Guide · Energy Efficient Rooflights
Solar gain, overheating and shading
Glass in a roof collects more solar energy per square metre than glass anywhere else in the house, and in June it collects it at the worst possible moment. That is the entire subject in one sentence. Everything else is arithmetic and sequencing: how much energy arrives, how much of it gets through, what the room does with it, and in what order the countermeasures are worth applying.
This page works through the numbers rather than the reassurance. It is written for Essex, where a rear extension in Chelmsford, Braintree or Maldon sits at a latitude of roughly 51.7 degrees north and takes a June sun almost overhead at midday, and where the same glass in December is one of the few things bringing free heat into the house.
How much energy lands on a roof plane
On a clear day in late June, a horizontal surface in southern England receives something in the order of 850 to 900 watts per square metre at solar noon. That is the total of direct beam and diffuse sky radiation, and it is close to the theoretical ceiling for this latitude.
Compare that with a south facing vertical window on the same day, which receives perhaps 400 to 500 watts per square metre because the sun is striking it at a glancing angle. The roof plane collects roughly twice as much. In December the position reverses completely: the horizontal surface receives about 100 to 150 watts per square metre while the south wall, taking a low sun nearly square on, receives more. The rooflight is the summer collector and the wall window is the winter one.
Solar altitude across the Essex year
The angle the sun makes with the horizon at noon is set by latitude and date, and at 51.7 degrees north it runs from about 61.8 degrees at the summer solstice down to about 14.8 degrees at the winter one.
That range explains a great deal. At 62 degrees the sun is close enough to vertical that a flat rooflight takes the beam nearly square on and a wall window barely sees it, which is why a south facing room with only wall glazing can be comfortable in July while the same room with a large flat rooflight is not. At 15 degrees, the same rooflight is taking the beam at a glancing angle and reflecting most of it away, while the sun is streaming through the patio doors. The rooflight is not a constant. It is strongly seasonal, in the unhelpful direction.
Turning the g-value into watts in the room
The g-value is the fraction of incident solar energy that ends up inside, counting both what passes straight through the glass and what the glass absorbs and then re-radiates inwards. It is the number that converts irradiance into a heat load.
The calculation is a multiplication. Four square metres of flat rooflight with a clear low-E unit at a g-value of 0.6, under 850 watts per square metre, admits 0.6 × 850 × 4, which is a little over 2,000 watts. That is the output of two fan heaters running continuously into a kitchen at the hottest part of the day. Specify the same unit with neutral solar control at a g-value of 0.30 and the load halves to about 1,020 watts. Nothing else available changes the arithmetic by that much for that little money.
Halving the g-value halves the solar load, and it does so before the heat is ever in the room.
Direct beam, diffuse sky and reflected light
The load arriving at the glass has three components and they behave differently, which matters because two of them are not stopped by anything you might put in the way.
Direct beam is the shadow casting component and it can be intercepted geometrically, which is what any overhang or external blind does. Diffuse sky radiation comes from the whole hemisphere, and on an overcast bright day in August it is nearly all of the load: it cannot be shaded selectively because it arrives from every direction at once. Reflected radiation comes off surfaces around the rooflight, and on a flat roof finished with light coloured chippings or a white single ply membrane there is more of it than people expect. Only the glass itself deals with the second and third components.
What the room does with the heat once it is in
Two thousand watts entering a room does not raise its temperature by a fixed amount. What happens next depends on how much material is available to absorb it and how quickly.
A masonry room with a solid floor, plastered walls and an exposed concrete or screed slab has high thermal admittance: it soaks up heat during the day, holds it, and gives it back overnight. The peak temperature is lower and later. A lightweight extension with a timber floor, plasterboard on dabs and an insulated ceiling has very little to absorb anything, so the air temperature follows the solar load almost immediately. The same rooflight in the same orientation behaves quite differently in a Victorian rear addition off Moulsham Street and in a timber framed garden room, and the reason is mass rather than glass.
The other things heating the room
Solar gain is not the only load, and in a kitchen it is often not the largest one at the moment it matters.
An oven at temperature contributes a few hundred watts into the room even when it is well insulated. A hob under a recirculating extract puts nearly all of its energy into the space. A dishwasher on a hot cycle, a fridge freezer rejecting heat from its condenser, downlights left on, and the occupants themselves at roughly 80 watts each of sensible heat all add up. A kitchen with four people cooking is carrying perhaps 800 to 1,200 watts of internal gain before the sun contributes anything, which is why the room with the largest rooflight in the house is frequently also the room with the highest baseline load.
What overheating feels like before a thermometer agrees
Discomfort under a rooflight starts before the air temperature is unreasonable, because a hot pane overhead is a radiant surface pointed straight at you.
Glass with a high absorptance under strong sun can reach 40 to 50 degrees on its inner face. You feel that as radiant asymmetry, the same sensation as standing near a bonfire on one side, and it registers as heat regardless of what the thermostat says. It is also why an internal blind, which stops the light but heats up itself, changes the appearance of the problem more than the physics of it. Anyone who has sat under an unshaded lantern at three in the afternoon in July recognises the feeling, and it explains why the complaint is usually about glare and heat together.
Part O, and where it applies
Part O of the Building Regulations in England addresses overheating in new residential buildings. It applies to new dwellings and to buildings created by material change of use, and it does not extend to an extension of an existing house. That distinction matters, because a great many rooflights go into extensions where Part O never formally bites and the physics applies anyway.
