External shading against internal blinds

The physics

Solar energy passing through glass is absorbed by whatever it lands on, and re-radiated at a longer wavelength that glass does not transmit back out. It is trapped.

An internal blind is inside the glass. It intercepts the light, absorbs the energy, and becomes a warm surface radiating into the room. The heat is already in.

An external shade is outside the glass. Energy is absorbed and re-radiated to the outside air, and it never enters the building.

Effectiveness

External shading typically reduces solar gain by 70 to 80 per cent. A good internal blind manages perhaps 30 to 40 per cent, and much of what it stops it converts to heat in the room.

Why external is rare domestically

Cost, because it is a mechanism mounted outside on a roof. Maintenance and weathering. Appearance, since it is visible from outside. And on a rooflight it has to be robust enough for wind and rain.

It is common on commercial glazing and rare on houses for those reasons rather than because it does not work.

What each is for

External shading: heat, on large glazed areas.

Solar control glass: heat, on domestic rooflights. Nearly as effective as external shading, permanent, invisible and needing no operation. This is the practical answer.

Internal blinds: glare, privacy and blackout. Legitimate reasons, all of them, and none of them is temperature.

The order

Glass first, at order, because it cannot be changed later. Blinds afterwards, for the jobs blinds actually do.

Where the heat is stopped decides everything

An internal blind stops sunlight after it has passed through the glass. An external blind stops it before. That sounds like a small distinction and it is the whole of the difference between the two products.

Once solar energy is through the glazing it is inside the room. An internal blind absorbs it, warms up, and re radiates much of that heat into the space. It has intercepted the light but not the energy.

An external awning intercepts both, outside the building envelope, where the heat it absorbs is carried away by the air rather than delivered to the room. Typically it stops the great majority of solar gain against roughly a third for an internal blind.

That ratio is why an internal blackout blind on an overheating south facing rooflight makes so much less difference than people expect.

What each one is actually for

Internal blinds are for light and privacy. Blackout in a bedroom, so an east facing rooflight does not wake the room at half past four in June. Diffusion in a bathroom or a study. Glare control over a desk or a screen.

At those jobs they are excellent, and nothing external does them as well, because an external blind either blocks the view entirely or does not black out.

External awnings are for heat. On a south or west facing unit in a room that overheats, they are the most effective retrofit measure available and the only one that does not require touching the glazing.

They are not blackout, they are not privacy, and they are not a substitute for an internal blind in a bedroom.

The third option, which beats both for heat

Solar control glass, specified at order.

It works permanently, needs no maintenance, has nothing to break, nothing to remember to operate and nothing exposed to weather. It stops the energy at the glass, which is the same place an external awning does.

Its limitation is that it is always on. It reduces gain in December as well as July, and it reduces visible light transmission from around 75 to 80 percent down to 60 to 70. On a room used all year that is usually an acceptable trade; on a room where winter light is precious it is a real cost.

An awning, by contrast, is deployed when wanted and retracted when not, which is a genuine advantage over a coating.

The pragmatic answer on most jobs is solar control glass as the baseline for anything facing south or west, with an internal blind where light or privacy needs controlling, and an external awning only where the glazed area is large enough that the glass alone will not cope.

Internal blinds in practice

Mass market roof windows have clips or channels built into the sash, so a purpose made blind fits exactly, goes on without tools, and costs far less than a made to measure blind fitted to a plaster reveal.

Blackout runs in side channels so light does not leak past the edges. That is what distinguishes it from an ordinary blind and it genuinely blacks out rather than dimming.

Pleated or translucent diffuses light and gives privacy without darkness.

Venetian allows light to be directed rather than blocked.

Order them with the window. They are matched to the model and size code, the sizing is certain, and they are fitted on the same visit at no extra access cost. Retrofitting is possible and means another visit.

Steel conservation rooflights generally do not carry these accessory ranges, so blinds become a separate made to measure item fitted to the reveal. Budget for it at the start rather than discovering it at the end.

External awnings in practice

A tensioned mesh fitted over the outside of the glazing, usually retractable, and available manual, mains powered or solar powered.

