Guide · Energy Efficient Rooflights
Triple glazing in a rooflight and when it pays
Triple glazing in a roof is a genuine upgrade with genuine costs attached, and the honest answer to whether it is worth it depends almost entirely on what you are buying it for. As a way of saving money on heating a rear extension in Chelmsford, the arithmetic is poor. As a way of making a loft bedroom comfortable to sit under in February, or of keeping a shower room dry, it is one of the better things you can spend money on.
The confusion comes from a marketing habit of quoting the thermal improvement as a percentage. Going from 1.2 to 0.8 W/m²K is a third better, which sounds decisive, and in absolute terms it is a few dozen kilowatt hours a year on a domestic rooflight. This page works through what the third pane actually changes, what it costs elsewhere in the specification, and the situations where it is clearly the right call.
What a triple glazed rooflight is built from
A triple unit is three panes and two cavities, and in a rooflight the build-up is constrained by the frame rebate rather than by what is thermally ideal.
A typical roof window triple runs something like a 4mm toughened outer, a 12mm argon cavity, a 4mm centre pane, a second 12mm argon cavity, and a 6.8mm laminated inner. That is around 39 millimetres overall against roughly 28 for a good double. Low emissivity coatings go on two of the four cavity facing surfaces, and the centre pane, which is coated on both faces in some products, is the one doing the extra work. In a flat rooflight or a lantern, where the rebate is deeper, the cavities can go to 14 or 16 millimetres and the unit performs correspondingly better.
The thermal step, in absolute numbers
Centre pane, a good argon filled double with one low-E coating reaches a Ug around 1.0 to 1.1 in the roof plane. The equivalent triple reaches around 0.6 to 0.7. As whole products, allowing for the frame and the edge, a double glazed roof window lands at a Uw of roughly 1.2 to 1.4 and its triple glazed equivalent at roughly 0.8 to 1.0.
So the improvement worth planning around is about 0.4 W/m²K on the whole product. On three square metres of glass with a twenty degree temperature difference across it, that is 24 watts. Twenty four watts is a bright bulb. It is real, it is continuous through the heating season, and it is nothing like the change the percentage figure implies.
Why the returns fall away so quickly
Heat loss is proportional to the U-value, so each successive improvement removes a fraction of what remains rather than a fixed amount.
Going from an old twin wall polycarbonate dome at 3.0 down to a modern double at 1.2 removes 1.8 W/m²K. Going from that double to a triple at 0.8 removes another 0.4. The first step is four and a half times the size of the second, for a smaller price difference. This is the shape of the whole subject: on a roof that still has a dome or an early double glazed unit on it, replacement is transformative and the glazing grade barely matters. On a roof that already has a current double glazed unit, the third pane is a refinement.
The largest thermal gain available on any rooflight is the first replacement, not the last upgrade.
Weight, and what it asks of everything holding it
Glass weighs about 2.5 kilograms per square metre per millimetre of thickness. A 6mm outer with a 6.8mm laminated inner comes to roughly 32 kilograms per square metre. A triple built 6, 6 and 6.8 comes to about 47.
That is close to fifty per cent more, and it lands on the sash, the hinges, the friction stays, the gas struts and the fixings into the timber. A manufacturer’s triple glazed unit is designed around this, with heavier ironmongery and often a slightly reduced maximum size in the range. What does not work is putting a triple unit into a frame engineered for a double: the sash droops, the compression seal loses contact along the top edge, and the opening gear wears out early. On a lifting or pivoting rooflight the weight is also felt by whoever operates it, which is one reason an electric opener is more common on triple glazed units.
Sightlines, and the frame the extra weight demands
Heavier glass needs a deeper rebate to hold it and a stronger section to carry it, so a triple glazed rooflight almost always has more visible frame than the double glazed version of the same external size.
On a roof window the difference is modest, perhaps ten to fifteen millimetres of extra sash depth. On a lantern it is more noticeable, because the glazing bars have to carry the load to the ridge and the eaves, and a bar that was 50 millimetres wide for double glazing becomes 65 or 70 for triple. Across eight or ten bars on a large lantern, that is a measurable reduction in glass area and a visible change in how heavy the structure reads from below. Anyone specifying triple in a roof lantern should ask for the bar dimensions of both options before deciding.
Light through a third pane
Every pane reflects roughly four per cent of the light at each of its two surfaces, and every coating absorbs a little more. A third pane and its two extra coated surfaces cost something in the order of eight to ten per cent of the visible light transmission.
A clear double glazed low-E unit transmits around 70 to 75 per cent. The triple equivalent transmits around 60 to 65. In a bright room that difference is imperceptible, because the eye adapts logarithmically. In a north facing room lit only from above on a grey December afternoon, it is not nothing. The rooflight is there to bring daylight in, so giving up ten per cent of it to save 24 watts deserves at least a moment’s thought.
Two cavities, and the gas in both of them
A triple unit has twice as much sealed edge, twice as much desiccant and two gas fills to retain, which means the manufacturing quality matters more than it does in a double.
Argon leaks slowly through any seal, and the standard assumption is a loss rate of around one per cent of the fill a year against an initial fill of 90 per cent. A well made unit is still performing close to specification after twenty years. A poorly sealed one loses its fill faster, and in a triple that degradation happens in two places rather than one. The practical response is to buy the unit as part of a complete manufactured rooflight from a maker who warrants the sealed unit, rather than as glass bought separately and glazed into a frame on site.
Inner pane temperature, and the comfort case
This is the argument that actually justifies triple glazing overhead, and it has nothing to do with the fuel bill.
