Casement or sliding windows: airtightness

Understanding a window's airtightness on site

What causes air leaks: frames, sashes, seals and hardware

On a window, air rarely passes "through the middle of the glazing." Leaks mainly come from the junctions. Between the frame and the sash, too much play, poor adjustments, or worn hardware prevent proper compression of the peripheral seals. Another classic: the frame-to-wall junction, if the installation isn't continuous and properly sealed.

The indicators to know: air permeability (A*), the AEV rating and the impact on comfort

Air permeability is rated A* (usually A1 to A4). The higher the class, the more the window limits stray air infiltration. The AEV rating covers Air, Water, Wind. It helps you choose based on the building's exposure. In practice, a better class reduces draughts, noise, and the risk of discomfort near the windows.

Sensitive points in renovation: irregular reveals, sills, shutter boxes and penetrations

In renovation, airtightness comes down to the details. Irregular reveals, damaged sills, roller shutter boxes, and cable or pipe penetrations create leaks. Aim for continuity with compression tape, suitable mastic, or a membrane, and secure the connections with sealing tape. A simple smoke test on site quickly identifies the weak zones.

Casement windows: why they're often easier to make airtight

Seal compression: the principle, adjustments and wear over time

On a hinged window, airtightness mainly comes from seal compression when the sash presses against the frame. It's mechanical, and so adjustable. With the strike plates and eccentric rollers, you can correct a slight gap without removing anything. Over time, the seals compress, harden or get cut. A simple check prevents air from slipping in like an uninvited draught.

Types of seals and profiles: double seal, centre seal, sill and drip mouldings

Depending on the profile, you'll find a double peripheral seal, or a centre seal on both the sash and the frame. The sill needs to stay continuous and clean, or water will rise by capillary action. The drip mouldings and drainage grooves handle the rest. If these outlets are blocked, even the best window will eventually let water in.

Common mistakes to avoid: out of plumb, poorly adjusted hinges, insufficient shimming

• Out of plumb or a twisted frame. The window no longer compresses the seals in the right place.
• Poorly adjusted hinges. The sash rubs, closes at an angle, and airtightness drops.
• Insufficient shimming. The weight is carried by the hardware, and the adjustments don't hold.

Sliding windows: the limits and the solutions to improve airtightness

Why sliding windows are more exposed to air infiltration: rail, brushes and overlap zones

A sliding window moves in a rail. It therefore doesn't compress its seals the way a hinged window does. Airtightness often relies on brushes and overlap zones between panels. With dust, wear or a slight adjustment gap, air more easily finds a way through, especially at the bottom near the sill.

Choosing the right options: lift-and-slide, reinforced seals, suitable sills and hardware

To gain comfort, lift-and-slide is a good option. On closing, it presses the sash against the seal. Also aim for a good AEV rating, reinforced peripheral seals and a sill suited to the use case. On the hardware side, a multi-point locking system and well-adjusted rollers limit air infiltration. If you're still deciding which type of glazing to pair with the frame, also see the criteria for choosing between single, double or triple glazing in renovation.

Special cases: large sizes, strong wind, wet rooms and exposed façades

In large sizes, a profile that flexes quickly loses pressure on the seals. In windy areas or exposed façades, favour high air and water performance and regular maintenance of the rails. In a wet room, check the water drainage and the ventilation to keep humidity from creeping in at the bottom.

Installation and sealing: the steps that make the difference between casement and sliding

Preparing the substrate: removal, reworking the reveals, checking levels and clearances

After removal, clean and rework the reveals. Check plumb, level and squareness. On a casement window, the slightest defect shows up as a seal that doesn't compress properly. On a sliding window, it's the rail that has to stay perfectly straight. Keep the planned installation clearances before fixing the window in place.

Ensuring continuous sealing: compression tape, membranes, controlled PU foam and adhesive strips

Work in three layers. On the outside, compression tape or suitable mastic for the rain. In the middle, a measured amount of PU foam, without distorting the frame. On the inside, a membrane and adhesive strips for airtightness. On sliding windows, don't block the water drainage. Aim for continuous sealing.

Treating the critical connections: sill, drip edge, jamb linings, flashings, connection to insulation (external/internal wall insulation)

Take care with the sill. A functional drip edge, shimming, slope and flashing guide the water outward. The jamb linings align the window with the external or internal wall insulation and limit thermal bridges. Finish with insulation returns and clean trim work for real durability.

Checks and adjustments in 2026: how to validate the airtightness of an installed window

Quick on-site checks: smoke test, paper-feeler test, listening for whistling, and inspecting the seals

On site, you can validate a window in 10 minutes. Run a smoke test all around the frame. The smoke shouldn't be drawn inward. Then do the paper-feeler test: pinch a sheet of paper in the seal, close the window, then pull. If it slides too easily, the compression isn't right. On the first windy day, listen for whistling. Finish with an inspection of the seals: mastic or compression tape continuous, with no gaps, and the water drainage holes clear.

When to request a blower-door test: comprehensive renovation, client requirements and trade coordination

In a comprehensive renovation, installing the windows and the membranes must be planned before a blower-door test, to keep real margin for correcting leaks. Request it if the client wants proof, if you're targeting a specific performance level, or if several work packages affect the continuity of the airtightness. Schedule it after the windows and membranes are installed, before the trim work.

Maintenance and adjustment plan: preserving airtightness season after season (seals, rollers, closing)

Plan for an annual check. Clean the seals, replace any that have hardened. Adjust the rollers and strike plates to restore even pressure all the way around. Lightly grease the hinges and mechanisms, and check that the closure presses firmly without forcing. A well-adjusted window keeps its comfort, winter and summer alike.