An internal blind and an external awning both reduce what comes through a rooflight, and they do different jobs. One controls glare very well and heat poorly. The other controls heat very well and is exposed to the weather. Choosing between them starts with which problem you actually have.
The physics, in one paragraph
Solar energy arriving at a rooflight either gets into the room or it does not.
An external shade intercepts it outside the glass. The energy it absorbs warms a surface that is in open air, and the air carries it away. Almost none of it reaches the room.
An internal blind intercepts it after it has passed through the glass and entered the room. The energy is already inside. The blind absorbs it, warms up, and re-radiates into the space from a surface directly above your head. It has stopped the light and kept the heat.
What that means in practice
| Problem | External shading | Internal blind |
|---|---|---|
| Overheating | Very effective | Limited. The heat is already in |
| Glare | Effective | Very effective |
| Blackout for sleeping | No | Yes, with side channels |
| Privacy after dark | No | Yes |
| Winter heat retention | No effect closed at night | A small benefit |
| Exposed to weather | Yes, entirely | No |
Two products, almost no overlap in what they are good at. Which is why the answer is frequently both rather than either.
The order to decide in
- Glazed area. Only available at design stage and the largest lever there is.
- Glass specification. Solar control chosen for the orientation, fixed at ordering.
- Ventilation. Removing heat that arrives anyway, at the highest point of the room.
- External shading. Where the first three leave a problem.
- Internal blind. For glare, blackout and privacy, which nothing above addresses.
Buying a blind to solve overheating is starting at five for a problem that belongs at two, three or four. It is the commonest mistake in this area and it is expensive because it does not work.
What external shading looks like on a rooflight
On a pitched roof window, an external awning blind: a fabric screen on the outside of the glass, running in guides, retracting into a cassette at the head. Usually powered, frequently solar powered, and sometimes with a wind sensor to retract it in a gust.
On a flat rooflight or a lantern, the equivalent is less common as a proprietary product, and the practical answers are shading from the building itself: a brise soleil, a deep reveal, a projecting canopy, or a pergola over the glass.
There is also planting, which is the oldest external shade there is and works remarkably well where a deciduous tree happens to sit in the right place.
How much better external actually is
Substantially, and it is worth being concrete about the shape of it.
An external shade over glazing rejects the large majority of the solar energy that would otherwise arrive at the glass. An internal blind of similar fabric rejects a much smaller proportion of the heat, because the energy is already through.
The difference is not marginal. It is the difference between a measure that solves overheating and one that makes the room darker while it overheats.
Why external shading is uncommon here anyway
Four reasons, and they are all practical rather than technical.
- Weather. An external fabric on a Sussex roof is in wind, rain and salt permanently, and it has a service life measured in years rather than decades.
- Wind. On this coast an awning is a sail. Most need a wind sensor to retract, which is another system to maintain.
- Access. Cleaning and servicing something on the outside of a roof needs roof access.
- Appearance. It changes how the roof looks from outside, which in a conservation area or the National Park is a planning consideration.
Which is why, on the roofs we work on, the usual answer is good solar control glass plus ventilation plus an internal blind, and external shading is reserved for the genuinely difficult cases.
Where external shading does earn its place
A west facing lantern over a kitchen used in the evening, unshaded, where the glass specification has been pushed as far as it usefully goes and the room is still uncomfortable in July.
A large horizontal glazed area over a single storey extension with a single aspect and no cross ventilation.
And any situation where the glazed area cannot be reduced and the room genuinely cannot be ventilated properly. In each of those the external route is the only one with enough effect left in it.
Shading from the building itself
The version that needs no maintenance and no mechanism, and it is decided at design stage or not at all.
A first floor set back over a single storey extension shades the roof below it for the middle of the day. A deep reveal on a pitched rooflight shades the glass at high sun angles and lets low winter sun straight in, which is exactly the seasonal behaviour you want. A pergola or a canopy over part of a flat roof does the same.
Fixed shading is geometry rather than equipment, and geometry does not need servicing. Where an extension is at design stage it is worth raising, because it is free then and impossible afterwards.
Deciduous planting
Worth a serious mention rather than a romantic one.
A deciduous tree to the south or west of an extension shades the roof in summer when it is in leaf and lets light through in winter when it is not. That is precisely the seasonal response you would design if you could.
