Does a New Rooflight Reduce Heat Loss?

Heat leaves a room through any surface that is colder outside than in. How fast it leaves depends on three things: the area of the surface, the temperature difference across it, and the U-value of the unit. The U-value, in watts per square metre per degree (W/m²K), is the rate of loss. A lower figure is better.

Here is a worked example, and the numbers are illustrative. Take a roof window 78 cm wide and 98 cm high, an area of about 0.76 m². Say it is 20 degrees colder outside than in.

The new unit loses about half the heat of the older one in this example. Your own figures depend on the actual units, which is why the label on the old frame and the specification of the new one both matter. Our guide to rooflight U-values explains how to read them.

The larger the gap between the old unit and the new, the larger the saving. A single glazed rooflight or an early double unit with a failed seal gives up far more heat than a current sealed unit. Replace one of those and the change is clear on a cold night, because the glass is warmer to the touch and the cold radiating down from the roof is reduced.

Replace a recent, well-made unit with another, and the saving is small. For that reason the best candidates for replacement are the oldest units, and any with misted glass.

A misted unit has a failed seal and may have lost some of its gas fill, so it insulates less than it did when new. Our answer on misted rooflights explains how to tell.

The U-value on the label describes the unit alone. Real heat loss also depends on how the unit sits in the roof.

• Insulation around the opening. Between the frame and the rafters there is a gap. Filled well, it carries on the insulation of the roof. Left empty or packed carelessly, it becomes a path for cold air.
• The lining. The inner reveals, or sides of the light well, should be insulated and finished so the warm side stays warm.
• Air tightness. A frame sealed to the vapour control layer on the room side keeps warm, moist air from reaching cold timber.
• The flashing. A flashing kit fitted for the roof covering and pitch keeps the weatherproofing at the edge in order, and the insulation around it dry.

These are done as part of the fitting. A correctly fitted unit can perform as its label describes, while a good unit poorly fitted loses heat around its edges.

A new opening in an insulated roof replaces a well-insulated area with glass. Even with the best glazing, the glass loses heat faster than the roof around it. Adding a rooflight lets in daylight and ventilation, but overall heat loss from the room rises a little.

That is a normal exchange, and the aim is to keep it small. The measures are the same as for a replacement: the best glazing specification the budget allows, careful insulation around the frame, and an opening kept to the size the room needs. The number of windows a loft needs is covered in our loft window answer.

A new opening is also a structural alteration, since rafters are cut and trimmed, and it needs Building Regulations approval.

You can often read the age and make of an old unit before any work starts. Look at the edge of the glass, where a stamp or a printed spacer label may give the maker and the year. On a VELUX roof window, the data plate on top of the sash, at the right-hand side, gives the model code.

Photograph both and keep them. When we survey, we record the model, the pitch, the rafter centres and the size of the opening, and the written sheet lets you compare quotes for the same opening.

A U-value for the old unit may not be listed anywhere. In that case the sensible comparison is between the new specification and the type of glazing you can see: single pane, early double unit, or a current sealed unit with a coated pane.

Check also whether the old unit has a blind or a roller fitted, since the sizes of new accessories depend on the frame.

For an existing house in England, Part L sets the minimum. Roof windows are held to 1.4 W/m²K, or alternatively Band B on the WER rating scale. Plain rooflights are held to a looser 2.2 W/m²K. Plenty of current units do better, and the extra cost of each glazing step is covered in our double or triple glazing answer.

The wider picture for homeowners sits with the energy efficient rooflights hub, including condensation and solar gain, which are linked to heat loss but are separate questions.