Weatherstripping a window: seals, insulation and stopping drafts
Windows are, along with the roof, the first point of heat loss in a home. Cold seeps in through worn seals, gaps in the casement and around the frame. Before replacing a window, often at a cost of several thousand euros, a simple, well-done weatherstripping job is usually enough to stop drafts and heat loss. Here is how to spot the leaks and choose the right seal for each situation.
Spotting air leaks
- Pass a lit candle or incense stick along the edges of the closed window: the flame flickers
- Check the gap between the casement and the frame: a sheet of paper should stay held when the window is closed
- Look at the condition of the existing seals: hardened, cracked or no longer recovering their shape
- Check the space between the frame and the wall, often the forgotten cause of leaks
- Test on cold days: a damp hand placed on the perimeter reveals the chill
The paper test is the most reliable: place the sheet between the casement and the frame, close the window and pull gently. If the paper slides out without resistance, the seal is worn or too thin. Repeat the test on all four sides and at the corners, since leaks are often concentrated on the hinge side and the bottom of the casement.
Choosing the right window seal
- Self-adhesive foam seal: economical, perfect for temporary weatherstripping
- Lip seal in EPDM or silicone: durable and effective on casements that close well
- Brush seal: ideal for sliders and casement windows
- Silicone sealant: for sealing around the frame-to-wall junction
- Clip-on profile without glue: the simplest to replace on PVC windows
Installing an adhesive seal on a casement
- Clean and degrease the mounting surface with a degreaser or alcohol
- Measure each side of the casement and cut the seal to the right lengths
- Remove the protective film and stick it on pressing evenly, without stretching the seal
- Handle the corners by cutting at 45° or by letting the seal follow the profile
- Close the window and check that the seal is compressed evenly
- Leave the window closed for a few hours so the adhesive bonds well
Sealing between the frame and the wall
- Scrape away the old crumbling sealant with a spatula or knife
- Vacuum and dust the perimeter of the frame
- Apply silicone or acrylic sealant into the cracks using a caulking gun
- Smooth with a finger moistened with soapy water for a clean finish
- Let it cure for 24 hours before painting if you use an acrylic sealant
The seal between the frame and the wall degrades faster than you might think: the caulk cracks, the old backing falls out and air gets through. An acrylic sealant, cheaper and paintable, suits interior use; a non-acidic silicone sealant, which does not attack PVC, is preferable outside where it must resist UV. Cut the nozzle at an angle and press evenly for a uniform bead.
When to replace a worn seal?
A worn window seal loses its elasticity and lets in air and noise. The most obvious sign: the paper slides out easily when the window is closed, or the seal detaches from the profile. Replacing a lip seal is simple: just pull on the old profile to tear it out, then snap the new one in pressing evenly, without glue on most PVC models. A new seal costs a few euros per meter, for immediate comfort.
Weatherstripping and airing: the right balance
Weatherstripping does not mean sealing off all air circulation. A home that is too airtight accumulates moisture, which condenses on the glass and grows mold in the corners of rooms. Keep the ventilation grilles and the mechanical ventilation system intact, and air out ten minutes a day. Weatherstripping aims to eliminate parasitic leaks, not to replace ventilation: it is this balance that guarantees healthy air and real comfort.
The mistakes to avoid
- Installing an adhesive seal on a greasy or dusty surface: it peels off within a few weeks
- Choosing a seal that is too thick, which strains and deforms the locking mechanism
- Using expanding foam to weatherstrip: it expands and deforms the frames
- Forgetting the ventilation ducts: sealing 100% causes condensation
- Leaving the old seal in place under the new one: poor adhesion and an uneven appearance
Weatherstripping a window is one of the simplest and most cost-effective insulation jobs there is. Adhesive seals on the casements, silicone around the frame and replacement of worn profiles: in a few hours and for a few euros, you stop drafts, reduce heat loss and noticeably improve the comfort of your home, without even touching the glazing.