Do skylights lose a lot of heat?

Heat leaves through the glass itself, which depends on the number of panes, the coating on them and the gas between. It also leaves through the frame, where a metal section without a thermal break conducts warmth to the outside face. The joint between the unit and the roof is the third route, and an old or poorly detailed installation can lose a good deal there.

The figure that sums it up is the whole-unit U-value counted as watts lost across each square metre for every degree between inside and out. A lower number means less loss. A catalogue may quote the middle of the glass only, which flatters the product, so we compare the whole unit at the exact size.

A skylight is also a small share of the roof. A single roof window in a loft bedroom covers a fraction of the slope. The window matters most in the room beneath it, where its cold inside face can feel noticeable at head height.

Age plays a large part. A sealed unit from a couple of decades ago may hold air between plain panes, with no coating, and the seal may have started to fail. Mist between the panes shows that has happened, and a misted unit insulates worse than it did new. Our guide on misted rooflights covers the replacement route.

Triple glazing lowers the figure further but adds weight that the frame and kerb must carry, so it suits some rooms better than others. A study or garden room used through the winter benefits, while a landing window above a stairwell often meets the limit with a good double unit.

Size plays a part as well. Frame width takes proportionally more of a compact roof window than of a big one, so two sizes from the same range can return different figures. We read the data sheet for the size we are fitting, not for the product in general.

A flat roof adds the kerb. If the upstand under a lantern is thinly insulated, a cold ring forms round the glass whatever its quality, which is why a replacement sometimes disappoints when only the glazing is changed. We check the kerb at the survey, and the guide to rooflight U-values explained shows how the numbers are built.

Loss in winter is half the picture. A skylight on a sunny slope lets in heat as well, and the number that describes it is the solar factor, or g-value. Solar control coatings lower it, and an external blind stops the sun at the glass. The balance between the two decides whether a loft room stays pleasant, so we weigh both with you. Whether a rooflight makes a room too hot has its own answer.

Condensation tells you about performance too. Water forms on the coldest surface in a humid room, so a bathroom skylight with tired glass runs wet on a frosty morning. Better glazing warms the inside face and the problem usually eases.

Daylight has a value of its own. A bright room needs fewer lights switched on during the day, and a hall or landing that looked gloomy can feel open with a single well-placed unit.

Ask for the whole-unit U-value of the exact product and size on any quote, and for the glass build-up to be named beside it. Check that the lining, flashing or kerb is part of the job. For a replacement, we measure the old unit against what the regulations now require, and tell you whether the surround can stay.

A new opening in a roof is building work, and we handle the Building Control notification where the work is notifiable. Planning is a separate question: on a house a roof window will normally be permitted development, while a lantern normally needs an application, and the skylight planning guide sets out the conditions. For the wider picture of glass, frames and limits, the energy-efficient rooflights section goes through them, or send us the roof details through the survey form and we will come and measure.