Do blinds reduce heat through a rooflight or skylight?
Yes, and the position matters far more than the fabric. An external blind or awning stops solar energy before it reaches the glass and is dramatically more effective. An internal blind stops the light after the energy has already entered the room, then warms up and re-radiates much of it inward. Both help; they are not comparable.
Why outside beats inside
Because of where the energy is stopped rather than what stops it.
An external blind absorbs and reflects sunlight in open air, where the heat it picks up is carried away by the wind. Very little of it reaches the room.
An internal blind absorbs the same energy inside a sealed room, where it has nowhere to go except into the air around it.
How big the difference is
Substantial rather than marginal. An external blind can reject the large majority of the solar energy falling on the glazing. A good internal blind removes a considerably smaller share, because it is fighting a battle already half lost.
Which is why in hot climates external shading is standard and internal blinds are a comfort item.
Why internal blinds are what people fit
Practicality rather than performance. An internal blind is cheap, easy to fit, never exposed to weather, and needs no maintenance. An external one on a roof is exposed to everything, harder to fit and considerably more expensive.
On a domestic rooflight most households end up with an internal blind, and it is a reasonable choice made for the right reasons.
What an internal blind is genuinely good at
Glare, privacy and darkness, which are not the same problem as heat.
Where a room is uncomfortable to work in because of brightness, or a bedroom is unusable in June because of early light, an internal blind is exactly the right answer.
Heat is the one thing it addresses least well, and it is frequently what it is sold on.
Reflective backing
The specification that makes an internal blind work harder.
A blind with a metallised or white reflective face toward the glass sends more of the energy back out through the glazing than a dark fabric does.
It is a modest improvement and it is free at the point of choosing, so it is worth asking for.
The gap around a blind
Where most internal blinds lose their effect, and it is fixable.
Air warmed between the blind and the glass rises and spills into the room around the edges. A blind running in side channels traps that air instead.
Channelled blinds are standard on most rooflight ranges and they perform noticeably better than a loose one.
Blackout against translucent
A choice about the room rather than about heat.
A blackout fabric blocks light entirely, which in a bedroom is the point. A translucent one diffuses it, which keeps a room usable while cutting glare.
Thermally the difference is small compared with whether the blind is channelled and what its backing is.
External options on a roof
Fewer than on a wall, and they exist.
External awnings made for rooflights are available on the main ranges, generally as a mesh that rolls down over the glass, operated manually or electrically.
They are visible from outside, which on a designated slope is a consideration, and they are the most effective single thing available for summer comfort.
Glass and a blind together
Complementary rather than alternative. Solar control glass rejects energy year round without anybody doing anything. A blind is deployed when it is wanted and put away when it is not.
On a south facing unit over an occupied room, both is the right answer.
What the glass does is on what is solar control glass.
Ventilation matters more than either
The point worth making, because blinds get asked about and ventilation does not.
A room that can purge warm air through a high level opening will be comfortable in conditions that would defeat any blind in a sealed room.
What actually causes overheating is on will a rooflight make my room too hot.
Deciding at order
The practical recommendation, and it saves money.
Manufacturer blinds fit into channels built into the frame, which makes them neat and specific to the unit. A generic blind added later rarely sits as well and can foul an opening sash.
Electric blinds
Worth considering on anything out of reach, and the cable is the decision.
A powered blind needs a supply at the opening, which is minutes of work while a ceiling is open and a separate job afterwards.
Even where a manual blind is wanted now, running the cable and capping it costs almost nothing.
Solar powered blinds
An option that removes the cable question entirely.
Some ranges offer blinds with an integral photovoltaic cell and battery, charged by the light coming through the unit they are fitted to.
On a retrofit where running a cable would mean opening a ceiling, they are a genuinely useful answer.
Condensation behind a blind
A consequence worth knowing about in a humid room.
A closed blind keeps warm room air away from the glass, which makes the glass colder, and moisture can then condense on it and sit there because the blind restricts air movement.
It is not a fault. It is a reason to open a blind occasionally in a kitchen or a bathroom.
Cleaning
The maintenance nobody mentions. A blind directly overhead collects dust on its upper face, which is invisible until it is lowered on a sunny day. Most are removable and washable, and it is worth checking before choosing.
What we specify as standard on the glazing
We specify a channelled blind with a reflective backing where heat is the concern, and we recommend an external awning where a room genuinely overheats and the slope is not visually sensitive.
The whole picture is on the energy efficient rooflights page.
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.