External shading against internal blinds
The short answer is that shade only works properly on the outside of the glass, and everything else on this page is the detail behind that sentence. An external awning stops solar radiation before it enters the room. An internal blind intercepts it after it has already come through, at which point the energy is inside the building and most of it stays there. The difference is not marginal. On the same rooflight, on the same July afternoon, an external fabric will typically cut the heat arriving in the room by three to four times as much as an internal blind of the same colour. Internal blinds still have work to do, but that work is privacy, blackout and glare rather than temperature. The physics of why glass admits and then traps solar energy is set out on solar gain, overheating and shading, and the wider specification picture is on the energy efficient rooflights hub. What follows is the comparison between the two positions.
Once the energy is through the glass, it is committed
Short-wave solar radiation passes through glass easily. It lands on carpet, furniture, plasterboard and the blind itself, is absorbed, and is re-emitted as long-wave heat. Glass is largely opaque to long-wave radiation, and a low-emissivity coating is designed specifically to reflect it back inward. So the same coating that keeps winter warmth in the room keeps summer heat in the room too.
An internal blind therefore sits on the wrong side of that one-way valve. It absorbs the radiation, warms up, and then heats the room by convection off its own surface and by radiation from it. A dark internal blind on a sunny July afternoon can reach 50 degrees, and it is a radiator hanging a few centimetres below the ceiling.
What an external awning blind actually consists of
On a manufactured roof window, an external awning is a woven mesh fabric on a spring or motorised roller housed in a slim cassette that clips to the top of the frame outside, with side guides running down the frame and a bar at the bottom. It sits 30 to 50mm off the glass, so the air behind it is ventilated and the heat it absorbs is carried away by outside air rather than delivered to the room.
VELUX, Fakro, Keylite and Roto all make them for their own frames, in manual, solar-powered and mains-powered forms. On a made-to-size flat roof rooflight or a lantern, external shading is usually a separate architectural element rather than a clip-on accessory, which is a different conversation covered further down.
What an internal blind is genuinely good at
Internal blinds do three things that no external product does. They give blackout, which matters in a bedroom under a north-facing slope in June as much as under a south-facing one. They give privacy at night, when an unshaded rooflight becomes a lit panel visible from surrounding houses. And they control glare, which is a visual comfort problem rather than a thermal one, and is the reason a home office under a rooflight needs something at the glass regardless of orientation.
They are also cheaper, fitted from inside in twenty minutes, unaffected by weather, and replaceable when the fabric dates. None of that is trivial. The mistake is only in expecting them to keep a room cool.
The two positions side by side
| External awning | Internal blind | |
|---|---|---|
| Typical reduction in solar heat entering the room | 60 to 80 per cent | 10 to 40 per cent |
| Effect on gtot, glazing plus shade combined | Brings a 0.50 unit to roughly 0.10 to 0.20 | Brings a 0.50 unit to roughly 0.35 to 0.45 |
| Blackout | No, mesh fabrics stay translucent | Yes, with a side-channel blackout fabric |
| Glare control | Good, diffuses rather than blocks | Good to excellent depending on fabric |
| View out when deployed | Retained, softened | Lost with blackout, partial with a screen fabric |
| Weather exposure | Fully exposed, retracts in high wind | None |
| Typical service life of the fabric | 8 to 15 years in this climate | 15 years or more |
| Access for cleaning and replacement | From outside, so scaffold or a tower on an upper storey | From inside, from a step ladder |
| Planning sensitivity | Visible externally, matters in conservation areas | None |
Fabric colour, openness factor and the mesh you can still see through
External awning fabrics are woven meshes with an openness factor, usually somewhere between 1 and 10 per cent, describing how much of the fabric area is actually holes. A lower openness factor blocks more and obscures the view more. Around 3 to 5 per cent is the common compromise: the sky stays visible as a soft grey field, and the direct beam is broken up.
