What a U-value tells you about walk-on glass
A walk on rooflight U-value is the rate at which heat escapes through the unit, in watts for every square metre of rooflight and every degree of difference between inside and outside. The lower the number, the less heat the room below loses through the glass.
The unit is written as W/m²K. A rooflight with a U-value of 2.0 loses two watts through each square metre for each degree of temperature difference. On a January night in Cambridge with the kitchen at 20°C and the terrace at 0°C, that is a 20 degree difference, so a one square metre pane at 2.0 W/m²K would be passing roughly 40 watts of heat outdoors, continuously, for as long as those conditions last. Halve the U-value and you halve that loss. It is a simple figure, and it is the one Building Control and every maker's data sheet use.
Two versions of the number turn up on specification sheets, and they are easy to confuse. The centre-pane figure, often written Ug, describes only the sealed glass unit in the middle of the pane, away from any frame. The whole-unit figure, often written Uw or simply given as the rooflight's U-value, includes the frame, the edge of the glass and the spacer between the panes. The whole-unit figure is always the worse of the two, because edges and frames lose heat faster than the middle of a sealed unit. When you compare a walk on skylight U-value between two suppliers, make sure you are comparing whole-unit figures, for similar sizes, taken in the same orientation.
That last point matters more for rooflights than for windows. Glass lying flat loses heat faster than the same glass standing upright, because the warm air trapped in the cavity circulates more freely when the unit is horizontal. A figure calculated for vertical glass can look noticeably better than the same unit will perform once it is lying in your roof. For a walk-on unit, which sits at or very close to horizontal, the horizontal figure is the only one that describes what you will actually get.
Part L and the 2.2 W/m²K limit
For a new or replacement rooflight in an existing home in England, Approved Document L sets a limiting U-value of 2.2 W/m²K, assessed in the horizontal plane. A walk-on unit is a rooflight for this purpose, however much it looks like a piece of floor.
The 2021 edition of Approved Document L changed how rooflights are assessed. Earlier guidance allowed the U-value to be quoted as if the rooflight were vertical, which flattered the figure. The current guidance asks for it in the horizontal plane, so a unit has to be genuinely better to pass. A walk-on unit that met the old rules on a vertical figure may fall short of the current limit once it is judged lying flat, which is worth knowing when an older data sheet is offered as evidence.
The limiting value is a floor, not a target. It is the worst performance the guidance accepts for an individual element in an existing dwelling. New dwellings are assessed differently, against a notional building in which rooflights are modelled at around 1.7 W/m²K, so a walk-on unit going into a new-build terrace or an extension designed alongside a new home may be expected to do better than 2.2 to help the whole design pass. Modern walk-on units intended for homes are generally designed to come in well under the limit, but the declared figure varies from one maker and one build-up to the next, so the number for your specific unit and size is the one to check.
New rooflights are normally notifiable building work, and where the work is notifiable we handle the Building Control notification as part of the installation. The inspector will want to see the declared U-value for the unit being fitted. Our guide to Building Regulations for walk-on rooflights sets out the structural, guarding and energy points together, so this page stays with the heat.
- Outer toughened pane: heat-strengthened to take hail, foot traffic during maintenance and thermal stress.
- Solar-control coating (optional) on the inside face of the outer pane, cutting heat gain on south and west roofs.
- Argon-filled cavity: a denser gas than air, so less heat crosses the gap.
- Warm-edge spacer: a low-conductivity edge that keeps the units’ edges warmer and cuts condensation at the frame.
- Low-E coating: a microscopically thin metal layer on a cavity face that reflects room heat back inside.
- Inner laminated pane: two sheets bonded to an interlayer, so if it ever breaks, the glass stays in place overhead.
- Triple glazing only: a third pane and a second argon cavity, usually with a second low-E coating. Warmer, heavier, and it lets in slightly less light.
Why the thick walking pane barely insulates
The heavy laminated glass you stand on is there for strength, not warmth. Glass conducts heat well, so adding thickness to the walking pane does very little for the U-value; the insulating work is done by the cavity and its coatings.
A domestic walk-on unit is usually built as a sealed unit with a thick laminated top pane, a cavity, and an inner pane below it. The top pane may be two or three layers of toughened or heat-strengthened glass bonded with interlayers, and it can be several centimetres thick in total. It is tempting to assume all that glass keeps the heat in. It does not, to any useful degree. Solid glass passes heat many times more readily than a still layer of gas, so a few extra millimetres of glass change the U-value by a very small amount compared with what the cavity does.
