticity and durability of the seal significantly. Without backer rod, deep caulk joints have a tendency to crack as the gap moves seasonally.

Expanding polyurethane foam — available in low-expansion window-and-door formulations — is appropriate for filling larger gaps around rough openings. Standard high-expansion foam can exert enough pressure to bow window and door frames; low-expansion products are specifically formulated to avoid this problem.

Interior window film for thermal improvement

Plastic window film kits, applied to the interior window frame with double-sided tape and then shrunk tight with a heat gun, create a secondary air space between the film and the glass. This additional still-air layer improves the effective insulating value of a single-pane window significantly, and provides a meaningful improvement to older double-pane units that have lost their inert gas fill.

The primary limitation of window film is that it prevents opening the window for the heating season. In rooms where ventilation by window opening is desired occasionally, film may not be practical. It is most useful in rooms with mechanical ventilation or in basement window openings that are rarely opened.

Timing and sequence for annual weatherproofing

The practical window for exterior caulking in most of Canada is September to mid-October — after summer heat has dissipated (which would prevent siliconized products from curing correctly) and before overnight temperatures drop below the minimum application temperature of most caulks (typically 5 °C for water-based products, 0 °C for silicone). Many products specify that the surface temperature must be above 5 °C at the time of application and for several hours afterward.

Weatherstripping replacement and interior air sealing can be done at any time of year. A late-autumn inspection after the first cold snap is often the most efficient timing — draughts are immediately detectable, and any deficiencies can be addressed before the coldest months.

The Natural Resources Canada weatherproofing guidance provides additional product-specific recommendations and installation notes for Canadian climate conditions.