Replacing aluminium windows with PVC: €6,000 and a 0.2 U-value gain

From a U-value of 3.3 to 1.5: what €6,000 actually buys you in windows

A semi-detached house in the province of Salamanca, at 800 metres of altitude, with seven aluminium windows without a thermal break and a U-value of 3.3 W/m²K. That is the starting point. The owner wants to know whether switching to PVC with low-emissivity double glazing and a U-value of 1.5 —€6,000— or to a 76 mm profile with a triple gasket and 1.3 —€6,500— really makes a noticeable difference. The short answer: it does, but not where he thinks.

The first mistake in approach is dismantled by a simple calculation. The wood-burning stove runs almost all winter and the warm air escapes up the open stairwell to the upper floor. Replacing the windows does not turn the ground floor into an airtight container: heat keeps rising. What it does do is slow the leak through the openings, which in an aluminium frame without a thermal break is noticeable to the touch in winter and summer.

Is it worth paying €500 more for 0.2 of U-value?

Here the technical consensus is more nuanced than it seems. A jump from 3.3 to 1.5 is huge; from 1.5 to 1.3 is marginal. The price difference —€500 on €6,000— is not recouped through the U-value, but through the third rubber gasket and the thicker profile, which affect airtightness and durability. Some argue that in a cold climate the standard for the whole assembly should not exceed 1.3, and that the glazing should be below 1.1. The most demanding scenario calls for 80 mm profiles from manufacturers like Schüco; the most pragmatic one points out that the glass is the largest surface area and that triple glazing on a 70 mm profile ends up causing weight problems over the years.

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The figure nobody looks at: the walls and the roof

The home's energy certificate gives façade walls a U-value of 1.40, the roof 0.96 and the floor 1.20. According to the benchmark used in the industry, the roof should be around 0.24. That is where the real leak is. Injecting rock wool or adding a layer of insulation moves the needle more than 0.2 of U-value in the frame. And if the joinery dates from the 1990s, the wall's air cavity may be blocked with debris or have a sagged 10 mm EPS board: it is worth carrying out inspections before signing the window quote.

The most sceptical camp goes further: if the effective energy is cheap —firewood and a 9 kW stove insert with fans—, window insulation is «small change» compared with what is lost when opening them to ventilate. It is a provocation, but it points to something true: without tackling the walls and roof, the return on €6,000 takes a long time to arrive.

Shutters, installation and the detail that gets forgotten

The quote includes 200 mm monoblock units with improved thermal and acoustic insulation and an 84 mm guide rail. Motorising the shutters costs around €120 per unit and avoids the holes made by the fixing tape, a plus for airtightness. The installation matters as much as the material: a good window poorly fitted loses much of its theoretical U-value.

The prediction, with reservations: he will notice less cold on the frame and fewer draughts through the openings, and the house will retain a little more heat. But if the goal is the energy efficiency of a home with a poor rating, windows are the first chapter, not the last.