Energy Efficient Rooflights

The U-value is measured in watts per square metre per kelvin, W/m2K, and it counts how much heat passes through one square metre of the finished unit for every degree of difference between the room and the outside air. Lower is better. A single-glazed Victorian rooflight sits somewhere near 5.0. A double-glazed unit from the 1990s runs 2.8 to 3.0. A current manufactured roof window is 1.0 to 1.4, and a triple-glazed one can reach 0.6 to 0.8.

The g-value, sometimes written as the solar factor, is a fraction between zero and one. It is the proportion of the solar energy striking the glass that ends up as heat in the room, counting both the radiation that passes straight through and the part absorbed by the glass and then re-radiated inwards. Clear double glazing is around 0.6 to 0.7. A solar control unit is 0.25 to 0.35.

The two numbers pull in opposite directions for part of the year. A low g-value keeps a loft room usable in July and gives up some free winter heat in exchange. A high g-value collects that winter heat and makes the same room unbearable by four in the afternoon in August. Which trade is correct depends on the orientation of the slope, the room underneath, and how much glass is going in, and that judgement is made at survey rather than on a website.

Triple glazing on a roof is a genuine improvement and it is not automatically the right answer. A good triple unit with two low-emissivity coatings and argon reaches a centre-pane value near 0.6 and a whole-unit value of 0.8 to 1.0 in a manufactured roof window. Against a double unit at 1.2 to 1.4 that is a real reduction in heat loss from the coldest surface in the room.

Three things push back. The first is weight: a third pane adds roughly 10 to 12 kilograms per square metre, which changes the manual handling on a scaffold, the load on the trimmer, and the effort needed to open a large top-hung sash by hand. On a steep Forest of Dean roof where the top of the frame is already high, an operating rod on a heavier sash becomes tiring, and an electrically operated unit starts to look sensible. That option is set out on the electric rooflight installation page.

The second is light. Every pane and every coating takes a slice of visible light. A clear double unit transmits around 78 to 82 per cent; a triple with two coatings is nearer 65 to 70. In a north-facing Stroud valley cottage with one small opening, that difference is visible in the room.

The third is that the unit stops being the weakest element. Once a roof window is at 0.8, the reveal, the perimeter seal and the surrounding roof insulation dominate the heat loss around that opening. Spending on the glass while leaving a cold bridge at the head of the reveal is money going in the wrong order.

Gloucester sits at about 51.9 degrees north. At noon on the winter solstice the sun reaches an altitude of roughly 14.7 degrees above the horizon. At noon on the summer solstice it reaches roughly 61.6 degrees. That swing of nearly 47 degrees is the single most important fact in specifying glass for a roof, and it is why a rooflight behaves nothing like a window in a wall.

Take a south-facing slope at 30 degrees, which describes a great many concrete-tiled rear slopes in Longlevens, Hucclecote and Quedgeley. A surface at 30 degrees has its perpendicular pointing at 60 degrees altitude. In June the midday sun sits at 61.6 degrees, which is within two degrees of perpendicular: the glass is very nearly square-on to the strongest sun of the year, collecting close to the full irradiance available, which on a clear June day is around 850 to 900 watts per square metre before the glass takes its cut. In December the same slope meets the sun at 45 degrees off perpendicular and collects about 70 per cent of what falls, from a much weaker sun, for barely eight hours of daylight against nearly seventeen in June.

A near-horizontal unit is more lopsided still. At five degrees of pitch the glass is 23 degrees off perpendicular in June and 70 degrees off in December, so it takes roughly three times the midday irradiance in summer that it does in winter. Overhead glazing is a summer collector. Any argument that a rooflight will pay for itself in winter solar gain has the geometry backwards.

The compass direction of the slope changes the specification more than any other single input, and on a lot of Gloucestershire houses it is not a choice.

A north-facing slope takes no direct sun at all for most of the year and delivers steady, even diffuse light with no glare and almost no solar gain. It is the easy case: pick the lowest U-value you are willing to pay for, take the highest light transmission available, and skip solar control entirely.

South is manageable because the peak arrives at midday when the room is usually empty and the outside temperature has not yet peaked, and because the high summer sun angle means a modest external shade cuts a lot of it.

West is the difficult one. The sun swings round to the west-north-west by late afternoon in July, dropping to a lower altitude as it goes, and a west-facing slope is presented more squarely to it exactly as the air temperature reaches its daily maximum around four to five in the afternoon. The room has already been warming all day. A loft conversion has plasterboard and insulation rather than masonry, so there is almost no thermal mass to absorb the peak, and the heat arrives at the hour the room is most likely to be occupied. A west-facing loft bedroom with clear double glazing overhead is the classic unusable summer room, and solar control glass plus external shading is not an upgrade there, it is the specification.

East warms a room hard between seven and ten in the morning and then releases it, which is tolerable in a bedroom and unhelpful in a home office.

Where the orientation is fixed by the building, and it usually is, the answer moves to the glass and the shading. Gloucester’s Victorian terraces are a good example: a two-storey rear addition has one lean-to slope facing whichever way the back of the house faces, the kitchen is underneath it, and there is no alternative elevation. The roof window installation page covers how position on that slope is worked out.

Since 2022 new residential buildings in England have had to be assessed for overheating risk under a dedicated part of the Building Regulations, which limits how much glazing can face east, south and west without shading, and requires a demonstrable route for hot air to leave. That requirement applies to new dwellings rather than to a rooflight going into an existing house, so it is context rather than a rule you have to satisfy.

