Is glass or polycarbonate better for heat?

The sheet used on older conservatories, lean-tos and porches is hollow. Thin walls divide it into long channels, and the still air in those channels does the insulating. A thicker sheet with more walls holds more heat than a thin, single-skin one, and the maker's data sheet gives the figure for each build.

The weak points are the edges and the bars. Sheets are clipped between aluminium or uPVC glazing bars, and the bars conduct heat across the roof wherever they run. Over a conservatory the bars run the length of the roof at regular spacing. Any gap at a closure strip or the ridge lets warm air leak straight out.

The open ends of the channels need tape and a cap or closure profile to keep water, dust and insects out. Where those have failed or lifted, the inside of the sheet can mist and the insulating value is affected.

Polycarbonate is also often opal or tinted. That diffuses the light and cuts the sun, which keeps a garden room cooler, but it gives a milky brightness and no view of the sky.

Age shows in the sheet

Sheet that has been up for years can scratch, cloud and yellow on its outer face, and dirt or algae can settle inside the channels where it cannot be wiped. The room under it gets dimmer without anyone noticing, and the heat figure on the original data sheet no longer describes what you have. If you are weighing a replacement, how the roof performs now is the useful starting point, and we look at that when we visit.

A sealed glass unit has two or three panes, a gap that can hold argon, and a coating on one surface that sends part of the warmth back down into the room. The spacer bar at the edge can be a warm-edge type. Put together, the unit can be specified to meet the Part L limit for a rooflight of 2.2 W/m²K, and the figure that counts is for the whole unit, frame included.

Clear glass also lets the sun in, so the same coating choice that saves heat in winter matters in summer. Our answer on whether rooflights overheat a room explains solar control glass for a south-facing roof.

The weight that comes with it

Glass is heavier than sheet. A frame that carries twin-wall polycarbonate may not take a glazed unit without strengthening, and the walls or beams under it have to carry the extra load. We check that at survey before any glass is ordered.

Take an orangery-style lean-to with a sheet roof. The old sheets come off, the bars may be reused or replaced, and a glazed roof or a roof lantern goes on. Because a new lantern will usually stand more than 150 mm above a flat roof, planning permission is normally needed unless the council confirms otherwise. The planning guide lays out those conditions in full.

Where the existing structure is sound, a glazed roof often brings the room into use for more of the year. The winter figure improves, the summer glare can be tamed with the right coating, and condensation lines under the bars tend to ease. A solid insulated roof with a lantern is another route, and it can be warmer still because the roof area around the lantern is insulation.

Where the work comes under the Building Regulations, the replacement glazing must meet the Part L limit for its type, so the written quote names the glass build-up and the whole-unit figure. Our page on Part L and rooflights explains where that limit comes from.

Start with what the room is for. A garden office used all winter justifies the best glass build-up. A porch roof may be fine in sheet. Think too about when the room is used: a sunroom enjoyed on summer evenings asks different things of a roof from an office occupied on dark winter mornings. Then look at what carries the roof: a thin aluminium frame, an old timber rafter or a new steel. We note the make-up at survey and put the glass, frame and fixings on a fixed written quote after a free survey.

For the numbers behind each choice, read energy-efficient rooflights, and see whether skylights lose heat. Or ask for a survey.