Colour behaves differently outside and inside. Outside, a light fabric reflects more radiation away before it is absorbed, so it performs slightly better thermally, but it scatters more light forward and can look brighter and hazier from inside. A darker external fabric absorbs more but sheds that heat to outside air, and gives a crisper view out with better glare control. Inside, the logic reverses completely: a light, reflective internal fabric is meaningfully better than a dark one, because reflecting radiation back out through the glass is the only mechanism an internal blind has for removing energy from the room.
Reflective backing, and the one internal blind that behaves half like an external one
An internal blackout blind with an aluminium-faced reflective backing, run in side channels that close the gap at the edges, is the best-performing internal option by a clear margin. The reflective face bounces a portion of the incoming radiation straight back out through the glass before it is absorbed, and the side channels stop warm air circulating around the edges of the blind and washing down into the room.
It still cannot match an external awning, because whatever the backing does not reflect is absorbed on the room side of the glass. But a side-channel reflective blackout blind reaching perhaps 40 per cent reduction is a different product from a loose pleated fabric reaching 15, and where an external blind cannot be used, it is the specification worth paying for.
Honeycomb and pleated blinds, and the still air behind them
Honeycomb, or cellular, blinds trap air in a row of closed cells across the width of the fabric. In winter that trapped air is genuinely useful, adding a small amount of insulation at the glass overnight and taking the edge off the radiant chill of a cold pane above a bed.
In summer the same trapped air works against you when the blind is dark and unbacked, because the cells slow the movement of heat away from the fabric and hold it against the glass. A honeycomb blind with a reflective outward-facing surface gets both benefits. One without it is a winter product being asked to do a summer job.
Blinds inside the sealed unit
Integral blinds, sealed between the panes of the glazing unit and operated magnetically or electrically, exist and have obvious appeal on a rooflight: nothing to clean, nothing to dust, nothing hanging below a ceiling that is three metres up.
Thermally they sit between the two other options, because the blind is behind the outer pane but in front of the inner one, so some of the absorbed heat is shed outward and some inward. The practical caution is that the blind is now inside a sealed unit with a service life of its own, so a mechanism failure means new glass rather than a new blind. On a roof, where units are already working harder than vertical glazing, that is a real consideration.
Fixed shading: overhangs, brise soleil and a tree that already exists
Movable shade is not the only external option. A projecting overhang above a near-vertical rooflight in a dormer cheek, a brise soleil fin above a lantern, or a deciduous tree to the south west of the house all shade without any mechanism to maintain.
Fixed shading works best where the sun angle it needs to exclude is high and the daylight it needs to admit is low, which is exactly the summer and winter split at this latitude. An overhang sized to cut out the 60 degree midsummer sun will let the 15 degree midwinter sun straight through. It is far less effective for west-facing glass, because late afternoon sun in July is low and comes in almost horizontally, under any overhang that would be practical to build.
Power, sensors and how an external blind actually gets operated
An external blind that is inconvenient to operate is an external blind that stays retracted through a heatwave. The manual versions on roof windows are worked by a rod from inside, which is fine at the head of a stair but tiresome in a bedroom used every day.
Solar-powered units carry a small photovoltaic cell on the cassette and a rechargeable battery, and they are the common choice because there is no cable to run and no notifiable electrical work. They charge from daylight, so a north-facing slope charges more slowly than a south-facing one but still charges. Mains-powered units are more reliable on shaded elevations and can be integrated with the same control as an electric opening unit, which is covered on electric rooflight installation. Either way, an automatic wind sensor that retracts the fabric in a gust is not optional on an exposed site.
Weather, cleaning and what Gloucestershire does to an external fabric
An external blind lives outside all year. The fabric is designed for it, but on a roof it sits in standing water when retracted badly, collects leaves under the cassette, and grows algae on a shaded elevation. The cassette gasket ages. The side guides collect grit and stiffen.
