Triple glazing in a rooflight and when it pays

What a triple unit is, millimetre by millimetre

A typical triple sealed unit for a roof reads 4-12-4-12-6.8, which is a 4mm toughened outer pane, a 12mm cavity, a 4mm clear middle pane, a second 12mm cavity, and a 6.8mm laminated inner pane. Total build-up 38.8mm. The equivalent double is 4-16-6.8, total 26.8mm.

Two spacer bars run round the perimeter instead of one, so there are two edge seals, two runs of desiccant and twice the perimeter to keep gas-tight. Both cavities are argon filled. Low-emissivity coatings sit on the cavity faces of the inner pane and, in most build-ups, on one face of the middle pane, so the unit carries two coatings rather than one. Six glass surfaces, two cavities, two spacers. Every one of those is a place where something can be specified well or badly.

Two cavities instead of one, and what that does to convection

Heat crosses a sealed cavity by conduction through the gas, by radiation between the two glass faces, and by convection as the gas circulates in a loop. The coatings deal with radiation. Cavity width and orientation decide the balance between the other two.

Convection only starts once the cavity is deep enough for a circulating loop to establish itself against the viscosity of the gas. Halve the depth and the tendency to convect falls away sharply, far faster than in proportion. Two 12mm cavities therefore transfer less heat than one 24mm cavity does, and rather less than one 16mm cavity does, even though the total gas thickness has gone up. The third pane also adds a second stagnant boundary layer of gas at each of its faces, and a low-emissivity coating on a face of the middle pane suppresses the radiant exchange in one cavity as well as the other.

Shallow slopes, where the second cavity does the most work

In a vertical window the warm pane and the cold pane stand side by side. Gas rises up the warm face and sinks down the cold one in a slow, stable loop that moves comparatively little heat.

Lie the unit down and the arrangement inverts. The warm surface is now underneath and the cold surface on top, which is the classic unstable configuration: warm gas rises in plumes, gives up its heat at the cold pane and falls back, and convective transfer rises steeply. Below about ten degrees of pitch the effect is at its worst, and that describes almost every made-to-size flat roof rooflight, every unit on a mansard’s shallow upper slope behind a Cheltenham parapet, and the near-flat rear areas on Gloucester’s Victorian back additions. A triple unit fights that directly by making each gas layer too thin to convect hard, which is why the gap between a good double and a good triple is wider on a rooflight than the vertical figures suggest. A double declared at 1.2 vertically might behave like 1.5 at eight degrees; a triple declared at 0.8 might behave like 0.95.

Weight, and the arithmetic on a scaffold

Glass weighs 2.5 kilograms per square metre for every millimetre of thickness. A double build-up of 4mm plus 6.8mm is 10.8mm of glass, so 27 kilograms per square metre. A triple at 4 plus 4 plus 6.8 is 14.8mm, so 37 kilograms per square metre.

On a unit 1340 by 1400mm, which is 1.88 square metres, that is 51 kilograms of glass against 70. Add the frame and a triple-glazed roof window of that size is comfortably a two-person lift on the flat and a difficult one up a ladder. In the Stroud valleys, where scaffold and materials are frequently hand-carried up a stepped path from a shared lane, and on the Forest of Dean terraces where the roof is steep and the ground drops away behind, that extra 19 kilograms is a genuine planning item rather than a footnote. It can mean an extra pair of hands, a longer day, or a mechanical lift, and it is worked out at survey rather than discovered on the morning.

What the extra mass does to a sash, a hinge and an operating rod

A fixed unit only has to be lifted once. An opening one has to be operated for the next twenty five years.

A centre-pivot sash is balanced about its pivot, so weight affects the friction in the hinges more than the effort to swing it, and a heavier sash needs its hinge friction set correctly or it will drift open. A top-hung sash is the harder case: the whole mass is being lifted against gravity and held on a stay or a gas strut, and a strut sized for a double unit will not hold a triple. On a steep Forest of Dean roof where the top of the frame already sits high, an operating rod on a heavy top-hung triple becomes tiring enough that people stop opening it, and a vent that is never opened is a ventilation strategy that has quietly failed. That is the point at which an electrically operated unit stops being a luxury, and it is set out on electric rooflight installation.

