Does a thermal blind keep a rooflight room warm?

Picture a loft bedroom at midnight in January. The glass above the bed is the coldest surface in the room, and warm air rising from the radiator meets it and sinks again as a cool draught.

That is the job a blind can soften. Glass loses heat by touch, because room air against a cold pane gives up its warmth, and by radiation, because your body and the walls send heat towards the cold surface. A blind interrupts both. It puts a second surface between you and the pane, and if it is shaped to trap air, it adds a cushion of still air that conducts heat poorly.

Still air is the useful ingredient. Moving air carries heat away, while a thin, sealed pocket of it holds heat in, which is the reason a double glazed unit has a cavity. A thermal blind is a smaller, movable version of the same idea, added inside the room.

The effect is easiest to feel as comfort. The surface you look at is warmer to the eye and the hand, the cool downdraught weakens, and the radiator has less work to do to hold the room steady. It is an improvement you notice before you can measure it.

Not every blind insulates. A plain roller blind of thin fabric does very little, and the difference lies in construction.

• Honeycomb, or cellular. The fabric is folded into rows of small hollow cells, like a beehive in section, each cell holding a pocket of air. Double-cell types have two rows and trap more.
• Thermal-backed. A roller or pleated fabric with a reflective or foam-like layer behind it, designed to turn heat back towards the room.
• Blackout with a backing. Some blackout blinds carry a coated back that also slows heat loss, which is useful in a bedroom.

The shape matters as much as the fabric. The benefit comes from the pocket of still air, so a blind that hangs loose with a wide gap at each side lets air circulate behind it and loses most of its effect. The best fits run in side channels or rails fixed to the rooflight frame, so the edges are close and the air behind stays put.

The gain varies with the room and the roof, which is why the same blind can seem worthwhile in one house and pointless in the next.

The north-facing case is the clearest. That window gains little from the sun, so closing the blind costs you almost nothing in free warmth, while the heat loss goes on all night. On a south-facing roof the trade is different. Low winter sun through the glass is welcome heat, so the blind should stay up in the day and come down once the sun has gone. Our page on whether a rooflight can warm a room in winter explains the daytime side, and choosing by orientation shows how facing changes the choice.

On a modern sealed unit with a low-E coating and a warm-edge spacer, the glass is already doing much of what a blind would, so the extra gain is smaller. A blind there is more about comfort and light control than a big change to the heating. The case is strongest where the glass is old, which is one reason to read how an old rooflight can be upgraded before you spend on a blind to compensate.

A blind keeps the room's warmth away from the glass, and that has a side effect worth knowing about.

With the blind down, the pane behind it gets colder than it would otherwise be, because the room's warmth no longer reaches it. If warm, moist air gets past the edges, it meets that cold glass and condenses there, out of sight. A blind that closes tightly but lets moist air leak round the sides is the worst combination, because it wets the glass and the frame without you seeing.

Two habits help. Raise the blind in the morning so the glass can warm and dry, and keep the room ventilated, especially a bedroom or a kitchen. A frame with a thermal break and a warm-edge unit is also far less likely to collect water at all, as we describe in cold bridging around a rooflight.

Rooflights are awkward for blinds, so the method of fitting is part of the choice.

Roof windows usually take a blind made by the window's own maker, running on side rails so it lies flat against the glass. Flat-roof rooflights and lanterns are harder, since the glass may sit high, be sloped or be made of several panes, and a custom blind in a frame is the usual answer. Where a rooflight is high above a stairwell or an extension, an electric blind on a remote saves climbing. Our hub on electric and opening rooflight installation covers powered units, and the roof window installation page covers the pitched case.

If you are fitting a new unit, it is best to decide on the blind first, so rails, wiring and a clear reveal are built in and not added awkwardly afterwards. A deep, square reveal can block a blind from sitting close to the glass, while a splayed one leaves room. It is easier to get right at the survey than after the plaster is done.

There is a sound angle too. A heavier cellular blind softens the tone of rain in a room, though it does not stop the glass vibrating, and it is no substitute for a laminated acoustic pane. That choice goes into the quiet spec. To talk through glazing and blinds for your room, get a quote.

Will a thermal blind cut my heating bill?

It may trim it, but the effect is modest and depends on the glass behind it. It is better seen as a comfort measure than a saving you can rely on, and it will not repay itself quickly on a good modern unit.

Is a honeycomb blind better than a blackout blind?

For warmth, a double-cell honeycomb usually does more, because it traps more air. A blackout blind is the choice where darkness matters most, and some carry a backing that helps with heat as well.

Can I leave the blind down all day?

It is not wise. The room loses daylight, and the glass stays cold and damp behind the blind. Raising it by day lets the pane warm, dry out and let light in, which is the reason you fitted a rooflight.

Would better glass beat a blind?

For the same money, yes, usually. The page on how heat escapes through a rooflight shows why, and the energy efficient rooflights hub sets out the options.