Solar gain, overheating and shading

Three streams of solar energy, and which one dominates on a roof

Solar radiation reaches glass by three routes. Direct beam radiation travels in a straight line from the sun and casts a sharp shadow. Diffuse radiation is sunlight scattered by the atmosphere and cloud, arriving from the whole sky hemisphere at once. Ground reflected radiation bounces off surfaces below.

A vertical window is fed heavily by the ground reflected stream and sees only half the sky. Overhead glazing sees the entire sky hemisphere, so it collects almost all of the available diffuse radiation and effectively none of the ground reflection. On a bright overcast Gloucestershire day, when there is no beam at all, a rooflight is still receiving 100 to 200 watts per square metre of diffuse energy from that whole sky view. It is also why a rooflight delivers two to three times the daylight of the same area of wall glazing, which is the reason it went in.

The cosine law, and the hours when a slope is square-on to the sun

Beam radiation striking a surface at an angle delivers energy in proportion to the cosine of the angle between the beam and the perpendicular to that surface. Square-on, the cosine is one and the surface gets everything. At 60 degrees off perpendicular the cosine is 0.5 and it gets half.

Gloucester sits at about 51.9 degrees north, so the midsummer noon sun stands around 61.6 degrees above the horizon. A slope at 30 degrees has its perpendicular pointing at 60 degrees of altitude, which means a south-facing 30 degree rear slope is within two degrees of square-on to the strongest sun of the year. Those slopes are everywhere across Longlevens, Hucclecote, Warden Hill and the Quedgeley estates. A near-horizontal flat roof unit at five degrees is 23 degrees off perpendicular in June and 70 degrees off in December, which is where the three-to-one summer bias comes from.

Reflection at the surface, and the angle where glass starts refusing light

The cosine law is only half of it. Glass also reflects a rising fraction of what reaches it as the beam gets further from perpendicular.

Up to about 50 degrees off perpendicular, transmission through clear glass is nearly flat, losing only the usual few per cent at each surface. Between 50 and 70 degrees it starts to fall away. Past 75 degrees most of the beam is reflected and very little gets through, which is why low winter sun glancing across a shallow rooflight contributes almost nothing to heating a room, and why any argument that overhead glazing pays for itself through winter solar gain has the physics the wrong way round. In summer the same effect works against you: the beam arrives close to perpendicular precisely when reflection would have been useful.

What a g-value is made of: direct transmission and the secondary term

The g-value is not simply how much sunlight passes through the glass. Under BS EN 410 it is the sum of two things: the direct solar transmittance, which is the energy that goes straight through, and the secondary internal heat transfer factor, which is the part of the energy absorbed by the panes and then released inwards.

That second term is the one people miss. Glass that absorbs energy gets hot, and a hot pane radiates and convects in both directions. Roughly speaking, the inner pane releases most of what it absorbs into the room and the outer pane releases most of what it absorbs to the outside air. A body tinted outer pane, which controls gain by absorbing rather than reflecting, can run 30 degrees above air temperature on a July afternoon and hands a good share of that back inwards. A reflective coating on the outer pane cuts the energy before it is absorbed at all, which is why the coating position matters as much as the coating itself.

Turning a g-value into watts standing in the room

The arithmetic is simple and it is the part that makes the problem real. Multiply the glass area by the irradiance on the plane of the glass by the g-value.

Take 1.5 square metres of glass on a south-west facing slope, taking 800 watts per square metre on a clear July afternoon, glazed with clear double at a g-value of 0.6. That is 720 watts entering the room, continuously, for hours. A 720 watt heat input into a loft bedroom of nine square metres is 80 watts per square metre of floor, which is around four times the input a heating system would use to hold that room at temperature in midwinter. Swap to solar control at 0.3 and the same glass delivers 360 watts. Add an external shade cutting 70 per cent of what reaches the glass and it drops to about 110. Those three numbers, 720, 360 and 110, are the entire summer conversation.

Internal gains, and the load that was already there

Solar gain lands on top of everything else the room is producing, and in a small space the rest is not negligible.

An adult at rest gives off around 75 to 100 watts of sensible heat. A desktop computer and a monitor together run 100 to 200. A television is 50 to 150. An oven with the door opened repeatedly, a hob with four rings, a dishwasher on its drying cycle and a fridge compressor all dump heat into a kitchen. In a home office under a south-facing roof window, two occupants and their equipment can be 400 watts before the sun contributes anything, and the sun is arriving at the same hours the office is occupied. Fabric gain adds more: a dark concrete tile roof surface can reach 55 to 65 degrees on a July afternoon, and heat conducts down through the insulation all day, peaking in the evening.