The features that bring the number down are the same ones found in any good sealed unit:
- a low-emissivity coating on one of the cavity faces, which reflects radiant heat back towards the room instead of letting it cross the gap;
- argon, or in some units krypton, filling the cavity in place of air, because these gases carry heat across the gap more slowly;
- a cavity width chosen for the gas and the orientation, since a wider gap is not always better once convection starts inside it;
- a warm edge spacer at the perimeter of the sealed unit, in place of an older aluminium spacer that conducts heat straight round the edge;
- a second cavity and third pane, where the weight and the depth of the opening allow it.
The practical result is that two walk-on units with the same walking pane can have quite different U-values, and a unit with a thinner walking pane but a better cavity can outperform a heavier one. When a supplier describes a unit, the load rating and the thermal rating come from different parts of it. Our hub page on walk-on rooflight installation covers how the whole unit is chosen, and the load side is dealt with in its own guide.
The edge, the frame and the terrace build-up
Most of the heat that a good walk-on unit loses goes out around its edges, not through the middle of the glass. The frame, the kerb and the way the terrace insulation meets them decide whether the declared figure is achieved in your roof.
A walk-on rooflight on a terrace is usually installed flush, or almost flush, with the finished walking surface. That means the unit sits down inside the depth of the roof build-up: the deck, the insulation, the waterproofing and the paving or decking on top. The rooflight's frame or upstand has to pass through all of those layers. If the insulation stops short of the frame, or the frame itself is a solid piece of metal running from the warm room below to the cold terrace above, you have a thermal bridge: a strip where heat escapes much faster than through the roof or the glass on either side.
Walk-on units designed for homes deal with this with an insulated or thermally broken frame, so there is a break between the inner and outer metal, and with an insulated upstand or kerb that the roof insulation can be taken up against. On site, the work is in making the insulation continuous: the terrace insulation must run tight to the unit, with no gap left for convenience around the perimeter, and the insulated kerb must be deep enough to carry that insulation up to the underside of the frame. A gap of a few millimetres around a two metre pane is a long cold line.
This is also where the structural opening for walk-on glass and the thermal design meet. The opening needs continuous, level bearing for the glass, and the engineer's steel or timber trimmers around it can themselves be a cold path if they run from inside to outside without insulation. It is far easier to plan the insulation around the trimmers before they go in than to box them in afterwards. On a roof terrace, where the build-up is often deeper and includes a paving system on pedestals, our guide to walk-on rooflights for roof terraces explains how the unit sits within the finished surface.
Condensation, inside and out
Condensation forms when moist air meets a surface colder than its dew point. A better U-value keeps the inner face of the glass warmer, and a continuous vapour control layer keeps moisture out of the kerb, so both the glass and the construction around it matter.
Inside the room, the risk is highest where warm, damp air collects under the glass: over a kitchen hob, in a bathroom, or in a basement room where moisture comes up from the ground and the air moves slowly. Warm air rises, so a horizontal pane in the ceiling is exactly where the most humid air in the room ends up. If the inner pane is cold enough, water forms on it and drips. The first place to fog is usually the edge, which is why a warm edge spacer and an insulated frame do more to prevent condensation than a small improvement in the centre-pane figure.
A well-insulated unit is not immune. In a steamy kitchen on a cold night, even good glass can reach its dew point for a while, and the answer is to deal with the moisture as well as the glass: an extractor that is used, a window or opening unit that can purge the air, and heating that reaches the room. A walk-on rooflight sits in the middle of a ceiling, often well away from any wall, so it is worth thinking about how air moves around it before deciding where it goes.
Hidden condensation is the more serious risk. Warm, moist air from the room can find its way into the kerb and roof build-up around the unit if the vapour control layer on the warm side is not sealed to the rooflight's frame. Once in, it can condense against cold surfaces inside the construction, where nobody can see it, and wet the timber or insulation over time. Sealing the vapour control layer to the unit, all the way round, is one of the details we pay most attention to when a walk-on rooflight goes into an existing flat roof.
Flat land and open sky around Cambridge mean clear, cold nights are common in winter, and a terrace or courtyard with nothing overhead radiates heat straight to the sky. It is a good reason to think about the outer surface as well as the inner one when choosing the glass and the finish.