Light, and the six surfaces it has to cross

Every glass surface reflects a few per cent of the light reaching it, and every low-emissivity coating takes a further slice. A clear double unit transmits around 78 to 82 per cent of visible light. A triple with two coatings is nearer 65 to 70 per cent.

Ten to fifteen percentage points does not sound like much and the eye adapts quickly, so nobody notices it in isolation. It shows up in two places. The first is a room that was marginal for daylight to begin with: a north-facing Stroud valley cottage with one small opening, or a Gloucester back addition kitchen with a single unit in a narrow lean-to slope. The second is where units of two specifications sit on the same elevation, when the difference is obvious side by side. Low iron glass recovers two or three percentage points and is worth asking about on a triple where daylight is the reason the opening exists at all.

The trap in a triple unit with two low-emissivity coatings

This is the part that surprises people. A triple unit with two standard low-emissivity coatings has a g-value in the region of 0.45 to 0.52. A double unit with a spectrally selective solar control coating has a g-value of 0.25 to 0.35.

So on a south or west facing slope, swapping a solar control double for a standard triple lowers the winter heat loss and raises the summer heat gain. The room gets warmer in July in exchange for being marginally warmer in January. On a west-facing loft conversion at Charlton Kings or a south-west facing rear slope on a Hucclecote semi, that is the wrong trade in both seasons, because the loft was already the hottest room in the house. Triple glazing is a heat-retention product. Choosing it for a room whose real problem is summer is spending money on the wrong number.

Combining solar control with a third pane

The two can be combined and it is worth knowing what it costs. Solar control goes on the cavity face of the outer pane, counting as surface two, so it reflects near-infrared before the energy is absorbed into the assembly. The low-emissivity coating then goes on a cavity face nearer the room, typically surface five in a triple.

A build-up like that reaches a g-value near 0.25 to 0.30 with a whole-unit U-value around 0.8 to 1.0, which is genuinely good on both counts. The price is daylight: light transmittance falls to somewhere around 50 to 58 per cent, so the room reads noticeably dimmer than the same opening in clear double glazing, and the coatings’ tint is more visible against a white plastered reveal. On a large opening over a kitchen that is usually acceptable. On a single small unit lighting a landing it is not, and the better answer there is a good double with the right single coating. How coatings are positioned by surface number is set out on low-emissivity coatings and argon fills.

Rebate depth, and the frames that cannot take it

A 39mm build-up has to sit in a rebate that will hold it with adequate edge cover, correct setting blocks and room for the gasket. Manufactured roof windows from the main makers are produced in triple-glazed variants with the frame designed around the unit, so this is settled at order.

Made-to-size flat roof rooflights and roof lanterns vary by system, and the aluminium profile has a maximum glass thickness written into its section. A slim sightline frame is slim precisely because there is not much material, and a system rated to 32mm will not take a 39mm unit whatever anyone would prefer. A conservation rooflight is the tightest case of all: a cast or pressed steel frame designed to sit almost flush in a 25mm Cotswold stone slate has a shallow rebate by definition, and a standard triple simply does not go in. Where a manufacturer offers a slim krypton-filled triple in a 28 to 32mm build-up, that is the route, and it is worth asking about specifically on a Painswick or Northleach roof.

The middle pane, deflection, and stresses only a triple has

The centre pane of a triple unit is unusual in that it is heated from both sides and cooled from neither. It sits between two sealed cavities, it often carries a coating, and it has no direct contact with either the room air or the outside air. On a sunny day it can run hotter than either of the outer panes, and where a shadow line crosses the unit that differential expansion concentrates at the pane edge. It is common practice to heat strengthen or toughen the middle pane of a triple for overhead use for exactly that reason, and it goes on the specification.

Two cavities also mean two gas volumes expanding and contracting independently. Units are sealed at the factory at the ambient pressure and temperature of that day, so on a cold morning both cavities contract and the panes bow inward, and on a hot afternoon they bow outward. With three panes the middle one is pulled both ways at once. The visible result is a faint distortion in reflections from the garden, more noticeable on a triple than a double, and slightly more so on large panes. It is cosmetic and it is normal. Units transported over a significant altitude change sometimes need pressure equalisation, which is a manufacturing matter that the fabricator handles.

