Guide · Energy Efficient Rooflights
External shading against internal blinds
External shading stops solar energy before it enters the building. An internal blind intercepts it after it has already crossed the glass and become heat in the room. That single difference decides most of the comparison, and it is why an external awning can cut solar gain through a rooflight by three quarters while an internal blind of the same fabric manages perhaps a third.
Both have a place, and on most jobs the answer is one of each doing different work. This page sets out what each position actually achieves, how the hardware mounts to a rooflight, what the operation and maintenance look like over a decade, and which decisions have to be made at order rather than later. The wider overheating strategy sits on the energy efficient rooflights hub and its shading page.
Where the energy is when the shade intercepts it
Sunlight arriving at a rooflight is mostly short wave radiation. Glass passes it readily. Once it lands on a surface inside the room, whether that is the floor, a worktop or the back of a blind, it is absorbed and re-radiated as long wave heat, and glass is largely opaque to long wave. The heat is trapped.
An internal blind therefore becomes a warm object inside the sealed envelope. It shields your eyes and your furniture, and it does reduce the rate at which the room heats, because a light coloured blind reflects a share of the incoming energy back out through the glass before it is absorbed. But the majority of what it stops has already entered the building and stays there.
The numbers on each side of the glass
The combined effect of glazing and a shading device is expressed as a total solar energy transmittance, the g-value of the whole assembly. It is the honest basis for comparison because it accounts for where the device sits.
| Assembly | Approximate combined g-value | Share of solar energy entering |
|---|---|---|
| Neutral double glazed unit, unshaded | 0.55 to 0.60 | Around 57 per cent |
| Solar control unit, unshaded | 0.25 to 0.40 | Around a third |
| Neutral unit with light internal blind | 0.35 to 0.45 | Around 40 per cent |
| Neutral unit with external awning | 0.10 to 0.15 | Around 12 per cent |
| Solar control unit with external awning | 0.05 to 0.10 | Under 10 per cent |
The row that decides arguments is the fourth. An ordinary rooflight with an external awning over it outperforms an expensive solar control unit left unshaded, and it does so while still passing plenty of diffuse daylight.
External awnings, and how they mount
The standard external device for a rooflight is a fabric screen in a cassette fixed at the head of the unit, running down guide rails or tensioned cables to the cill. Retracted, it sits in a slim housing barely wider than the frame. Extended, it covers the full glazed area a short distance above the glass.
The fabric is an open weave mesh rather than a solid sheet, typically passing between five and ten per cent of visible light. That sounds low and reads as surprisingly bright, because the sky it is filtering is extremely luminous. The mesh matters for a second reason: it lets the view out survive, which a solid external shutter does not. The mounting brackets are part of the frame, which is the reason this is an order decision.
External shutters and louvred systems
A rigid external shutter, usually an aluminium roller or a set of adjustable louvres over the opening, is the heavier option. It rejects almost everything, gives full darkness on demand, adds a security layer where a flat roof is accessible from a boundary, and takes wind far better than fabric.
The costs are cost, weight and view. A closed shutter is a closed lid, so the room goes dark rather than dim, and adjustable louvre systems that keep some daylight are considerably more expensive than an awning. They suit exposed positions, large lanterns and rooms where a full blackout is genuinely wanted, which in practice means bedrooms and rooms holding screens.
Powering the outside, and why it is decided early
Anything on the outside of a rooflight needs power, and the two routes are a solar module on the cassette with an internal battery, or a mains spur run to the unit. Solar is the common choice on retrofits and on units in the middle of a roof, since it needs no cable and it charges perfectly well on the light available at roof level even in a British winter.
A mains fed device is more responsive, unaffected by a long dull spell and easier to integrate with a control system. The cable is trivial to run at first fix and disruptive afterwards, which is the same argument that applies to an electric opener. Where either might be wanted later, run the cable now and cap it.
Wind, and the device that retracts itself
Fabric on the outside of a roof is a sail. Every awning carries a wind class, and above its rated wind speed it has to be retracted or it will be damaged. Automatic systems handle this with an anemometer or a vibration sensor that pulls the screen in without being asked.
This is a live consideration in Essex rather than a footnote. The Blackwater and Crouch estuaries, Canvey, Maldon and the open arable country north of Braintree carry noticeably more wind than the sheltered town centres, and a roof plane facing south west on an exposed plot is the worst case. In those positions a rigid shutter or louvre system earns its extra cost, because it stays put.
Internal blinds, and the channel that holds them
An internal rooflight blind runs in side channels within the frame, for the simple reason that gravity works against it. On a horizontal or shallow surface a hanging blind sags away from the glass, gaps at the edges and stops being a blind. The channels hold the fabric flat against the plane and close the light leak at the sides.
Those channels are part of the unit, not an accessory bought later. Retrofitting a proper integrated blind to a rooflight ordered without the provision usually means an external cassette fixed to the reveal or a compromise on light leakage at the edges. The honest moment to decide is at order.
Blackout against a pleated screen
Internal blinds come in two broad families and they solve different problems. A pleated or translucent screen diffuses light and takes the hard edge off glare while keeping the room bright. A blackout blind is an opaque fabric, usually with a reflective backing, in a channel tight enough to stop light at the edges.
