You're watching the installer fit your new window. He places it in the opening, grabs the foam gun and fills the gap all around with that yellow, expanding foam. It swells, cures, and the window looks rock-solid. He hastily drives a screw or two, trims the excess and leaves.
At first glance, the job looks fine. But you've just fallen victim to one of the most dangerous practices: using foam as a structural fixing material. Foam insulates - it does not support.
PU foam is an excellent material, but it was designed for one purpose only: filling voids and providing thermal and acoustic insulation. When installers rely on its adhesive properties to hold the window in place, they ignore three massive forces.
Modern energy windows are nothing like the lightweight aluminium frames of the '90s. A contemporary balcony door with triple glazing can weigh 100–150 kg per sash! Foam is spongy and soft. Under this continuous load it gradually compresses. The window "droops", catches on the floor and won't lock.
In summer, a dark-coloured aluminium profile in the sun can reach 60–70 °C. The metal expands with enormous force, pushing against the wall. Hard PU foam cannot accommodate this movement - it cracks, splits and detaches.
A window held primarily by foam is every burglar's dream. A strong shove or crowbar can rip the entire frame out of the wall in seconds, no matter how good the locking hardware.
Before any screw is driven, the window must be laser-levelled in three axes (horizontal, vertical and depth). To keep it in place, the installer uses special rigid plastic (or impregnated timber) shims.
The most critical ones sit at the bottom corners. Their role is vital: they take the hundreds of kilograms of glazing weight and transfer it directly to the concrete or brick, completely bypassing the joint gap. The foam applied later carries zero weight!
Correct installation demands levelling in three dimensions using a laser or digital spirit level. Plumb/level tolerance must not exceed 1.5 mm per metre of frame. This precision ensures the sashes close hermetically with no sticking for decades.
Once the window is perfectly levelled on its shims, it must be anchored to the wall. This is done exclusively with screws.
Professionals use special, long plug-free screws that drive directly into brick or concrete, delivering absolute structural adequacy. Each screw must penetrate at least 40 mm into the wall body.
There is a strict rule: screws must be placed 15 cm from each corner, then every 50–70 cm along the entire frame. For large sliders, the spacing may decrease to 40 cm due to higher wind loads.
Only after the window is mechanically locked to the wall with screws and shims does the installer reach for the foam gun. And even then, not just any foam - but special elastic closed-cell foams.
Foam's role is strictly insulating: it fills the joint, blocks air and absorbs thermal expansion. It carries zero mechanical load. This distinction - shims + screws for support, foam for insulation - is the fundamental rule of correct installation.
A top-quality window can be ruined on day one if the installation is rushed. Next time you see a crew fitting windows, glance under the frame. If you don't see support shims and perimeter screws, but only mountains of foam, ask for an explanation. Foam insulates - it does not support. That one small sentence holds decades of correct operation.
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