Thermal mass, decrement delay, and the loft that has neither

A heavy building absorbs a peak and releases it slowly. That property has two names worth knowing. Admittance describes how readily a surface takes up heat as the room temperature swings. Decrement delay describes how long a pulse of heat takes to travel through a construction and arrive on the inside.

A 225mm solid brick wall in a Gloucester terrace has a decrement delay of around eight hours, so the afternoon peak arrives in the room near midnight, heavily damped. A stone flagged kitchen floor in a Cotswold cottage soaks up a substantial part of the day’s gain and gives it back overnight. A loft conversion has neither: 12.5mm of plasterboard on insulation on rafters has a delay of about an hour and almost no capacity, so the peak arrives in the room essentially as it happens, undamped. The same rooflight in a stone-floored ground floor kitchen and in a plasterboard loft bedroom produces two completely different afternoons.

The temperature criteria a designer actually works to

Overheating has agreed definitions rather than being a matter of opinion, and knowing them makes a conversation about glazing concrete.

For homes without cooling, the widely used benchmark treats a bedroom as failing if the operative temperature exceeds 26 degrees for more than one per cent of the annual occupied night hours between ten at night and seven in the morning. For living spaces the criteria are adaptive: the acceptable temperature rises with the running mean of the outdoor temperature, on the reasonable basis that people accept more heat in a warm spell than in a cold one. Since 2022 new dwellings in England have had to be assessed against a dedicated overheating requirement, using either a simplified method that caps glazing areas by orientation and demands a minimum free ventilation area, or full dynamic thermal modelling. That requirement covers new homes, so for a rooflight going into an existing house it is a design reference rather than a rule to satisfy.

Absorbed heat in the frame, the blind and the reveal

Not all the heat arrives through the glass. Anything dark and sunlit near the opening becomes a secondary emitter.

A dark grey or black powder coated aluminium frame absorbs most of the radiation falling on it and can reach 60 degrees or more, conducting inward through the frame section and warming the reveal. A dark internal blind pulled across the underside of the glass absorbs energy that has already passed the glazing line, and the air trapped between fabric and glass heats up and convects into the room around the edges of the blind unless the head of that cavity can vent. A deep plastered reveal in a dark colour does the same on a smaller scale. None of these is a reason against a dark frame or a blind, and all of them are reasons why the biggest single lever sits outside the glass rather than inside it.

Removing heat with air: what a purge actually has to shift

Ventilation does not stop radiation arriving. It removes air that has already been heated, and the quantity needed surprises people.

Air carries roughly 1,200 joules per cubic metre per kelvin. To remove 720 watts of gain while holding the room only four degrees above the incoming air needs about 0.15 cubic metres per second, which is 150 litres per second, or 540 cubic metres an hour. In a loft bedroom of 29 cubic metres that is nearly nineteen air changes per hour. No trickle vent will do it. It needs a large opening, and it needs the outside air to be cooler than the room, which through a Gloucestershire July afternoon it usually is not. The hours when it works are between about ten at night and seven in the morning, which is exactly why night purge is the mechanism that matters and daytime opening is not.

High opening, low inlet, and the stack that drives it

A single opening at high level moves very little air on a still night, because there is nowhere for replacement air to enter. Two openings at different heights turn the room into a chimney.

Warm air inside is less dense than cooler air outside, so it rises and leaves through the highest opening, drawing outside air in through the lowest. The driving pressure grows with the height difference between the two and with the temperature difference across them, so a rooflight at the top of a stairwell paired with a window on the floor below is far more effective than the same rooflight on its own. In a loft conversion the useful arrangement is the roof window open at the top of the stairs and a window open in a room below. Where the unit is out of reach, a rain-sensing electric opener on a timer turns night purge from a thing you remember into a thing that happens, and that is covered on electric rooflight installation.

Splitting one opening into two, and what it does to the peak

Daylight and solar gain do not scale together, which gives one genuinely free improvement.

Daylight in a room is limited by the darkest corners rather than the brightest spot. One large rooflight in the middle of a ceiling produces a bright pool underneath it and leaves the edges dim, so the eye reads the room as adequately lit only when the pool is very bright. Two smaller units spread apart light the same room more evenly, and the total glazed area needed to reach the same perceived brightness is smaller. Less glass for the same light means less peak gain, and on an interwar hipped roof at Charlton Kings or Warden Hill, where the slope narrows towards the corner anyway, two units in adjacent rafter bays is often both the structurally simpler answer and the cooler one.