In a bedroom under a roof window, blackout is close to essential. An east facing slope in June makes that argument at half past four in the morning, and a west facing one keeps a child’s bedroom light until nine at night. Elsewhere the pleated screen is usually the better daily tool, because it manages glare without turning the lights on.
Honeycomb blinds, and the air they hold
A honeycomb or cellular blind is a pleated fabric folded into closed cells that trap a layer of still air against the glass when the blind is down. That air layer does real thermal work: it can improve the effective U-value at the rooflight by a useful margin overnight, and it takes the chill off the surface in a room where the glass runs cold.
The caution is the other side of the same effect. Holding still, cool air against the inner pane in a humid room raises the condensation risk on that pane, particularly in a kitchen or bathroom. Where a honeycomb blind is used over a wet room, ventilation and a warm frame matter more, not less.
Glare, which the inside wins outright
Solar gain and glare are different problems and they are habitually confused. Gain is energy. Glare is a contrast ratio between a bright source and the surrounding field of view, and it is what makes a screen unreadable or a kitchen uncomfortable to work in.
An external device reduces glare only in proportion to how much light it stops. An internal blind sits between the eye and the source, which is exactly where a diffusing screen does its best work, and it can be adjusted to the task rather than to the weather. This is the clearest case for having both: the outside handles heat, the inside handles the view.
What each one costs to keep working
Internal blinds are indoors, which means they stay clean, stay dry and last as long as the mechanism. A pull cord or a small motor in a dry environment is a long lived thing, and the fabric is replaceable in the channel.
External fabric lives outdoors on a horizontal surface. Modern acrylic and glass fibre meshes hold their colour well and shed dirt reasonably, but they collect pollen in May, leaf debris in October and, under a tree, an organic film that wants washing off. Expect to clean an external screen annually and to expect a fabric life measured in a decade or so rather than in the life of the unit. Rigid shutters and louvres are far less demanding.
Automation, sensors and the rules worth setting
Both positions can be automated, and automation is what makes shading actually work, because the device has to move at the right time on a weekday when nobody is home. A basic setup uses a roof mounted solar sensor with a threshold and a delay, so the screen extends when irradiance stays high for a few minutes and retracts when it drops.
Better setups add an internal temperature input, so the shade does not deploy on a bright cold March morning when the gain is welcome. Adding a wind sensor and, on an opening unit, a rain sensor completes the picture. Keep the rules simple. Over-complicated logic gets overridden manually within a fortnight and then never gets switched back.
Using both, which is the usual answer
The specification that satisfies most rooms is an external device on the planes that get strong summer sun and an internal blind everywhere, doing different jobs. The awning or shutter is set to manage the July afternoon and is largely forgotten. The blind is the daily control for glare, privacy at night and, in bedrooms, darkness.
Where the budget only stretches to one, the deciding question is what the room actually suffers from. If it becomes unusable in a heatwave, the money goes outside. If it is comfortable but you cannot see a screen at three in the afternoon, the money goes inside. Guessing between the two is how people end up buying both anyway, in the wrong order.
Provision on the order, item by item
Almost every decision here has to reach the manufacturer before the frame is made. A rooflight order that keeps both options open names the following.
- Whether the frame carries external mounting provision, and for which system.
- Whether internal blind side channels are included, and for which blind type.
- Power route for each device: solar module or a mains feed, and where the cable enters.
- Fabric openness factor and colour on any external screen.
- Wind class of the external device against the exposure of the plot.
- Control method, and whether it is to be integrated with anything else.
The last three are the ones that get left blank, and they are the ones that decide whether the thing is still in daily use three summers later.
Matching the device to the Essex plot
Exposure varies more across thirty miles of Essex than people expect, and it changes the sensible answer. A sheltered rear extension in a Chelmsford terrace, hemmed in by neighbouring roofs, takes a fabric awning without concern. A bungalow on reclaimed land at Canvey, or a plot on the open ground towards Southminster, sees estuary wind that will exercise a wind sensor several times a month.
Conservation areas add a different constraint. In Thaxted, Coggeshall or the older core of Maldon, an external cassette sitting proud of a slate slope is a visual intervention that a planning officer will have a view on, and the internal route is usually the one that keeps the roofline unchanged. On those roofs, solar control glass plus an internal blind is the practical combination, and it is worth checking before anything is ordered.
The mounting provision for external shading is built into the frame, so it is decided with the unit and not after it.
What to settle at the survey
The survey is where orientation, pitch, exposure, room use and reach all get recorded together, and those five inputs decide the shading answer between them. A north facing slope over a hallway needs nothing. A flat rooflight over a south facing kitchen island in a hard finished extension needs solar control glass, an external device and an opening unit for night purging.
Sunspire sets the shading provision out on the written specification alongside the glazing make-up, so the frame arrives with the right channels and the right brackets whether or not the devices are fitted on day one. That keeps the cheap decision cheap and the later one possible. Indicative figures for each route are on the costs page.
Keep reading
More on energy efficient rooflights
Solar gain, overheating and shading
Read the guide →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 →
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