PVC or aluminium with thermal break: the battle that decides your heating bill

PVC or aluminium with thermal break: the battle that decides your bill

PVC conducts heat 1,200 times worse than aluminium. That figure, which appears in any materials manual, explains why a growing share of the market has stopped looking at aluminium with a thermal break as the definitive solution. The conclusion attributed to the IDAE (Spain's Institute for Energy Diversification and Saving) is blunt: three-chamber PVC frames with low-emissivity glazing are the enclosures with the best thermal performance, without ruling out series in other materials with the same U-value.

The comparison starts with an energy-saving table pitting PVC against aluminium with a thermal break and aluminium without one. On that basis, the technical consensus is quickly reached: aluminium without a thermal break is a radiator on hinges. Anyone who has it can feel it in their hand when touching the frame in January. The break, a small piece of plastic between the outer and inner part of the profile, prevents direct contact of the metal, but does not eliminate the inner air gap. PVC, on the other hand, is plastic along its entire length.

What it costs and how long it takes to pay off

A window with a thermal break costs between 30% and 60% more than one without. The range comes from a participant with a payback criterion: five years. With a couple of windows, the extra cost is recovered; with a whole flat, the bill stretches out. PVC works out slightly cheaper than high-end aluminium, and considerably cheaper than two-tone aluminium, where the exterior finish means paying for the façade's aesthetics.

Some argue that the best thermal-break aluminium insulates as well as ordinary PVC. The argument has a basic flaw: it compares the ceiling of one material with the average of the other. A poor PVC profile is still a better insulator than standard aluminium, because the plastic's conductivity does not depend on the air chamber.

Colour, expansion and the myth of the melted plastic

Two-tone PVC pushes up the budget, but it is not the only problem. Expansion in direct sunlight is the classic complaint: windows that will not close until they cool down. Against that narrative, accumulated experience points the other way. High-end profiles with more than fifteen years exposed to the elements, north-facing and with frosts, still close without play. One striking case: dark grey PVC in a village in inland Málaga, five kilometres from the coast, with oven-like summers and no signs of wear.

Yellowing of the white does not appear in fifteen-year-old installations. Dark colours are another matter, and there caution is reasonable. The anodised aluminium alternative also loses its shine over the years, even though the market sells it as everlasting.

The glass, the shutter box and the sealing matter more than the profile

The profile is a small part of the window's surface. The glass, the shutter box, the hardware and the installation decide the final result. A 4-16-4 double glazing with low emissivity is the reasonable standard; widening the cavity beyond that does not pay off. The inner gas is lost over the years, around a decade, and solar-control glass such as Guardian Sun protects against summer heat at the cost of giving up winter solar gain.

Orientation rules. On a south-facing façade with many hours of sun, solar-control glass can be a miscalculation: in winter the sun heats for free and the glass blocks it. The decision depends on whether heating is cheap or expensive. With a heat pump or geothermal system, protecting against July heat matters more than capturing January's.

Installation matters more than the label

The best profile on the market comes to nothing with a poor installation. Polyurethane sealing, filling the shutter box and flush alignment with the wall affect insulation as much as the frame's U-value. Some point to the workshop as the decisive variable: the same profile system installed by different professionals gives different results.

The air cavity of the façade completes the picture. A 70-millimetre PVC window is of little use if the wall around it is a sieve. Interior lining with insulation takes up centimetres and can create condensation; insulating on the outside is the right approach, but the homeowners' association rarely approves it.

Noise, the argument nobody disputes

On a noisy avenue, PVC wins by a landslide. The combination of a casement closing system, a double EPDM gasket and panes of different thicknesses damps the frequencies that aluminium lets through. A 4+6 unit performs better than a 6+6 for noise, and a special butyral in the laminated pane finishes the job. Sliding windows, in any material, are a leak for water and sound.

With the drop in oil prices, PVC is cheaper today than a few years ago. The only recognised drawback compared with high-end aluminium is the width of the profile, which costs a little light. In homes with small openings, that centimetre counts.

The reasonable prediction is that PVC will keep gaining share in new builds and renovations, except where façade regulations require two-tone finishes or the architect demands slim profiles. Thermal-break aluminium will keep its niche in design projects and communities with protected aesthetics. The battle is not decided in the catalogue, but in the sealing line.