Is triple glazing worth it in a rooflight?
Sometimes. Triple glazing takes a rooflight from around 1.4 W/m²K to around 1.0, which removes roughly a third of the heat lost through the opening. On a large lantern over a room that is occupied every winter evening, that is money well spent. On a single 550mm roof window in a landing, it is a premium paid for a number nobody will ever feel.
The decision turns on area, on how the room is used, and on whether the frame and the structure can carry what the third pane brings with it. The wider thermal picture is on energy efficient rooflights.
What the third pane actually buys
A triple unit has two cavities instead of one and usually two low emissivity coatings instead of one. Both cavities can be argon filled. The result on a well made rooflight is a whole unit figure somewhere between 0.8 and 1.1, against 1.3 to 1.5 for the equivalent double.
Expressed as heat, a square metre at 1.4 loses about 24 watts at a seventeen degree temperature difference. The same square metre at 1.0 loses about 17. Over four square metres of lantern that is roughly 28 watts, continuously, through the heating season. Over 0.6 square metres of small roof window it is about four watts, which is a phone charger.
Two narrow cavities behave differently from one wide one
The gas gap in a sealed unit has an optimum width, and for argon it is around 16mm in a vertical plane. Go wider and convection currents set up inside the cavity that carry heat across faster than conduction would.
Lying near horizontal, that optimum narrows. Warm gas rises against the inner pane, cools at the outer, and falls, which is a more efficient loop than the same gas gets standing upright. This is why rooflight units are often built with cavities around 12 to 14mm rather than the 16 used in a wall window, and it is one of the reasons triple works well overhead: two smaller gaps suppress the convection loop that one large gap encourages.
Weight, and the structure carrying it
A third pane is a third sheet of glass. A double glazed unit in 4mm glass with a toughened outer runs around 20 to 22 kg per square metre. A triple in similar make-ups runs 30 to 34, and if the outer pane is laminated for rain noise or security it climbs further.
On a two by one metre flat rooflight that is a difference of around 25 kg. The kerb and the deck beneath it need to be designed for the loaded weight, not the catalogue weight, and on an existing roof where a like for like swap was assumed, a change from double to triple can turn a simple exchange into a structural check. Worth raising at survey rather than at delivery.
The frame becomes the limiting part
A triple unit at 0.6 W/m²K in the centre of the pane, sitting in a frame that performs at 2.0, produces a whole unit figure disappointingly close to what a good double would have achieved for less money.
To make triple pay, the frame has to keep up: a laminated timber or polyurethane roof window frame, or an aluminium section with a deep polyamide thermal break, plus a warm edge spacer on both cavities. Fitting triple glass into an ordinary aluminium frame with an aluminium spacer is the most common way the upgrade fails to show up in the finished number.
Daylight lost to a third pane
Every pane and every coating takes light out. Clear double glazing transmits around 78 to 80 per cent of visible light. Triple with two low emissivity coatings typically transmits 68 to 72 per cent, and a triple that also carries a solar control coating can fall to the low fifties.
Ten per cent of daylight is not noticeable on a bright day and is noticeable in a north facing room in December. If the rooflight exists mainly to fix a dark room, that loss is working directly against the reason it was fitted, and a good double glazed unit with the daylight kept high is often the better call.
Surface temperature, and how a room reads it
This is the argument that gets underplayed. On a cold night the inner pane of a 1.4 rooflight might sit at 14 degrees while the room air is at 20. A 1.0 unit holds the inner pane nearer 16.
Two degrees on a surface directly overhead changes how the room feels, because a person under it is radiating heat upwards to that surface. It also raises the pane above the dew point on more nights, so the unit runs dry when a colder one would be wet. In a bedroom under a west facing slope, or a kitchen with a large lantern over a seating area, that comfort effect is usually a better reason to go triple than the fuel saving is.
Opening units, hinges and actuators
An opening triple rooflight has to move its own extra weight. Manufacturers rate hinges, friction stays and electric actuators for a maximum sash weight, and the same model that opens a double glazed sash comfortably may be at the edge of its range with a triple.
Two practical consequences. A manual pole operated unit becomes noticeably heavier to work, which matters on anything reached from below. And an electric unit may need a higher rated actuator or a second one, which changes both the price and the cable route. Fixed triple units carry none of this.
Rooms where it pays, and rooms where it does not
Triple earns its place where the glazed area is large, the room is occupied in winter evenings, and the rest of the envelope is already good. A four square metre lantern over a kitchen and dining space in a recently built or well upgraded house is the clear case. So is a bank of roof windows in a loft bedroom on an exposed north or east facing slope, where surface temperature drives comfort.
It pays less on small openings, where the frame proportion is high and the absolute saving is tiny. It pays less on landings, hallways, utility rooms and bathrooms that are heated intermittently. And it pays less in a solid wall Victorian terrace where the walls are losing far more than the roof, since the same money spent on the fabric elsewhere does more.
Comparing two quotations honestly
Ask for four things in writing on each option: the whole unit U-value in the plane the rooflight will sit in, the light transmittance, the total unit weight, and the spacer and frame construction. Then compare the pairs rather than the headline.
A triple at 1.0 with 70 per cent light transmission against a double at 1.3 with 79 per cent is a real choice with a real trade in it. A triple quoted at 1.2 against a double quoted at 1.3 is not a choice at all, and it usually means the frame is holding the unit back. The U-value target for a rooflight gives the figures worth holding a supplier to.
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