In brief
Yes. Current conservation rooflights are made with sealed double glazed units fitted into slim frames, and some makers offer triple glazing as well. The slim look comes from the frame and the profile, not from a single sheet of glass, so the energy performance is designed to meet the limits in Part L of the Building Regulations. The exact glazing is named in the written quote for your unit.
- The glass is a sealed double glazed unit, not a single pane
- Triple glazing is offered by some makers for conservation styles
- The slim frame and low profile are what make the unit look traditional
- Part L sets a 2.2 W/m²K limit for rooflights in existing homes
- The quote names the glazing for the unit that has been chosen
How a slim frame holds double glazing
A conservation rooflight looks light because the metal frame is narrow and the glass sits close to the roof slope. Inside the frame there is a sealed unit with two panes of glass and a gap between, spaced and sealed at the edge. The frame is made slim by careful engineering, not by leaving out a layer of glass.
The gap is usually filled with an insulating gas, and a coating on one pane reduces heat loss. These are the same ideas used in modern windows, scaled to fit a thin frame.
What you see from the room is a narrow border of metal around the glass. From the street, the unit sits low and does not stand proud of the slates or tiles.
Double or triple glazing
Double glazing is the usual specification for conservation rooflights. Some makers also make triple glazed versions, which add a third pane and a second gap. A triple unit is heavier and needs a deeper frame, so its look and price are different, and not every size is offered with it.
Which suits you depends on the room and the roof. In a heated loft conversion it may be worth considering. In an outbuilding or a store, double glazing is normally enough. We talk through the options at survey.
Glass finishes are another choice. Coatings can reduce heat gain in summer as well as loss in winter, and self-cleaning coatings can help on a roof that is hard to reach. The cost page explains how these add to the price: see what goes into the cost of a conservation rooflight.
Whichever you choose, ask for the glass specification in writing. It should name the number of panes, the coating and the gas fill, so that you can compare one quote with another on the same terms.
What Part L asks for
In an existing home in England, the energy rules in Part L set limits for new and replacement glazed units. Rooflights are held to 2.2 W/m²K, and roof windows to the window standard, which is 1.4 W/m²K or WER Band B. The lower the figure, the less heat escapes.
Which limit applies depends on how the product is classed. A conservation rooflight is normally a rooflight, with the limit that goes with it, but the product data from the maker confirms the class and the figure for each size.
A whole new rooflight, meaning frame and glass, is a controlled fitting, so Building Control is notified and we arrange that. A change of the sealed unit alone does not need that. The Building Regulations guide sets out the rest.
Replacing an old single glazed rooflight
Older cast-iron and steel rooflights were often single glazed, and many roofs still have them. Fitting a modern conservation rooflight in the same opening brings the glazing up to current standards while keeping the look of the original.
The opening is measured, not the old glass. The old frame is removed, the kerb and upstand are checked and the new unit is flashed into the covering. If the old rooflight sits in an iron frame set into the roof, the frame may need to come out with care.
On a listed building, replacing an old rooflight may need consent even if the new one is similar. See listed building consent for a rooflight for how to check.
The bar, the frame and the look
The look of a conservation rooflight depends on the frame colour, the profile and whether it has a central bar. These choices do not change whether the glass is double glazed, but they do change what a council may accept. See the glazing bar page for when a bar is asked for.
In a National Park, the authority is the park authority, and in a conservation area it is the council. Our guides to conservation area rooflights and National Park rooflights explain what each looks at.
What goes in the quote
The written quote states which rooflight is being supplied, whether the glass is double or triple, the frame and finish, and what the builder must have ready. The maker gives the delivery period, and we confirm it when ordering.
The builder or architect is given the opening dimensions ahead of the roof being formed, so the structure is cut once and fits. The conservation rooflights hub, our conservation rooflight range, sets out the other choices.
A free survey is where this starts.
Ask us about the finish on the frame as well. Most conservation rooflights are supplied in black, and other colours are available from some makers, which matters if the roof has a particular look.
Related questions
- Will a conservation rooflight make the room warmer?
- A well made sealed unit loses less heat than an old single glazed one, so replacing the old rooflight helps. How warm the room feels still depends on the insulation around the opening and on the rest of the roof, which is why we look at the whole build-up at survey.
- Can the sealed unit be replaced on its own later?
- Yes. If the glass unit in a good frame has misted, only the sealed unit can be replaced, and that does not need a Building Control notification. Measure the existing unit and send the details through for a quote.
- Is condensation on the inside a sign that something is wrong?
- Not usually. Warm, moist air in a room meets the coolest surface, and that can be the glass in winter. An opening rooflight and good ventilation in the room help to move that air out.
