Roof windows in a bedroom and blackout

What blackout means on a sloping window

Blackout is a property of a system, not of a fabric. A blackout fabric blocks essentially all light through its face. That is the easy part and almost every manufacturer offers it. The difficulty is the perimeter: the gap down each side, the gap at the head, and the gap at the cill where the fabric ends.

On a vertical window a blind hangs against the wall and the gaps are narrow. On a roof window the blind is on a slope, so unless something physically holds the fabric down against the frame, it sags away from it and light runs along both edges. A blackout fabric in a system with open sides produces a dark rectangle with two bright lines beside it, which is arguably worse than no blind at all.

So the specification to ask for is not blackout fabric. It is a blackout system with side channels, which is the piece of hardware that actually does the work.

Side channels, and why they decide everything

A proper roof window blackout blind has aluminium channels screwed to the side frames of the sash, running the full travel of the blind. The fabric carries a thin bead or wire at each edge that runs inside those channels, so the fabric is held down against the frame for its entire length and cannot lift.

The channels do two things. They close the side gap, and they keep the fabric under slight tension so it stays flat instead of bowing. A blind without them, however dark the fabric, is a light-reducing blind rather than a blackout blind.

They are visible. Two slim grey or white channels sit either side of the glass, roughly 20mm wide, and they are part of the appearance of the window from inside. People occasionally reject them on that basis and then find the room is not dark. It is worth seeing one before deciding.

The bottom bar and the last gap

With the sides closed, the remaining leak is at the bottom bar where the fabric stops. Good systems land the bar into a recess or against a stop at the cill, and the bar itself is deep enough to close the gap when it is parked.

The failure here is a bar left short of its stop, which happens when a blind is fitted slightly high or when the cord has stretched. A 5mm gap along the full width of a window is a bright line at floor level and it is exactly the thing that wakes somebody at dawn.

Check it at fitting by standing in the room with the light off and the blind down in daylight. That takes two minutes and it is the only test that matters.

Summer sunrise, and the hours a bedroom is actually fighting

Around Oxford, midsummer sunrise is a little before five in the morning and civil twilight starts around forty minutes before that. So a bedroom is dealing with usable daylight from roughly a quarter past four in mid-June, and that is the specification driver for anybody who does not want to be awake then.

Direction matters less than people expect on a roof window, because the glass faces upward and picks up sky brightness from a wide arc rather than direct sun from one bearing. A north-facing slope still fills with light at four in the morning in June. It simply does it without the sun in the frame.

East-facing slopes get direct low sun on top of that, and they get it at the worst possible hour. Where a bedroom roof window faces broadly east, the blackout decision is not optional and an external option is worth considering alongside it.

Where the window sits relative to the bed

Directly above the head of the bed is the position people regret most. Lying on your back you look straight into the brightest part of the sky, cold air falls off the glass onto your face in winter, and rain lands two metres above your head.

Across the room from the bed is the easiest position to live with. The window gives light and view without being in the sightline from the pillow, and any residual light leak is at the far end of the room rather than immediately overhead.

Where the bed position is fixed by the shape of the loft, and it often is, then the blackout specification has to be better rather than adequate. A window directly over a bed with a partial blind is a permanent low-level annoyance.

Operating a blind you cannot reach from the bed

A bedroom blind gets operated twice a day for years, and a control that is awkward is a control that stops being used. Reach matters more here than in any other room.

Manual blinds with a bar you push and pull are fine where the window is low enough to touch, which generally means a head height under about 1,900mm. Above that you need a control rod, which works but is a stick that has to live somewhere in a bedroom.

Electric blinds solve it properly and are worth the cable in a bedroom more than anywhere else, because a blind on a switch or a timer beside the bed will actually be used. The general case for and against powered blinds is covered on our page about when to specify roof window blinds.

Running the cable before the ceiling closes

The decision that costs money if it is missed is the power supply. An electric blind needs a supply at the window head, and once the plasterboard is up and skimmed, getting a cable there means opening a sloping ceiling.

Run a spur to the head of each bedroom roof window during first fix, even if the blind is not being ordered yet. The cost at that stage is trivial. The cost afterwards is a decorator.

Solar-powered blinds exist and they remove the cable question, with a small panel on the external face of the sash and a battery in the head. In a north-facing bedroom they charge more slowly than the marketing suggests, so they suit brighter slopes better.

Overheating, which is a separate problem

Darkness and heat are not the same specification. An internal blackout blind stops light entering the room but the solar energy has already passed through the glass by the time it reaches the fabric. The blind heats up, and the heat goes into the room anyway.