Rain noise, and the assumption that more glass is quieter

More panes does not automatically mean less noise. A sealed unit behaves acoustically as masses separated by springs, and a symmetrical build-up with two equal cavities has resonances that a well chosen asymmetric double does not.

For rain and hail on overhead glass, which arrive as impacts across a wide frequency band, the thing that helps most is a laminated outer pane with a damping interlayer, because it absorbs the impact energy in the interlayer rather than radiating it into the room. A triple unit with a thin toughened outer pane can drum more than a double with a 6.8mm laminated outer. If rain noise is the priority, the interlayer choice matters more than the pane count, and that is dealt with properly on acoustic glazing and rain noise.

Putting a number on the winter saving

Heat loss over a heating season can be estimated with degree days, which sum how far and how long the outside temperature sits below a base temperature. Gloucestershire runs around 2,100 degree days in a typical year.

Take a 1.5 square metre unit and an improvement from 1.3 to 0.9 W/m2K, which is a fair representation of a good double against a good triple in the same frame. The annual saving is 0.4 multiplied by 1.5 square metres, multiplied by 24 hours, multiplied by 2,100 degree days, which comes to roughly 30 kilowatt hours a year. That is the honest figure, and it is small. Two or three units on a loft conversion take it to 60 or 90 kilowatt hours. Triple glazing overhead is not bought back through the heating bill within any sensible period, and anyone presenting it that way is overselling it. What it does buy is a warmer inner surface, which is worth several degrees of margin above dew point on the coldest mornings, and a room that feels different to sit in because there is no cold radiant surface overhead. Those are comfort arguments and they are real ones.

Vacuum glazing, and where it sits between the two

Vacuum insulating glass is a genuine third option and it is worth understanding, particularly on heritage work. Two panes are sealed at the perimeter and the air between them evacuated, leaving a gap under a millimetre held apart by an array of tiny support pillars. With no gas there is no conduction and no convection, and a low-emissivity coating deals with the radiation, so a total build-up of around 6 to 8mm can reach a centre-pane value near 0.7.

The attraction on a Cotswold or Cheltenham roof is obvious: near-triple performance in a thickness that will go into a slim steel conservation rebate or an original cast frame. The reservations are real too. The support pillars are visible as a faint regular pattern in certain light. The perimeter seal is the whole product, so edge detailing and handling matter more than with a conventional unit. Overhead applications need the manufacturer’s specific approval because the panes are thin and the loading case is different from a wall. Where it is offered for a roof and the officer will accept it, it solves a problem nothing else solves.

Where it pays across Gloucestershire, and where it does not

The pattern across the county is fairly consistent once the roof and the room are known.

Triple earns its place on shallow made-to-size units over kitchens and dining spaces in the vale, where the pitch is five to ten degrees and cavity convection is at its worst. It earns it on north-facing slopes, where there is no solar gain to trade away and the light penalty is the only cost. It earns it in continuously heated rooms and in houses on a heat pump, where the system holds a steady state and has little spare capacity to overwhelm a cold surface. It earns it on high exposed sites in the wolds and in the Forest, where wind speed raises the external heat transfer and the outer pane runs colder. And it earns it on the valley floors around Chalford and Brimscombe, where cold air drains downhill after dark and pools, so a unit that copes at zero degrees is being asked to cope at minus four.

It does not pay on a small conservation unit in a slim steel frame, where the frame is the dominant term and the rebate will not take the glass. It does not pay on a south or west facing loft window, where a solar control double is the better specification in both seasons. It does not pay on a garage, a store or an occasionally heated room. And it does not pay anywhere the reveal detail has not been sorted out first.

The decision is made at survey from the orientation, the pitch, the size the roof will take, the frame system’s maximum glass thickness, how the room is heated and how the glass has to be lifted onto the roof. From there an indicative range comes back by email and a fixed price follows the survey, itemised so the glazing step shows as its own line. Email info@severnskylightsgloucester.co.uk with the pitch and the direction of the slope, and see the costs page for how the two figures work.