The short answer
Yes. Most conservation rooflights are sold double glazed, with a sealed unit that sits inside a slim frame designed to look like the single-glazed cast-iron originals. Double glazing is also what you need to meet current Building Regulations on a new or replacement unit, so a single pane is rarely an option on a house you live in.
How a slim frame holds a sealed unit
A modern double-glazed unit is thicker than one pane. Two sheets of glass, a cavity between them and a spacer around the edge add up to a build-up of around two dozen millimetres or more, depending on the make-up. Conservation rooflight makers deal with that by making the frame so that the outside face looks thin while the depth sits below the line of sight. The sealed unit is set into a rebate, and the visible edge stays narrow.
From the roof, you see a fine frame and a single sheet of glass. From inside, you see the depth of the unit in the reveal, which is a reason to think about the lining. A neat reveal hides it and gives the opening a finished look.
What is inside the unit
A typical conservation rooflight sealed unit has these parts:
- an outer pane of toughened glass, which may carry a self-cleaning coating;
- a low-emissivity coating on one inner face, which reflects heat back into the room;
- a cavity, often filled with argon, which slows heat transfer;
- a warm-edge spacer around the perimeter, which cuts the cold bridge at the edge;
- a laminated inner pane, which holds together if it cracks.
The laminated inner pane is good practice and industry guidance for glazing overhead, not a requirement of the Building Regulations. It is worth asking for because, if the outer pane is ever damaged, a laminated inner layer stays in the frame.
Triple glazing
Some makers offer triple glazing in their conservation ranges, which can reduce heat loss further. The trade-off is depth and weight. A triple unit is thicker, so the frame has to be deeper or the visible edge wider, and that can undermine the slim look that the product exists to deliver. On a listed building, the planning officer may be more concerned about appearance than about a lower U-value. Our guide to double or triple glazed rooflights weighs the options, and it is worth reading before you decide.
Why not stay with single glazing
Single glazing has an appeal on an old building because it matches the original, but in a lived-in room it has real drawbacks. A single pane loses heat quickly, so the room below is colder in winter and warmer in summer. Warm air from the room meets the cold glass and condenses on it, and over time that moisture can mark the lining and the ceiling around the opening. Those are the very surfaces we are trying to keep looking good.
A single pane also has to meet the same Building Regulations as any new rooflight in a house you live in, and a plain pane will not reach the thermal limits. There are places where single glazing is acceptable, such as an unheated outbuilding or a store, but for a bedroom, a kitchen or a landing, double glazing is the sensible baseline.
What you will notice from the room
A sealed unit changes how the opening looks from below. The extra depth shows as a slightly thicker edge around the glass, and a slim bar in the glazing may sit on the outer pane while the inner pane runs behind it. Some makers add a slim bar between the two panes so that it appears on both faces. Ask to see a sample section or drawing so you know what will show at ceiling level.
On a bright day, a low-e coating can give the glass a faint tint when you look at it at an angle. It is subtle and varies by manufacturer, and it is not a fault. If the colour of the daylight matters to you, ask for a sample of the glass rather than relying on a catalogue image.
Listed buildings and secondary glazing
Where a building is listed, the conservation officer may have views on the glazing as well as the frame. Some prefer a single pane with a sealed secondary pane inside it, to keep the exterior as close to the original as possible. Others accept a standard double-glazed unit, provided the frame profile is slim. There is no single rule, and the answer depends on the building and the officer. Our guide to conservation rooflights for listed buildings covers how that conversation usually goes, and our answer on planning permission explains when consent is needed at all.
Building Regulations and Part L
For a rooflight added to an existing roof or a complete unit replaced, the 2021 edition of Approved Document L (Volume 1) sets the limits. Double glazing with a low-e coating is the usual way to do that. Historic buildings can in some cases be treated with more flexibility, as the regulations allow for special considerations where energy-saving measures would harm the character of a protected building, but that is a decision for Building Control and not a reason to specify a poorer unit.
Changing only the sealed unit and leaving the frame in place is not notifiable, which helps when an older rooflight has a failed seal but a sound frame. Every other case needs a Building Control notification, and we make it.
Choosing the glazing for your roof
Think about three things. First, how visible the rooflight is: on a street-facing slope, the frame profile matters most, and a slim double-glazed unit usually satisfies that. Second, what the room is used for: a bedroom or a landing under a north-facing slope will benefit from a better thermal performance more than a utility space. Third, how the opening will look from inside, since the depth of a sealed unit is part of that picture.
We note the glazing type, the lining and the finish in your finish schedule, so you can see what is going into the roof and how the ceiling will look. For the product range itself, see our conservation rooflights page, and when you want a price, ask for a quote.