Each of these materials is sold as the obvious choice, which cannot be true of all three. They genuinely differ, and the right answer depends on the opening, the climate where you live, and how long you intend to keep the building.
Aluminium
Where it wins: slim frames and large openings. Aluminium is strong for its section, so you get more glass and less frame than uPVC of equivalent capability. It does not warp in heat, and a powder-coated or anodised finish lasts well. It is also the easiest of the three to get modified or repaired locally.
Where it loses: sealing and heat. A basic aluminium slider does not seal as completely as a good uPVC casement, so it admits more dust and noise. Aluminium also conducts heat readily — thermally broken sections solve this and cost considerably more.
Best for: large openings, balcony sliders, anywhere the view matters, and most ordinary urban homes where dust is manageable.
uPVC
Where it wins: sealing. A well-made uPVC casement closes onto a gasket all round, which makes it the best of the three against dust, driving rain and traffic noise. It does not corrode or need painting, which is a real advantage in coastal and high-humidity locations.
Where it loses: frames are bulkier for the same strength, so you see more frame and less glass. Quality varies enormously — an unreinforced profile bows in Indian summer heat, particularly in a dark colour and a large frame. And it is essentially unrepairable: damage usually means replacing a sash or the unit.
Best for: dusty or noisy locations, coastal homes, bathrooms and utilities, and anyone who wants to stop maintaining windows.
Mild steel
Where it wins: strength and cost, and a genuine aesthetic. The thin black frames in period and industrial-style designs are steel, and nothing else achieves those sightlines. It is also the strongest option for security, and simple sections are cheap.
Where it loses: rust. Steel outdoors needs a serious coating system and ongoing attention, and near the coast it is a losing battle unless galvanised. It conducts heat and cold freely, and condensation on steel frames in cool weather is common.
Best for: secure openings, grills, a deliberate industrial aesthetic, and dry inland climates where maintenance will actually happen.
Honest cost ranking
Over a twenty-year horizon rather than at purchase: mild steel is cheapest to buy and costs the most to keep. Aluminium sits in the middle on both. uPVC costs most up front and least to keep. If you are selling the property in three years, the purchase price is what matters; if you are living there for twenty, it is not.
The questions that actually decide it
- How close to the sea are you? Within a few kilometres, uPVC or properly treated aluminium. Not plain steel.
- How dusty is the location? A main road or a dry, windy city pushes strongly towards uPVC casements.
- How large is the opening? Beyond a certain size, aluminium's strength-to-section advantage becomes decisive.
- Will anyone maintain it? Be honest. Steel assumes somebody repaints it.
- Does appearance lead? Slim steel or slim aluminium look quite different from any uPVC system.
A note on mixing
There is no rule that a house must use one material throughout. uPVC in the bathrooms and the road-facing bedrooms, aluminium on the large balcony sliders, and steel grills where security matters is a perfectly sensible specification — and usually a cheaper one than forcing a single material to do every job.