Energy efficient rooflights in SouthamptonWarm glass, warm kerb.

When people ask for an energy efficient rooflight they usually mean the glass, and the glass is the biggest part of the answer. But a rooflight is a small assembly, and the heat leaves by several routes at once. It goes through the panes, along the edge where the glass meets the frame, through the frame itself, and around the outside where the unit meets the roof. A good result needs all four to be considered together rather than one chosen and the rest left to chance.

The panes are the part most people picture. A sealed unit is two or three sheets of glass separated by a gap that is filled with gas and closed round the edge. One of the sheets carries a thin low-emissivity coating, which is a clear layer that reflects heat back towards the room instead of letting it pass out through the glass. You cannot see it on a normal day, but it is the reason a modern unit feels different from an old single pane.

The edge of the unit is where a good pane can be let down. The spacer is the strip that holds the sheets apart, and a metal one carries cold across the glass edge into the room. A warm-edge version uses a material that conducts far less, which keeps the glass at the perimeter warmer and helps stop moisture forming there. We explain the part in what a warm edge spacer is.

Frame and kerb

The frame is the second route. Aluminium is strong and slim, which gives more glass for the opening, but bare metal conducts heat, so a good frame has a thermal break, a strip of insulating material that separates the inner and outer faces. Roof windows in timber or coated wood bring their own insulation with them, which is one reason they suit habitable loft rooms.

The kerb is the part nobody photographs. On a flat roof the unit stands on an upstand of timber or a preformed kerb, and if that upstand is thin or uninsulated, the cold from the roof edge reaches the frame and the plaster reveal below. That is why we talk about the rooflight and the kerb as one thing.

A third sheet of glass gives a second sealed cavity, and each cavity slows the passage of heat. Triple glazing therefore loses less than double of the same type, all else being equal, and the difference is greatest on the large flat panes that sit horizontally and face the cold sky on a winter night. A rooflight is a more demanding position for glass than a wall window, because warm air rises to meet it.

The extra pane is not free of consequences. A triple unit is heavier, which matters for the frame and for how the unit is lifted on to the roof. It is also thicker, so the frame must be deep enough to hold it, and the kerb or upstand needs to be sized for that depth from the outset. If the choice is made late, a builder may have formed a kerb for a double unit that cannot take the thicker one.

Choosing for the room below

The best way to decide is to think about what happens under the glass. A kitchen-diner with underfloor heating and a large pane above the table is a good candidate for triple, because the occupants sit close to the glass for hours and a cold surface is felt as a draught. A small pivoting roof window in a spare bedroom may be well served by a good double unit with a warm-edge spacer and a coating.

There is also the question of sound and solar gain. A third pane tends to quieten rain noise a little, which some people notice in a lightweight extension roof. It also changes how much of the sun's heat passes through, and on a south-facing roof that is something to weigh against a solar control coating, which we cover in a later chapter.

If you are weighing the cost against the benefit, our answer to is a triple glazed rooflight worth it looks at where the third pane earns its place. For a broader view of whether glass roofs lose too much heat in the first place, see do skylights lose a lot of heat. Both are written so you can read them before you speak to us, and then ask for the options priced side by side.

A rooflight that keeps heat in during winter can let too much in during summer, particularly on a roof that faces south or west and receives the sun for most of the day. The room beneath may warm quickly, furniture can fade and the space becomes uncomfortable to use. Solar control glass addresses the second half of the problem by limiting how much solar energy passes through the pane.

It works through a coating on one of the glass surfaces. The coating is selective: it admits visible light, so the room stays bright, but reflects a share of the infra-red part of the sun's radiation that produces heat. The result is a pane that looks a little more neutral or slightly tinted, depending on the coating, and a room that stays cooler on a sunny afternoon.

A good specification is one that nobody has to ask about later. For an energy efficient rooflight that means the glazing build-up, the frame type, the declared U-value, the kerb size and the upstand insulation are all set down in one place, ahead of the order and ahead of the roof. The place is usually the architect's drawing, with the quote as its companion.