Realistically it wants a wipe and a check once a year, and that means being able to reach it. On a single-storey rear extension that is a step ladder. On a second-floor loft window in a Stroud valley terrace where the ground drops away behind the house, it is a tower or scaffold, and the honest position is that the maintenance cost over fifteen years may exceed the cost of the blind. That belongs in the decision at the outset, not afterwards.
Cheltenham conservation areas, listed stucco and what is visible from the street
External shading is a visible alteration to the outside of a building, and in central Cheltenham that changes the conversation. Lansdown, Montpellier, Pittville and the Bath Road terraces are stucco with Welsh slate mansards, much of it listed, much of it inside a conservation area, and a fabric cassette bolted to the outside of a rooflight on a visible slope is not going to be welcomed.
The geometry helps more than the rules do. A great many of these roofs are mansards with a shallow upper slope hidden behind a parapet, so the rooflight is invisible from the pavement and so is anything fitted to it. Where the unit sits on the steep visible face, external shading is difficult and the specification moves instead to solar control glass plus a reflective internal blind. On a listed building, consent covers alterations whether or not they can be seen, so the sensible order is pre-application advice with photographs before anything is ordered.
Solar control glass compared with a shade you can put away
A solar control coating on the outer pane reduces the solar factor permanently, typically taking a unit from around 0.50 down to 0.25 or 0.30. It needs no operation, no power, no maintenance and no consent, and it works on the days nobody thinks to close a blind.
The cost is that it works in February as well as in July. On a south-facing Cotswold slope where the winter sun is genuinely useful, permanently rejecting three-quarters of it is a loss. On a west-facing loft conversion, where the July afternoon is the problem and the February afternoon is worth very little, the permanent coating is close to free. The trade is a real one and it is decided by orientation, not by preference. How the g-value itself is chosen is on U-values and glazing specification, and which slope you are on is on specifying glazing by which way the slope faces.
What a south-west facing Cotswold loft conversion actually needs
Take the common Gloucestershire case: a loft conversion on an interwar semi in Warden Hill or Charlton Kings, or a cottage on a south-west facing slope above Painswick, with two roof windows in a bedroom under an uninsulated-looking ceiling and a July afternoon sun that arrives at 30 degrees above the horizon and shines straight down the slope.
The layered answer is solar control glass because it works unattended, an external awning because it is the only thing that will hold the room down on the worst five afternoons of the year, and a blackout internal blind because it is a bedroom and the sun rises at a quarter to five in June. Each is doing a different job and none substitutes for another. Adding purge ventilation, an opening unit high in the room that lets the hot air out, completes it, and is the part most often left out.
What to fit on the day, and what can sensibly be added later
Glass specification is the one decision that cannot be revisited without new glass, so the coating, the laminate and the solar factor are settled before the unit is ordered. Everything else can be sequenced.
External awnings clip to the frame and can be added in a later year, provided the frame is one the accessory is made for, which is a reason to note the size code at the time of installation and keep it. Internal blinds are entirely independent and can be chosen after living with the room through a summer, which is often the more sensible order because people are consistently wrong about which slope will bother them. Where a mains-powered external blind is even a possibility, the cable is worth running while the reveal is open, because a chased cable in finished plaster later is the expensive version. That is the same principle as everything else on the roof: the things that are cheap while the scaffold is up are expensive once it has gone.
Next on this subject.
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- Guide Solar gain, overheating and shading Overhead glazing collects far more solar energy in July than in January, and the ratio is not close. A rooflight…
- Guide Condensation, ventilation and rooflights Surface condensation on a rooflight is a whole-house ventilation problem showing up at the coldest point in the building. The…
- Guide Triple glazing in a rooflight and when it pays Triple glazing in a roof is worth more than triple glazing in a wall, and for a reason that has…
- Guide Warm-edge spacers and where condensation starts Watch any rooflight through a cold week and the water always arrives in the same place first: a band following…
- Guide Low-emissivity coatings and argon fills Roughly three quarters of the heat crossing an uncoated sealed unit travels as infrared radiation from the warm inner pane…