What is the most energy efficient rooflight?

The short answer

The lowest whole-unit U-value available on a domestic roof today comes from a fixed, triple glazed, made-to-size flat rooflight with two low-emissivity coatings, a krypton or argon fill and a warm-edge spacer, set in an insulated composite frame on a properly wrapped kerb. That specification lands around 0.6 to 0.8 W/m2K. A fixed triple glazed roof window is close behind at 0.8 to 1.0. Those are the numbers, and they are the easy part of the answer. The harder part is that on a real roof the perimeter detailing, the size of the unit and which way the slope faces move the delivered result further than the choice between two good units does, and a superb unit in a badly detailed opening performs worse than a merely good one done properly. The whole thermal picture is on the energy efficient rooflights hub.

The build-ups, ranked

Unit	Typical whole-unit U-value	Notes
Fixed triple glazed flat rooflight, composite frame	0.6 to 0.8 W/m2K	The lowest figure generally available, and heavy
Fixed triple glazed roof window	0.8 to 1.0	Standard sizes, manufacturer flashing kits
Opening triple glazed roof window	0.9 to 1.1	The sash frame adds a little
Fixed double glazed flat rooflight	1.0 to 1.4	The common specification, and a good one
Double glazed roof window, current generation	1.1 to 1.4	Comfortably inside the limiting standard
Roof lantern, double glazed	1.2 to 1.8	Every glazing bar adds frame and edge
Conservation rooflight, slim steel frame	1.5 to 1.9	Frame material chosen for appearance
Double glazed roof window from the 1990s	2.8 to 3.0	What is on many Gloucestershire roofs now
Twin-wall polycarbonate dome	2.6 to 3.4	The lowest performing thing still in service

Vacuum glazing is the outlier worth knowing about. A vacuum unit achieves a low figure in a total thickness of about 8mm, which suits a slim heritage frame where a triple unit will not physically fit, though the cost and the limited size range keep it a specialist choice.

Fixed beats opening, and by how much

An opening unit has two frames instead of one: the outer frame fixed to the structure and the sash carrying the glass. That doubles the frame section in the U-value calculation, adds a gasket line and adds an air permeability path. The penalty is typically 0.1 to 0.2 W/m2K on an otherwise identical specification.

That is a real difference and it is rarely the deciding factor, because ventilation matters more to how a room actually behaves than a tenth of a watt does. The sensible arrangement in a run of several units is one opener for purge ventilation and the rest fixed, which gets most of the thermal benefit and keeps the room controllable. Where a room in the roof needs an escape opening, the unit serving that duty has to open, and that requirement outranks the U-value.

Size, and why a small unit is always worse per square metre

The glass is the good insulator and the frame and the edge of the glass are the poor ones. A whole-unit figure is the weighted average of those, so it depends on the ratio between the perimeter and the area, and that ratio gets worse as units get smaller.

Take the same glass and the same frame in two sizes. A unit 550mm by 780mm has 2.66 metres of perimeter for 0.43 square metres of glass. A unit 1500mm by 2500mm has 8 metres of perimeter for 3.75 square metres. The second has less than half the perimeter per square metre, so the frame and edge terms are diluted and the whole-unit figure comes out substantially better on identical components. This is why one large rooflight outperforms three small ones covering the same area, and why the narrow units that fit between trusses on the Quedgeley and Bishops Cleeve estates always quote worse figures than the datasheet headline. The arithmetic is worked through on U-values and glazing specification.

Frame material, and the four in common use

Timber with a polyurethane facing, used by the roof window manufacturers, performs well because timber conducts poorly and the facing keeps water off it. A pure polyurethane frame with a timber core is better again in a bathroom.

Aluminium is the standard for made-to-size flat rooflights and lanterns because it takes slim sightlines and large spans, and it conducts heat readily, so a good unit uses a thermally broken profile with a polyamide bar isolating the inside from the outside. A composite frame combining an aluminium outer with an insulated inner or a timber inner is the best of the group. Slim cast or pressed steel is used on conservation rooflights for appearance rather than performance, and its figure reflects that. Frame comparisons are on timber and polyurethane roof window frames.

