Are Conservation Rooflights Double Glazed?

A double glazed conservation rooflight has two panes of glass with a sealed gap between them, held apart by a spacer. Many people assume that a frame as narrow as a Victorian cast-iron rooflight could only hold a single sheet. Modern makers have solved that by shaping the frame so the sealed unit sits in a shallow rebate, with the bulk of the section hidden under the slates.

That is why conservation skylight glazing looks so different from the thick white frame of a standard roof window. The glass is still a thermal unit, and the inner pane is normally laminated so that it holds together if it breaks overhead. The outer pane is toughened. A coating on one of the inner faces reflects heat back into the room, and the gap between the panes is often filled with a gas that conducts less heat than air.

The spacer is the part you see. A dark, warm-edge spacer is less obtrusive than a bright aluminium one, and when a central glazing bar is used, the bar is aligned with the spacer so the two read as one line. Our guide to conservation rooflight design covers how the bar and profile are chosen.

For an existing English home, the 2021 edition of Approved Document L, volume 1, sets the limits in its Table 4.2, and they apply to a new unit and to one replacing an old unit alike. A rooflight must reach 2.2 W/m²K. A roof window must reach the tighter 1.4 W/m²K, or Window Energy Rating band B, and a roof lantern is classed as a rooflight. The 2026 edition takes over on 24 March 2027, keeping the existing-dwelling figures as they are and renumbering the table as 3.2.

Which limit applies turns on how the unit is classed. Approved Document L defines a rooflight as a glazed unit set out of the plane of the roof on a kerb or upstand, and a roof window as one fitted in the plane of the surrounding roof. A conservation rooflight is usually sold as a rooflight, but it lies close to the plane of a pitched roof, so the tighter roof window figure can apply to it. Where a product sits between the two, the maker's declared U-value and the building control body settle which limit applies. We check that at survey, since the answer changes the glass specification.

The U-value that counts is for the whole unit, frame and glass together. A glass-only figure is always better than the figure for the finished window, so ask for the whole-unit number, not the centre-pane one. For a wider look at how those figures work, our page on rooflight U-values and Part L goes through them.

Welsh homes in Flintshire or Wrexham fall under the Welsh version of the Building Regulations 2010 and Approved Document L (Wales), 2022 edition. Building control is handled by the county council or by a registered building control approver. We do not quote English figures for a Welsh address; the requirement for your roof comes from the Welsh document and the unit is specified to match it. The 2026 Welsh edition takes effect from 4 March 2027.

There is a real trade-off here, and it is better to see it before you order. Thinner frames carry less insulation than chunky ones, and a very narrow glazing bar lets the edge of the glass sit closer to cold metal. The frame and the spacer lose heat faster than the middle of the pane, so a slim frame leaves less margin.

Makers answer this in three ways. They put a thermal break into the frame, so the inside metal is separated from the outside metal. They use warm-edge spacers. Third, the glass itself gets better: a low-emissivity coating, a gas fill or, occasionally, a third pane. A triple-glazed unit needs a deeper frame, which works against the flush look, so it is usually kept for the rare site where the thermal result matters more than the profile. Our answer on double or triple glazed rooflights goes into that choice.

A planning officer will rarely ask for a thicker frame, but may press for slimmer sightlines, a single central bar and a dark finish. If the frame is slim enough to satisfy the council, the glass has to carry the thermal work. We raise this at the survey so the unit is picked for both tests at once.

• Get the U-value for the finished unit rather than the centre of the glass
• Check that the inner pane is laminated
• Choose a warm-edge spacer in a dark colour
• Line the glazing bar up with the spacer
• Keep the council's approved product details to hand

Double glazing helps a conservation rooflight with more than the energy rules. A sealed unit keeps the inner pane warmer, so condensation forms less readily on the glass in a loft room. It also cuts the noise of rain on the slope, which is a feature of a western roof in a gale off the Irish Sea.

Listed buildings are treated with some flexibility under the regulations where the standard figures would harm the building's character, and that is decided with the conservation officer. That route is explained in listed building consent for a rooflight. The conservation rooflight installation page covers the work as a whole, and Get a free quote if you would like the glass specified for your own roof. We price only after a survey, put the fixed quote in writing, and back the fitting with a written guarantee on our workmanship that runs for ten years.