Replacing windows: PVC or aluminium and why the 16 mm cavity matters

Replacing windows: the 16 mm cavity nobody respects

An apartment with sliding windows that let in draughts of air. That is the starting point of a decision that ends up costing thousands of euros, in which every salesperson swears their profile is the only one that works. Three quotes on the table: Rehau 70 mm with five chambers and 4-16-4 glazing, Gealan 74 mm with seven chambers and 4-14-6, and Kömmerling 70 mm with five chambers and 6-20-4. On paper, all three profiles declare the same insulation: **Uf of 1.3 W/m²K**, according to the data provided by one participant. The real difference is not in the brand, but in the glazing and in a detail many installers overlook.

The 20 mm cavity best ruled out

The seemingly most generous option is also the most problematic. A 20 mm cavity exceeds the threshold at which the air inside starts circulating by convection and loses much of its insulating capacity. The technical consensus in the discussion puts the **limit at 16 mm**: below it, insulation is reduced; above it, internal currents and thermal bridges are created. That is why the Kömmerling proposal with 6-20-4 is flagged as the least advisable despite its well-known brand.

The most frequently cited calculation points to **6-16-4** as the winning combination: two panes of different thickness filter different frequency ranges and improve acoustic insulation. A 4 mm pane has one critical frequency and a 6 mm pane another; together they compensate for each other. That is why the Rehau option, with 4-16-4, falls short on the glazing even though the profile is correct.

PVC versus aluminium: who recommends what and why

The second front in this matter is the material. The accumulated experience is clear: **PVC insulates better than aluminium with a thermal break**, and aluminium quotes come out **20% to 25% more expensive**, according to the figures the participants are working with. Suspicion about the installer's advice keeps coming up: metalwork shops sell what they manufacture in their own workshop, while PVC usually arrives from the factory. It is no coincidence that the biggest defenders of aluminium are precisely those who do not work with PVC.

The claimed lifespan of PVC is also debunked as a myth. There are cases of windows installed right on the seafront that, almost two decades later, still had not yellowed or pitted. The bad fruta comes from the first extremely low-quality profiles installed in Spain. In Andalusia, foil-wrapped anthracite grey profiles hold up without warping years later.

The blind spot: the shutter box

There is no point spending money on the best profile if the shutter opening is still there. The box is the Achilles' heel of any window: **two millimetres of plastic separate the living room from the street**. The most frequently mentioned solution is to seal up the box and fit motorised roller shutters with solar power, or opt for an insulated box with an electric motor, which costs around **150 euros**. It is also suggested to add the same type of glazing to the shutter casing and seal it against the subframe.

What really decides the bill

The conclusion that prevails: the profile brand matters least. What guarantees a good window is the workshop that assembles it, the hardware and a properly done installation. No profile manufacturer —not Rehau, not Gealan, not Kömmerling— answers for the finished window; the workshop does, with its CE marking. And when it comes to glazing, the differences between manufacturers are minimal.

One doubt remains: whether it is worth paying more for a seven-chamber profile when the declared insulation is identical. With the data in hand, the answer is not in the salesperson's brochure.