Windows and exterior doors are among the most thermally vulnerable parts of a home's envelope. Even in a house with well-insulated walls and attic, poorly sealed window frames, worn weatherstripping, and single-pane glass can account for a substantial portion of total heat loss during a Canadian winter. Addressing these points is often the most accessible retrofitting work a homeowner can do, requiring modest materials and no structural changes.
This article covers the main techniques for reducing air infiltration and radiant heat loss through and around windows and doors, with a focus on methods appropriate for the range of housing stock found across Canada — from century homes in Ontario to mid-century bungalows in the Prairies.
Understanding Where Infiltration Occurs
Air leakage around windows and doors happens at two distinct locations: the junction between the window or door frame and the surrounding wall assembly (the rough opening), and the operable components of the unit itself — where sashes meet frames, or where door panels meet the frame and threshold.
The rough opening gap is often addressed once during installation and then ignored for decades. Caulking applied at installation cracks and separates over time as the building moves seasonally. In older homes, the original installation may have used oakum, fibreglass scraps, or other materials that have long since compressed or degraded.
The operable junction — between a sliding sash and its frame, or between a hinged door and its jamb — relies on weatherstripping that wears out more rapidly than most homeowners expect, particularly at high-use locations like main entry doors.
Caulking: Materials and Placement
Caulking seals fixed gaps at the perimeter of window and door frames. The exterior bead of caulk seals the frame-to-cladding joint; an interior bead seals the frame-to-drywall (or plaster) joint on the inside. Both are necessary in a cold climate because air infiltration can enter from the exterior while interior-side gaps allow warm moist air to reach the cold outer portions of the assembly.
Silicone-based caulks perform well for exterior applications because they remain flexible through wide temperature swings, adhere to most substrates including glass and painted wood, and are UV-resistant. They are more difficult to paint over, however, so a paintable latex-silicone hybrid is often used where aesthetics require it.
For interior applications around window trim and door casings, paintable acrylic latex caulk is standard. It can be applied cleanly with a wet finger, sands and paints easily, and handles the moderate movement of interior framing.
Application Technique
Effective caulking requires a clean, dry surface. Old caulk should be removed entirely rather than caulked over, because adhesion to existing caulk is unreliable. A utility knife, a plastic scraper, and a caulk removal tool are sufficient for most jobs. The bead should be continuous, pressed firmly into the gap, and tooled smooth before it skins — typically within 10 to 30 minutes depending on formulation and temperature.
In low temperatures (below about 5°C), most water-based caulks will not cure properly and should not be applied. Some silicone formulations are rated for low-temperature application, but even these require the substrate to be above freezing. Planning exterior caulking work for late summer or early fall avoids these constraints.
Weatherstripping: Types and Selection
Weatherstripping fills the moving gaps in operable windows and doors. The appropriate type depends on the gap geometry, frequency of use, and the material of the surfaces it will seal against.
Foam Tape
Adhesive-backed foam tape is the lowest-cost option and is suitable for low-friction applications where the surfaces compress against the foam without sliding. It is commonly used on double-hung window sashes where the top and bottom rails contact the frame stops. Its lifespan is typically two to five years before compression fatigue reduces its effectiveness.
V-Strip (Spring Bronze)
V-strip weatherstripping is a folded metal or plastic strip that creates a spring tension against the mating surface. It is particularly effective for the sides of double-hung windows and door jambs, where the sealing surface slides along the strip during operation. Spring bronze is the most durable version and, when properly installed, can last 20 years or more. It requires more careful fitting than foam tape but performs far better over time.
Door Sweeps and Thresholds
The gap at the bottom of exterior doors is the single largest infiltration point in most door assemblies. A door sweep — a flexible fin or brush strip attached to the interior face of the door bottom — compresses against the threshold when the door closes. Automatic door bottoms, which retract when the door is open and lower when it closes, provide better performance but are more expensive and require a threshold adjustment to function correctly.
The threshold itself should create a tight seal against the door bottom. Adjustable-height thresholds allow this seal to be re-established as the door sags or the flooring changes over time.
Window Glazing Options
For windows that are not being replaced, interior window insulation film offers a meaningful improvement in thermal performance at low cost. These polyester films are adhered to the interior window frame and then shrunk tight using a hair dryer. They create a still-air buffer between the film and the glass, reducing both convection and radiant heat transfer. In tests conducted at cold temperatures, properly installed window film on a single-pane window can reduce heat loss through the glazing by 25–40%.
For windows with operational sashes that cannot be covered with film, interior magnetic-mount acrylic panels offer a removable alternative that preserves the ability to open the window in warmer months. These are more expensive than film but can be reused year over year.
Windows with double-pane units that have developed seal failures — visible as fogging or moisture between the panes — can sometimes be addressed by replacing the glazing unit alone rather than the entire window frame. This is an option worth investigating before replacing a window that is otherwise structurally sound and well-installed.
Identifying Gaps Using a Blower Door Test
For homeowners who want a systematic assessment of air leakage, a blower door test — available through energy auditors certified under the EnerGuide program administered by Natural Resources Canada — identifies leakage pathways by depressurizing the house and measuring airflow. This can reveal infiltration around windows and doors that is not obvious under normal conditions, including leakage through the rough opening behind the interior trim.