Are aluminium roof lanterns better than uPVC?

A roof lantern is a structural object. It has to span an opening in the roof, carry its own glass, and take wind uplift and snow load without the bars sagging or twisting. That is where the two materials part company. Aluminium is stiff for its weight, so a slim aluminium bar 50 to 60 mm wide will span a distance that would need a uPVC bar half as wide again, often reinforced with a galvanised steel core threaded down the middle to stop it bending.

The practical result is sightlines. On an aluminium lantern the ridge and the hip bars are thin, and more of what you see when you look up is sky rather than frame. On a uPVC lantern of the same footprint the bars are wider and deeper, so a larger share of the opening is plastic and a smaller share is glass. On a small lantern the difference is easy to live with. On a three or four metre lantern over a kitchen it changes the whole feel of the room, because the extra frame is exactly the part that is not letting daylight in.

Span is the other limit. uPVC lanterns are usually held to a maximum size, often around three metres on the long side, because beyond that the frame cannot stay rigid and flat. Aluminium comfortably reaches larger openings, which is why the big rectangular lanterns over open-plan rear extensions are almost always aluminium. If you want the glass, the material more or less chooses itself.

uPVC is not the wrong answer everywhere. On a small lantern, say up to about two metres over a modest extension or a utility room, the frame is not being asked to do much work, so the bulkier bars matter less and the sagging risk barely applies. The saving is real: a uPVC lantern typically costs noticeably less than an aluminium one of the same size, and if the budget is tight and the opening is small, that can be the deciding factor.

The frame material itself also insulates well. Solid uPVC is a poor conductor of heat, so a uPVC frame does not need engineering to stop the cold coming through the way a metal one does. That is a genuine point in its favour, though as the next chapter explains it is not the whole thermal story, and it matters far less than the glass you put in either frame.

If a lantern is small, north-facing or shaded, and the priority is a decent rooflight at the lowest sensible price, uPVC does the job. It is the large, sunny, glass-led installations where its limits start to show.

People often assume aluminium must be cold because metal conducts heat, and bare aluminium would be. Every serious aluminium lantern today is thermally broken, which means a strip of insulating material sits between the outer and inner faces of the frame so heat cannot travel straight across the metal. A thermally broken aluminium lantern and a uPVC lantern of the same size will have frame U-values close enough that the choice rarely turns on it.

What actually decides the thermal performance of the whole lantern is the glass, because the glass is most of the area. A lantern glazed with clear double units at a U-value around 1.6 will lose heat faster on a January night than one glazed to nearer 1.0, whatever the frame is made of. So the sequence that matters is glass first, then frame. Ask for the whole-unit U-value of the finished lantern rather than a figure for the frame or the glass on its own, and remember that a new or enlarged rooflight is notifiable under Building Regulations, with Part L setting the thermal standard the installation has to meet.

Over a long life the two materials also age differently. Powder-coated aluminium holds its colour and shape for decades, does not swell in heat, and comes in effectively any RAL colour, so the lantern can match the windows or the guttering rather than being white by default. uPVC is usually white or a foiled woodgrain finish, and while modern grades are far better than older ones, large white uPVC sections in full sun can discolour over many years and are more prone to expanding and contracting with temperature, which is part of why the spans are kept modest. Neither is high-maintenance: both want no more than an occasional wash. The difference shows at the top end of size and exposure, where aluminium has more in reserve.

Here is where the material choice quietly feeds into the thing that actually goes wrong in south Essex. Southend faces south across the Thames Estuary, and a great many rear extensions face south with it. Choosing aluminium so you can fit a bigger, glassier lantern is the right instinct for daylight, but a bigger area of glass pointed at the summer sun is also a bigger solar collector over the room below.

A lantern sits close to horizontal, so it catches the high summer sun almost square on and orientation barely protects you. Fill that opening with clear double glazing at a G-value around 0.6 and roughly 60 per cent of the sun’s energy lands in the room as heat, right over a kitchen that is already making its own heat from cooking. The frame material has no say in that. The glass specification does. This is the point our roof lantern installation work is built around: choose aluminium for the slim frame and the span, then specify solar-control glazing at a G-value nearer 0.3 so the light comes in and most of the heat does not. Glare off the water is part of the same problem, and the same coating helps with it. The longer version of that argument sits on our energy-efficient skylight installation page.

Work backwards from the opening. If the lantern is small, shaded and the budget is the priority, uPVC is a sensible buy. If it is large, if it faces anywhere near south, or if you want slim frames and as much glass as the roof will take, aluminium earns the extra cost, provided the glass is specified to match the orientation. See where both land on our skylight installation costs page, then tell us the size of the opening and which way the roof faces. Ring 01702 898232 or request a quote, and we will specify the frame and the glass together rather than one without the other.