The mesh is not opaque: you can still see out through it, which surprises people who imagine a shutter. What it does is scatter and absorb the direct beam.

Wind is the design consideration. An awning on a roof is exposed, and a good one retracts automatically above a set wind speed. On an exposed or coastal site that is essential rather than optional, and it is worth confirming the specific product does it rather than assuming.

Maintenance. It is outside, so it collects debris and it has a mechanism. Expect to keep it clear and expect it to have a shorter life than the window.

Access. Fitting and servicing means reaching the outside of the rooflight, which on a first floor slope means a tower or scaffold. Retrofitting an awning to a high level unit costs more in access than in product.

Power and control

Manual blinds are operated by hand or with a pole, and above about two and a half metres a pole becomes a chore and the blind stops being adjusted. That is the practical threshold for motorising.

Mains motorised blinds need a cable, which into a finished room is the same disruptive job as motorising the window itself. Solar powered blinds carry their own panel and battery and need no connection at all, which makes them the sensible retrofit choice.

Where the window itself is already motorised, check whether the control unit has a spare output before assuming new cable is needed: adding a blind to an existing system is frequently a matter of connecting and configuring.

If any work is happening near the rooflight position, run a capped cable to it. Twenty minutes at first fix, and it covers a future motor and a future blind.

How to decide for a given room

Bedroom, any orientation: internal blackout blind, ordered with the window. Add solar control glass if it faces south or west.

Kitchen or living space, south or west: solar control glass and an opening vent at the highest point. Add an external awning only if the glazed area is large.

Kitchen or living space, north or east: no solar control, no shading. Spend the saving on the kerb insulation.

Bathroom: obscure glazing rather than a blind, because a blind in a shower room gets wet.

Study or home office: internal blind for glare, solar control if it faces the sun. A north facing rooflight over a desk needs neither and is the best light there is.

An existing rooflight that already overheats: external awning, because it is the only effective measure that does not mean replacing the glazing.

Why the position decides the physics

Solar energy that passes through glass is inside the building. Whatever it meets next absorbs it and re-radiates it, and most of that re-radiated heat stays in the room.

An internal blind absorbs solar energy on its room side of the glass. It stops the light and the glare, and much of the heat it absorbs ends up in the air of the room anyway.

An external blind absorbs the same energy outside the glass, where the heat is carried away by moving air and radiated back to the sky. It never enters the building.

That is why the difference between them is not marginal. For overheating, external shading does several times what an internal blind does.

What external shading looks like on a roof

An awning blind: a mesh fabric fitted to the outside of the unit, running in side rails, retracting into a housing at the head.

It is a mesh rather than a solid fabric, so it cuts the solar energy substantially while still letting you see out and letting diffuse light in. The room does not go dark; it stops being an oven.

Operation is manual by pole, solar powered with a remote, or mains where the unit is already wired.

What internal blinds are for

Darkness, glare and privacy, and they are the right product for all three.

A blackout blind in side channels is what makes a loft bedroom usable in June, and nothing external substitutes for it.

It also has a genuine thermal effect in winter, trapping a layer of still air against the glass and reducing heat loss on a cold night.

So the two are complementary rather than alternatives. In a south facing bedroom the complete answer is an awning outside for summer heat and a blackout blind inside for darkness and winter insulation.

The third option, specified at order

Solar control glazing, which reduces the g value so less solar energy passes in the first place. Permanent, no moving parts, nothing to operate, and it cannot be retracted on a dull February afternoon when you would welcome the gain.

The sensible combination on a difficult orientation is solar control glass for the baseline and an awning for the peak.

The practical drawbacks of external shading

It is exposed to weather, so it needs to be a product designed for it and it will need occasional cleaning and eventual replacement of the fabric.

It is visible from outside, which on a designated building may be a consideration.

And it needs to be retracted in high wind, which is what the sensors on powered versions are for.

Wind and the awning

An external blind is exposed, and it needs retracting in high wind.

Powered versions have wind sensors for that reason. On a manual one it depends on somebody remembering, which on an inaccessible rooflight is not a plan.