Inside the glass: coatings, gas and the edge

Three components do the work between the panes and they act independently, so all three belong on an order.

A low-emissivity coating on the cavity face of the inner pane reflects long-wave heat back into the room and takes roughly a third off the loss across the cavity on its own. Argon replacing air in the cavity adds a further useful step for very little money, and krypton adds a little more in a narrow cavity, which is why it turns up in triple units where the two cavities have to fit into a fixed rebate depth. A warm-edge spacer raises the temperature at the perimeter of the glass and improves the whole-unit figure by around 0.1. Coatings and gases are on low emissivity coatings and argon fills, and whether the third pane pays on triple glazing in a rooflight and when it pays.

The perimeter, which is usually the bigger term

This is where the honest answer sits. A junction is described by a psi value in watts per metre kelvin, and around a poorly detailed kerb it can reach 0.25 or more. On a rooflight of one metre by one and a half, the five metre perimeter at that figure loses more heat than a third of the loss through the glass, and it can exceed the glass loss entirely on a small unit.

Buying a 0.7 unit and setting it on a bare timber kerb, with the deck insulation stopping at the timber and a void behind the plasterboard reveal, throws away most of what the glass was bought for. The detail that avoids it is insulation wrapped around the outside of the kerb through its full height, the deck insulation lapped onto that wrap, and the control layer taped to the frame. That is set out on thermal bridging at the rooflight perimeter.

The winter number against the summer behaviour

Efficiency is not only heat retained. A rooflight is also an aperture for solar energy, and on a shallow or horizontal plane it takes the June sun close to square-on. Specify purely for the lowest U-value on a west-facing loft slope in Gloucester and you get a room that holds its heat beautifully in January and is unusable in July.

So the pair of numbers is the specification, not one of them. On north-facing slopes, take the highest light transmittance and the lowest U-value available. Between east and west, bring the solar factor down to around 0.25 to 0.35 with a selective coating, which costs about a fifth of the daylight and no meaningful winter performance. That balance is on solar gain, overheating and shading and by aspect on specifying glazing by which way the slope faces.

Where the extra money stops buying anything

The steps are not equally valuable. Moving from a 1990s unit at 2.9 to a current double glazed unit at 1.2 removes the majority of the loss and costs the price of a normal replacement. Moving from 1.2 to 0.8 with a triple unit removes a further useful slice and costs more in glass, weight, scaffold handling and sometimes a heavier frame. Moving from 0.8 to 0.7 is a change most households will never detect on a bill.

Beyond a certain point the same money does more good elsewhere: getting the perimeter right, adding depth to the rafter insulation around the opening, or upgrading the second and third units on the roof rather than perfecting the first. Where a household is choosing, that is the order Severn suggests, and the reasoning is recorded in the survey notes rather than left as an opinion.

The best unit for a Gloucestershire roof is not always the lowest number

Weight decides more than people expect. A triple unit is roughly half again heavier than a double, and on a Stroud valley cottage where scaffold is hand-carried in along a footpath and the glass has to be lifted by hand, that difference changes the job rather than the price alone. On a Painswick cottage with diminishing courses of Cotswold stone slate, the conservation rooflight at 1.7 is the correct answer because it is the only unit the roof and the conservation officer will accept, and the effort then goes into the perimeter and the reveal.

On the trussed estates around Quedgeley and Ashchurch, the constraint is width between trusses, so the honest specification is two narrow units in adjacent bays with the best glass available rather than one wide one needing a designed trimmer. And on a west-facing rear slope at Hucclecote or Warden Hill, the solar factor decides whether the room works, and the U-value is the second question. The best rooflight is the best one for the roof it is going into, which is why the survey happens before the order.