With the room at 20 degrees and the outside at zero, the inner pane of a good double glazed rooflight sits at around 15 to 16 degrees. The triple equivalent sits at around 17 to 18. Comfort depends on the mean radiant temperature of the surfaces around you as much as on the air temperature, and a large cold surface directly overhead radiates heat away from the top of your head and shoulders. Two degrees on that surface is felt. In a room where people sit still, under glass, in winter, it is the difference between a rooflight that is pleasant in January and one that is admired in May and avoided in January.
Condensation headroom in a humid room
The same two or three degrees buys margin against surface condensation, and here it can be decisive rather than merely pleasant.
Room air at 20 degrees and 65 per cent relative humidity has a dew point around 13.2 degrees. A double glazed inner pane at 15 degrees clears that by under two degrees, and the edge of the pane, which always runs colder than the middle, may not clear it at all. A triple glazed inner pane at 17.5 degrees has more than four degrees of margin. Over a shower room, a utility room or a kitchen where laundry gets dried indoors, that margin is worth more than the energy saving. The full picture of moisture and how to manage it is on the condensation and ventilation page.
How much energy the third pane actually saves
The calculation is straightforward and the answer is usually a surprise, so it is worth doing before ordering.
Heating degree days for this part of Essex run at roughly 2,000 kelvin days a year to a base of 15.5 degrees. Annual energy saved equals the U-value difference, times the area, times the degree days, times 24, divided by a thousand. For a three square metre rooflight and a 0.4 W/m²K improvement, that is 0.4 × 3 × 2000 × 24 ÷ 1000, which comes to about 58 kilowatt hours a year. On a large lantern of six square metres it is about 115. Against the price difference between a double and a triple glazed unit, those are long payback periods on energy alone. Buy triple glazing for the comfort and the condensation margin, and treat the energy as a bonus.
External condensation on the outer pane
A well insulated unit lets very little heat out, which means the outer pane stays close to the outside air temperature. On a clear, still night the sky radiates heat away and the outer surface can drop a degree or two below the surrounding air.
The result is dew forming on the outside of the glass, exactly as it forms on a car windscreen or a lawn. It appears on clear autumn and spring mornings, it clears within an hour of the sun reaching the roof, and it happens far more on triple glazed units than on double. It is not a fault. It is proof the unit is working, though it is a genuine nuisance on a rooflight over a kitchen where the view of the sky is part of the point, and it is worth knowing about before it happens rather than after.
Deflection, and the way a near horizontal unit sits
Sealed units flex. The gas in a cavity expands when it warms and contracts when it cools, and barometric pressure changes push the panes in and out as well.
In a triple there are two cavities acting on a centre pane that is supported only at its edges, so the middle of the sandwich has the least restraint and the most movement. On a near horizontal unit, gravity adds a permanent small sag to the same pane. The visible consequence is a faint distortion in reflections seen from the garden, and on very large panes a slight optical waviness looking up. It is cosmetic rather than structural, and it is one reason very large triple glazed flat units are less common than very large double glazed ones.
What it does to noise
A third pane helps less with sound than people assume, because three panes of similar thickness resonate together rather than damping each other. The reliable acoustic gains come from asymmetry and from a laminated pane with an acoustic interlayer, both of which are available in a double glazed unit.
Where a triple does help is at low frequencies, and it helps most when the panes differ in thickness and the cavities differ in width. The full treatment of rain drumming and traffic noise through roof glazing is on the page covering acoustic glazing and rain noise, which is where to look if noise rather than heat is the reason for the enquiry.
Fitting triple glazing into an existing frame
Replacing the sealed unit in an existing rooflight with a thicker triple unit is rarely practical, and the reasons are mechanical rather than commercial.
The rebate depth is fixed, the edge cover required to protect the seal from ultraviolet cannot be reduced, the gaskets are sized for a specific unit thickness, and the sash and hinges were selected for a lighter pane. Where the existing frame is sound and a thermal improvement is wanted, a like for like double glazed unit with current coatings is usually a substantial gain over a unit from twenty years ago. Where the whole rooflight is coming out anyway, that is the moment the triple glazed option is genuinely open, and it is one of the arguments for planning a replacement rather than a glass swap.
The rooms in Essex where it earns its place
Four situations make the case on their own. A loft bedroom where the rafter line is the ceiling and the roof windows are a large fraction of the room’s envelope, common in the Victorian terraces of Chelmsford and Colchester. A shower room or wet room with a rooflight over it, where the condensation margin matters more than the energy. A north facing plane where there is no solar gain to offset the winter loss. And any house being built or refurbished to a low energy standard, where the rooflight would otherwise be the weakest element in an otherwise excellent envelope.
Outside those, a good double glazed unit with argon, a proper warm edge spacer and the right coatings is the sensible product, and the money is better spent on the perimeter detailing and the ventilation. That order of priorities is set out across the energy efficient rooflights hub.
What to ask before ordering
Ask for the Uw of both options at the actual size being fitted, not the centre pane figures and not a range wide reference size. Ask for the light transmission of both, because that is where the cost is paid. Ask for the weight per square metre and confirm the sash and ironmongery are rated for it. Ask what the glazing bar dimensions become on a lantern.
Then ask the question that decides it: is the reason for the upgrade the heating bill, or is it the comfort of sitting under the glass and the risk of a damp room. If it is the first, the numbers above argue for double. If it is the second, triple is the right answer and the energy saving is a footnote.
Keep reading
More on energy efficient rooflights
Solar gain, overheating and shading
Read the guide →Condensation, ventilation and rooflights
Read the guide →Warm-edge spacers and where condensation starts
Read the guide →Low-emissivity coatings and argon fills
Read the guide →Thermal bridging at the rooflight perimeter
Read the guide →
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