uPVC vs Aluminium Windows

There is no universally better material. We fit both, and the recommendation changes house by house. What follows is the honest set of trade-offs rather than a sales case for one side.

This is the most visible difference and the one most people notice immediately. Aluminium is strong enough to carry glass in a much slimmer section than uPVC, which needs more bulk and internal steel reinforcement to achieve the same rigidity.

The practical result is that for the same hole in the wall, an aluminium window gives you more glass and less frame. On a large picture window or a bank of windows across a rear elevation, the difference is striking. On a small cottage casement it matters far less, and a well-made flush uPVC casement can look entirely right.

uPVC is a poor conductor of heat, so the frame insulates well by its nature. Aluminium is an excellent conductor, which would make it a poor window frame were it not for the polyamide thermal break — an insulating barrier bonded between the inner and outer sections that stops heat travelling straight through.

Every aluminium window we fit is thermally broken. With that in place, both materials comfortably meet the Part L requirement for replacement windows in an existing dwelling of 1.4 W/m²K or Window Energy Rating Band B.

Measured on frame alone, uPVC still holds a small advantage. Measured on the whole window, aluminium claws much of it back precisely because the slimmer frame means a greater proportion of the opening is glass, and a modern sealed unit insulates better than any frame section. Do not compare frames — compare whole-window U-values, which is what the regulations are written around.

uPVC is the cheaper of the two, typically by a meaningful margin on a like-for-like specification. The gap varies with size, colour, opening configuration and glass, and it narrows on smaller windows where the material content is a smaller share of the total.

The honest way to think about it: if budget is the binding constraint and the property is a standard modern house, uPVC will give you a better overall result than stretching to aluminium and economising on the glass or the installation. If the apertures are large or the property justifies it, aluminium is where the money is well spent.

Aluminium is the more durable material. It does not warp, does not discolour with UV exposure, and a good polyester powder-coated finish holds its colour for decades. Frames of forty years and more are common.

Modern uPVC is far better than the material that gave it a poor reputation in the 1980s and 1990s. Current formulations resist yellowing well, and a well-specified uPVC window has a long service life. It remains the case that on a very exposed south-facing elevation, aluminium will look newer for longer, and that white uPVC is more forgiving in this respect than dark foiled finishes.

On both materials, the sealed units and the hardware tend to reach the end of their life before the frames do.

Where a window is tall, wide, or carries a large single pane, aluminium's strength becomes the deciding factor. uPVC sections rely on steel reinforcement and there is a practical limit beyond which the frame has to get visibly heavier to stay rigid. Aluminium simply does not have that problem at the sizes found in domestic work.

This is why almost all bifold and large sliding door systems are aluminium, and why picture windows and full-height glazing tend that way too.

A rough guide, and only a rough one — the survey decides it.

Whichever material you choose, the specification of the glass, the quality of the hardware and the standard of the installation matter more to the finished result than the frame material does. A well-installed uPVC window outperforms a badly installed aluminium one on every measure that matters — draughts, condensation, security and how it looks in ten years.

Both are covered by the same guarantee with us, both are self-certified under Assure, and both come with a Building Regulations Compliance Certificate.