Self-shading from the reveal, the roof and the horizon

Some shading is already built into the geometry and costs nothing to specify.

A rooflight set on a 300mm kerb has an upstand that shades the glass whenever the sun is low and to one side, which on an east or west facing unit removes part of the early and late load. A deep splayed reveal does a little of the same from below. A chimney stack, a neighbouring gable, a taller terrace next door or a mature lime tree can remove the summer problem entirely, and on a Stroud valley site the hillside itself blocks a large arc of sky. That is why overshading is surveyed rather than assumed: two identical houses two hundred metres apart in Chalford can need different glass. The caution is that a shadow line crossing a pane sets up differential expansion, so a known partial shadow goes on the survey sheet and the glass is specified with it in mind.

South-facing Cotswold slopes, and the shade that arrives from the landscape

The wolds villages sit high and open. A south-facing stone slate roof at 50 degrees in Northleach or Chipping Campden has its perpendicular pointing at 40 degrees of altitude, so it is square-on to the sun in April and September rather than in June, and the midsummer beam actually arrives at a fairly oblique angle. The steep pitch is doing useful work.

What follows is that the hard months on a steep southern Cotswold slope are the shoulder seasons rather than high summer, and the room warms in spring when nobody expects it. The other constraint is that the conservation rooflights these roofs take are small, which caps the total gain, and that external shading over a slim steel unit on a visible slope will rarely be acceptable. So the control has to live in the glass, and a spectrally selective coating on the outer pane is the specification rather than an upgrade. Thick stone walls and a solid ground floor supply thermal mass that a modern extension does not have, which helps considerably.

West-facing Severn Vale extensions, and the four o’clock problem

A flat roof extension across the vale, in Gloucester, Longford, Tewkesbury or the villages between, typically carries one or two made-to-size rooflights at five to ten degrees over a kitchen and dining space. The construction is blockwork and plasterboard with a timber deck above, and there is usually a large glazed door onto the garden as well.

The load builds through the day rather than peaking at noon. Near-horizontal glass takes its maximum from the high midday sun, the west-facing garden doors then take the low afternoon beam almost square-on, and the outside air temperature reaches its daily maximum around four to five in the afternoon. All three peaks stack up in the hours the room is most used. The vale also sits low with a wide open horizon over the river and very little to shade a rear elevation, and still evening air means the outside temperature falls slowly after sunset. The answers that work here are solar control on the rooflight glass, a genuine high-level opening in the rooflight itself so the stack has an exit, and shading over the vertical glazing, which is where most of the afternoon energy is actually arriving.

Cheltenham conservation constraints, and shading that has to be invisible

Central Cheltenham complicates the summer answer in a way no other part of the county does. The Regency terraces at Lansdown, Montpellier, Pittville and along the Bath Road are largely within conservation areas and a great many are listed, so an external awning, an external roller shutter or a retractable fabric shade on a visible elevation is a difficult proposition and often not one worth starting.

The geometry does hand back something. These roofs are frequently mansards with the shallow upper slope hidden behind a parapet, and the parapet itself shades the glass in the early morning and late evening. But at midday in June nothing intercepts the beam, and that shallow slope is close to the maximum collection angle. The workable specification is therefore a spectrally selective solar control coating carrying the whole load, a high-performance unit to keep the winter side sound, and a ventilation route designed properly, because with shading removed from the toolkit the remaining two levers have to do more. Where an internal blind is being considered as the visible answer, the honest ranking of what each option delivers is set out on the sibling page.

What gets measured before the glass is chosen

The summer decision is made from a short list of facts taken on the day: the orientation off a compass and the pitch off a digital level, the horizon and any overshading from buildings, hillsides or trees, the floor area and use of the room below, whether the construction has any thermal mass, what other glazing the same room already has, and whether a second opening exists at a lower level to make night purge work.

From that comes a target g-value, a target light transmittance, a shading recommendation and a ventilation strategy, written down rather than assumed. Send photographs of the roof and the room to info@severnskylightsgloucester.co.uk with the direction the slope faces, and an indicative range comes back before anyone stands on the roof. The costs page sets out how the fixed price follows, and the written guides go further into individual questions.