The costs are real: leaf fall onto the roof and into the outlet, debris in a recessed rooflight, and eventually roots and shade in places you did not want them. On this coast, salt limits what will thrive close to the sea.
Where a tree already exists in the right place, it is worth valuing before removing it. Where one is being planted, the position wants thinking about for thirty years rather than five.
Internal blinds, and what they are genuinely for
Three things, all of which matter and none of which is heat.
Glare. Direct sun through a rooflight produces a bright patch that moves across the room, and on a screen or a work surface it is the thing that makes a room unusable regardless of temperature.
Blackout. A rooflight faces the sky and the sky is bright from before four in the morning in June. In a bedroom this is the reason blinds exist.
Privacy after dark. An overhead window is private in daylight and not at night, particularly on rising ground where a rear roof is overlooked from above.
The small thermal benefit internal blinds do have
In one direction, at one time of year, and it is worth acknowledging.
A blind closed over a rooflight at night in winter traps a layer of still air against the glass, which reduces heat loss through it a little. On a large area of overhead glazing in a bedroom, that is a small but genuine improvement in comfort as well as in heat.
The same still layer makes the glass behind the blind colder, which means more condensation forms on it, which is covered on the condensation pages. It is a trade rather than a free gain.
Reflective backings
Some blind fabrics carry a reflective, usually aluminised, backing on the outward facing side.
It helps, by reflecting part of the solar energy back out through the glass before the fabric absorbs it. It is still working on the wrong side of the glazing, so it improves an internal blind rather than turning it into an external one.
Where an internal blind is being fitted in a room with a solar gain problem and no external option, a reflective backed fabric is worth asking for. It is not a substitute for the glass specification.
Between glass blinds
Blinds sealed inside the cavity of the unit exist and turn up in enquiries.
They are protected from dust, they never need cleaning, and they are operated magnetically or electrically. Thermally they are between the two: better than internal, because the energy is intercepted before the inner pane, and worse than external, because it is already inside the outer one.
The considerations are that the unit is a sealed assembly with a mechanism in it, so a failure means replacing the glass, and availability in rooflight products is more limited than in doors and windows.
What we would specify for a Sussex kitchen extension
The common case, and the answer is fairly consistent.
Neutral solar control glass chosen against the orientation. An opening rooflight at the highest point of the room, with a rain sensor. A low opening on the opposite side of the room for a flow path. And an internal blind for glare and for the evenings.
That combination handles the great majority of extensions here. Where it is not enough, the conversation moves to external shading or to glazed area, in that order.
What to do about a room that already overheats
Where the extension is built and the glass is in, the levers are fewer.
Ventilation is usually the most effective retrofit and the least often considered, because everybody looks at the glass first. Adding an opening rooflight, or an extract, changes a room more than a blind will.
Then external shading, then applied film on the glass with the caveats on the solar control page, then internal blinds for the glare that remains. An internal blind first is the natural instinct and the least effective order.
Wind, and the reason external shading needs a sensor
An external awning extended over a rooflight is a fabric panel in the wind, held by a cassette and two guides.
On the scarp or the seafront that is a genuine load, and the standard answer is a wind sensor that retracts the shade automatically above a threshold. Which means it retracts on exactly the bright, breezy afternoons when you wanted it, and that is a real limitation rather than a quibble.
It is one more reason external shading is less common here than in a calmer climate, and one more reason the glass specification is doing the heavy lifting.
Getting the timing right
Shading is only useful when it is deployed, and on anything manual that means somebody being there.
The peak solar load on a south facing rooflight is around the middle of the day, when a good many houses are empty. A blind that gets closed when somebody comes home at six is closing after the heat has arrived, which is most of why internal blinds disappoint on the heat question.
Powered shading on a timer or a sensor closes before anybody is there, which is the arrangement that actually works. It is another argument for deciding the cabling question while the ceiling is open, and another case where solar powered products earn their place on a retrofit.
The question we start with on the glazing
We ask whether the problem is heat or glare before recommending anything, because the two have different answers and they get conflated. Where it is heat we look at glazed area, glass and ventilation before shading of any kind. Where a blind is wanted for glare, blackout or privacy we specify it with the window so the fixings are concealed, and we say plainly that it is not a heat measure.
Stopping energy outside the glass works. Stopping it inside makes the room darker and no cooler.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.