The physics does not care about the scope of the regulation. A loft room is the hottest room in any house before a single square metre of glass is added, because heat rises, because the roof is the surface with the largest solar exposure, and because a rafter-line insulation build-up has little thermal mass. Add overhead glazing facing south-west and the room can run ten degrees above the rest of the house on a July afternoon.

The approach that works is the same one the regulation uses. Limit the glazed area to what the daylight actually needs rather than the maximum that fits. Put the solar control in the glass. Add external shading on the exposed slopes. And design a ventilation route that lets hot air out at high level and draws cooler air in low down, which on a loft conversion usually means an opening rooflight at the top of the stairs working with a window in the room below. A single opening at high level with no low-level inlet moves very little air.

A whole-unit U-value describes the unit. It says nothing about the junction between the unit and the roof, and that junction has its own heat loss, expressed as a linear value in watts per metre per kelvin along the perimeter.

On a well detailed installation that linear loss is small. On a badly detailed one it can add more heat loss than the difference between double and triple glazing. The failure is nearly always the same: insulation between the rafters is cut short of the frame, leaving a gap of 20 to 50mm filled with air, plasterboard is fixed over the top, and the finished reveal looks perfect. Around the full perimeter of a 1340 by 980mm window that is over four and a half metres of uninsulated line.

Doing it properly means insulation carried tight to the frame all the way round with no voids, rigid board cut to fit rather than mineral wool stuffed into a gap, the vapour control layer taped to the frame itself rather than trimmed to the plasterboard edge, and continuity maintained between the reveal insulation and the main roof insulation so there is no interruption in the line. Where the rafter depth is too shallow to take the full thickness, insulated plasterboard over the reveal makes up the difference.

Ventilation and insulation look like opposites and are not. A house needs a continuous, small, controlled air exchange to carry away the moisture generated by cooking, washing and drying clothes indoors. Without it, the moisture lands on the coldest surface available, which in a loft room is the glass overhead.

Controlled ventilation is cheap in energy terms because it is small and steady. A trickle vent or a ventilation flap in the head of a roof window, left open, exchanges a modest volume continuously and costs very little heat. Uncontrolled leakage through gaps is expensive because it is driven by wind and stack pressure and is largest exactly when the weather is worst.

Purge ventilation is the other half. Opening a unit fully for a few minutes after a shower clears far more moisture than an hour of trickle, and cools the room surfaces very little because the fabric holds its heat. Where a unit is high above a stairwell and nobody can reach the handle, a rain-sensing electric opener is what turns a theoretical vent into a used one. Where daylight is wanted in a windowless bathroom or landing without adding a full opening, a sun tunnel is often the better answer, since a sealed tube with a diffuser at ceiling level adds very little heat loss.

Water on the room side of the glass on a cold morning is condensation, not a defect. The controls are the vent flap, the purge, and a horizontal cill lining that lets warm air rise up the face of the glass rather than a splayed cill that traps a pocket of still cold air against it.

The Stroud valleys have a microclimate that shows up in condensation rather than in heating bills. On a clear, still night, air in contact with the hillsides cools, becomes denser, and drains downhill into the valley bottoms, where it pools. The result is that Chalford bottom, the Frome valley floor and the lower parts of Brimscombe can be several degrees colder overnight than Rodborough or Minchinhampton a couple of hundred metres above them, and frost forms there when the ridge stays clear.

For a rooflight, every one of those extra cold degrees on the outer pane pushes the inner surface closer to dew point. A unit that never condenses on a hillside above the valley will run wet on the same house type at the bottom of it. So in those addresses the useful moves are a warm-edge spacer as a minimum, a triple unit where the budget allows, and particular attention to the temperature factor at the reveal junction, because a marginal detail that copes at zero degrees will not cope at minus four.

The access side of these valleys is a separate matter and a real one. Terraces step up the hillside, lanes are shared, and scaffold is often hand-carried, so the same unit takes longer to install in Chalford than in Quedgeley. That is looked at properly at survey so the figure holds.

Homeowners often ask whether new rooflights will move an EPC band. The straight answer is that they seldom do on their own. A reduced-data energy assessment is dominated by the heating system, the wall construction, the loft or rafter insulation and the fuel, and glazing enters as a type and an age band rather than as the specific U-value on the certificate. Replacing one or two units in a roof is a small change to a small term in a large calculation. Convert a loft, insulate it to current standards, and fit good rooflights as part of that, and the band moves, though the rooflights are the smallest contributor to the move.

The genuine energy case for a rooflight is daylight rather than heat. A horizontal or shallow opening sees a much larger area of sky than a vertical window of the same size, which is why the accepted rule of thumb is that a rooflight delivers roughly two to three times the daylight of an equivalent area of wall glazing. In a Gloucester rear-addition kitchen or a windowless landing, that is the difference between electric light on all day and electric light on at dusk.

Being straight about the balance: a rooflight is a hole in the insulated envelope and a well specified one still loses heat faster than the roof it replaced. The winter heating cost goes up slightly. The lighting cost goes down, the room becomes usable at times it previously was not, and on a south-facing slope there is some useful winter gain on bright days. Anyone selling a rooflight purely as an energy-saving measure is overselling it. Specified properly it is close to thermally neutral and a substantial gain in how a house works, which is the honest claim.