Where it does apply, there are two compliance routes. The simplified method sets prescriptive limits on glazing area and on openable free area, split by whether the location is high risk or moderate risk. Most of Essex sits in the moderate risk category, with the high risk designation concentrated in parts of London. The alternative is dynamic thermal modelling.
The simplified method, and the limits it sets
The simplified route works by capping two things: how much glazing faces which way, and how much of the facade can be opened.
It distinguishes cross ventilated dwellings, where openings exist on more than one facade or orientation, from single aspect ones, and the limits are considerably more generous for the former. Rooflights count towards the glazing area and, where they open, towards the free area for removing heat. That second point is the useful one for a rooflight specification: an opening rooflight is not only a comfort feature, it is a compliance asset, because the free area at high level is exactly what the calculation is looking for.
Dynamic thermal modelling, and when it earns its fee
The second route runs a simulation of the actual building against a reference weather year and tests the result against the criteria in CIBSE TM52 and TM59.
It is more work and it costs more, and it is worth it in two situations. The first is where the simplified method rules out a design the client genuinely wants, because modelling frequently shows that a large glazed area with good solar control and generous purge ventilation passes comfortably. The second is where the building is unusual: a deep plan, a single aspect flat, or a room where the rooflight is doing all of the daylighting. Modelling gives you a number for a specific design rather than a rule written for the general case.
Reducing the load before it reaches the glass
The most effective interventions happen outside, because energy stopped before it enters the room never becomes a heating load at all.
Geometry is free. A rooflight set back under an existing overhang, or positioned on a plane that a neighbouring gable shades from two in the afternoon, has a solar profile that no coating can match. Pitch is nearly free: a lantern at 25 degrees takes a June sun more squarely than one at 35 degrees, and the steeper option also gathers more of the useful December sun. External shading devices work well and are covered in detail on the page comparing external shading against internal blinds, which sets out what each does and what each costs.
Reducing the load at the glass itself
Once the geometry is fixed, the coating is the main control, and the modern products are a great deal better than their reputation.
A neutral solar control unit at a g-value of 0.33 still transmits around 60 to 65 per cent of visible light, so the room stays bright while the heat load falls by nearly half against a clear unit. The figure worth asking about is selectivity, the ratio of light transmission to g-value: above 1.7 the glass is doing real work, below about 1.3 it is a tint. The way coatings achieve this, and which cavity surface they sit on, is set out under low emissivity coatings and argon fills.
Removing heat that has already arrived
Glass rejects heat. It does not remove heat that is already in the room, and by mid afternoon in August the room is holding a day’s worth of accumulation.
An opening rooflight sits at the highest point in the space, which is where the hottest air collects, so opening it lets that air leave while cooler air is drawn in through doors and windows at low level. The driving force is the density difference and it scales with both the temperature difference and the height between the openings, which is why a tall extension shifts far more air than a shallow one. Practically, an electric opener on a rain sensor is the difference between a vent that gets used and one that does not.
Night purging, and the hours it actually works
The most useful cooling in an English summer is available between about ten at night and seven in the morning, when the external air in Essex typically drops to 13 to 16 degrees even after a hot day.
Running that air through the building overnight discharges the heat stored in the structure and resets the starting temperature for the following day, which is worth two or three degrees on the afternoon peak in a room with any mass in it. The obstacles are practical rather than technical: security at ground floor level, rain, and insects. A high level rooflight solves two of the three, since an opening three metres up is not an entry route and a rain sensor closes it before the shower arrives.
Ceiling fans, and the degrees they buy back
Air movement across skin increases evaporative and convective loss, so moving air feels cooler than still air at the same temperature. The usual working figure is that 0.8 metres per second of air movement is equivalent to a reduction of about two to three degrees in perceived temperature.
A ceiling fan draws perhaps 30 watts to achieve that, against a few thousand for air conditioning, and in a room with a high ceiling under a lantern it also breaks up the layer of hot air that collects above head height. It does nothing for the actual temperature of the room, which is why it belongs after the glass and the ventilation rather than instead of them.
Sizing the glass to the room in the first place
The single most reliable protection against overheating is not putting in more glass than the room needs.
The usual daylight starting point is a glazed roof area of 15 to 20 per cent of the floor area below. In a north facing or heavily obstructed room the upper end is right. On a south or west facing plane with no external shading, the lower end is right, and going beyond 20 per cent commits you to strong solar control, opening vents and probably a blind as well. Sunspire raises orientation before size at survey for exactly this reason, and the broader case for treating the whole specification as one decision sits on the energy efficient rooflights hub.
The order to work through it
Deal with the items in the sequence in which they cost least and do most: position and pitch first, since both are free at design stage. Then glazed area, kept proportionate to the room. Then the g-value, chosen for the orientation. Then opening vents at high level with a rain sensor. Then external shading if the design still wants it. Then, and only then, internal blinds and fans.
Working the list in reverse, which is what happens when the glass is ordered before anyone has thought about July, produces a room that needs a blind closed for four months of the year. At which point the daylight the rooflight was fitted for is being deliberately shut out, and a large amount of money has been spent on a ceiling.
Keep reading
More on energy efficient rooflights
Condensation, ventilation and rooflights
Read the guide →Triple glazing in a rooflight and when it pays
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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