Double or triple glazing on a walk-on unit
A triple-glazed walk-on unit loses less heat than a double-glazed one, but it is heavier and deeper, and it only earns its place where the structure and the build-up can take it. Double glazing with a good cavity is often enough to sit well inside the Part L limit.
Adding a third pane and a second cavity cuts heat loss meaningfully and warms the inner face of the glass further, which also helps with condensation. The cost is weight and depth. The walking pane is already the heaviest part of the unit, and a triple-glazed build-up adds another pane and another spacer below it. That means a deeper frame, a deeper kerb and more load on the trimmers around the opening, which the engineer has to allow for. On an existing flat roof, the extra depth can also be hard to fit within the available build-up without raising the terrace surface. The answer on whether a walk-on rooflight can be triple glazed goes into the depth and weight in more detail.
| Glazing | Heat loss | Trade-off |
|---|---|---|
| Double, air, old spacer | Highest | May not meet Part L |
| Double, low-E, argon | Low | Usual choice |
| Double, low-E, warm edge | Lower at edges | Less edge fogging |
| Triple, two cavities | Lowest | Heavier, deeper |
The table gives the direction of travel rather than numbers, because the declared figure depends on the maker, the size of the pane and the frame. Larger panes generally achieve slightly better whole-unit figures than small ones, because the edge makes up a smaller share of the area. When we set out the options in a written light plan, we give the declared horizontal U-value for the actual unit and size being proposed, not a figure from a brochure for a different size.
Summer heat is a different number
The U-value describes heat going out in winter. The heat coming in on a July afternoon is described by the g-value, and a walk-on unit needs both considered, because horizontal glass faces the high summer sun almost directly.
A U-value says nothing about solar gain. The g-value is the share of the sun's energy striking the glass that ends up inside the room, and a clear, low-iron walking pane with standard coatings can let through a large share of it. Because a walk-on rooflight lies flat, it sees the sun when it is highest and strongest, around the middle of the day in June and July, and the room below can warm quickly. Solar control coatings reduce the g-value, usually with some loss of visible light, and obscure or fritted walking surfaces scatter the light and change how the heat lands in the room.
Improving the U-value and reducing solar gain are separate choices, and the best glass for one is not automatically the best for the other. A north-facing basement lightwell under a shaded garden and a south-facing roof terrace over a kitchen extension call for different balances. That is the heart of how we plan the light before quoting. The answer on whether walk-on glass gets hot in the sun looks at the surface you walk on, and our guide to glare and pools of sunlight below walk-on glass covers what the direct sun does to the room itself.
Replacing an older walk-on unit or pavement light
Older walk-on glazing, and especially glass block pavement lights, often performs far below today's limit. Replacing one is normally treated as a new rooflight for Part L, so the new unit has to meet the current figure.
Pavement lights set in concrete or cast iron, common over the basements and vaults of older properties, were made to let light down, not to keep heat in. They are usually single-layer blocks in a solid frame with no thermal break, and the room below loses heat freely through them. Early walk-on sealed units can also disappoint: an air-filled cavity, an aluminium spacer and a vertically quoted U-value were all common. If the sealed unit has failed and misted, it will also have lost whatever gas fill it had.
When we replace one, the opening, the kerb and the insulation around it are assessed along with the glass, because a new unit set into an uninsulated surround will not deliver its declared figure. It is a good moment to improve the vapour control and insulation at the edge, and to reconsider the glass for the light the room now needs. The answer on whether a pavement light is the same as a walk-on rooflight explains the difference between the two.
Questions people ask
What U-value should a walk-on rooflight have?
For a rooflight in an existing home in England, it must not be worse than 2.2 W/m²K, measured in the horizontal plane. Modern units designed for homes generally declare figures well below that, but the exact number depends on the maker, the glazing and the size, so check the declared horizontal whole-unit figure for the unit you are offered.
Does thicker walk-on glass keep more heat in?
Very little. The thickness of the walking pane is set by the load it has to carry. The insulation comes from the sealed cavity, the low-emissivity coating, the gas fill, the warm edge spacer and an insulated frame and kerb.
Why does my walk-on rooflight get condensation on the inside?
Moist air is meeting glass or frame colder than its dew point. Better glazing, a warm edge and an insulated surround reduce the risk, and good extraction deals with the moisture itself. If an older unit fogs regularly, replacing it with a modern insulated unit and sealing the vapour control layer around it usually changes things. To see what that would mean for your room, book a light survey.