A loft bedroom under a south or west slope can be unusable on a July afternoon, and pulling the blackout blind down at three in the afternoon makes it darker and no cooler. This surprises people every summer.

If overheating is the issue, the answer is to stop the energy at the outside of the glass, which means an external awning or solar-control glazing rather than any internal fabric.

External awning blinds on a bedroom window

An external awning is a mesh screen that sits on the outer face of the sash, held in guides, and it intercepts solar gain before it reaches the glass. It reduces heat entering the room dramatically, far more than any internal blind, and it still allows a view out because the mesh is translucent.

What it does not do is darken the room. Awnings are made for heat, not for sleep, and the two products are frequently used together: an awning outside for summer afternoons, a blackout blind inside for the night.

They are exposed to weather and to leaves, and they need retracting in high wind unless the unit does that automatically. On an accessible slope that is easy. On a high slope, specify a version with a wind sensor.

Glazing choices that reduce the load

Solar-control glass carries a coating that reflects a portion of solar energy at the outer pane, reducing what enters without visibly tinting the room. It is specified as a g-value, and a lower g-value means less solar heat through the glass.

For a bedroom on a south or west slope, solar-control glazing is worth specifying at order rather than trying to correct with fabric later. It costs a modest amount at the point of purchase and nothing afterwards, whereas an awning is an ongoing object to manage.

It has no effect on darkness. Solar-control glass at night is ordinary glass, and the sky at four in the morning comes through it exactly as before.

Light from streetlights and neighbours

Roof windows collect light from sources that a wall window never sees, because they look upwards at a hemisphere. A neighbour’s security light, a streetlight on a tall column, and general sky glow all arrive at a roof window without anything in the way.

The practical consequence is that a bedroom roof window in a built-up part of Oxford can be noticeably brighter at midnight than a ground-floor bedroom in the same house. Somebody who sleeps fine downstairs may not sleep fine in a converted loft.

The remedy is the same blackout system, with the difference that these sources run all night rather than at dawn. It reinforces the case for doing the side channels properly rather than relying on a fabric alone.

Rain over a bed, and what people actually notice

Rain on a roof window is audible, and directly over a bed it is more audible than anywhere else in the house. Some people find it pleasant. Some find it keeps them awake, and there is no way to know which you are until the window is in.

The variable that matters is the glazing. A laminated inner pane damps the sound noticeably compared with a standard toughened one, and it is a specification made at order. There is a fuller treatment on our page about rain noise.

An internal blind makes essentially no difference to the sound. It is a light control, not an acoustic one, and expecting otherwise leads to disappointment.

Ventilation in a dark bedroom

Two adults asleep put out roughly a litre of water vapour overnight, and a loft bedroom has fewer routes for it to leave than any room downstairs. Closing a blackout blind does not stop ventilation, because the blind sits inside the sash and the vent slot is in the head of the frame above it.

What does interfere is a badly fitted blind box that sits over the vent flap and stops it being operated. Check that the vent can still be reached with the blind fully retracted, because a vent nobody can operate is a vent nobody uses.

The detail of vents and their filters is covered on our page about ventilation flaps and filters. In a bedroom, the flap should be left open through the night.

Escape openings in a loft bedroom

Where a roof window is providing the escape opening for a loft bedroom, the blind must not obstruct it. That means a blind that retracts fully clear of the openable area, and no fixed screen, secondary glazing or shutter arrangement across the aperture.

It also means the household needs to know how to get the blind out of the way in the dark, which is a fair argument for a manual blind that can be pushed up by hand as well as an electric one that depends on power.

The requirements themselves are set out on our page about escape openings and the rules. Where escape applies it constrains the window before anything else does.

Children’s bedrooms

Two things change in a child’s room. Darkness matters more, because a child’s sleep is more easily disrupted by early light, and blind cords become a safety question rather than a convenience one.

Roof window blinds are generally operated by a bar or a rod rather than a looped cord, which is one of the genuine advantages of the format. Where a system does use a cord, it needs a cleat or a breakaway device fitted and used, and the cord kept out of reach of a cot or a bed.

Restricting the opening of the window itself is a separate matter, and is dealt with on our page about handles, catches and child safety.

Fitting a blackout blind to an existing window

Most current roof windows have moulded blind fixing points already in the frame, so a blackout system with side channels goes on without drilling into the sash face. Older units may not have them, and the channels then need screwing to the frame, which is straightforward but permanent.

You need the model code and the size code from the data plate, which on most units is on the top rail of the sash and visible when the window is rotated. Ordering a blind by measuring the aperture rather than reading the plate produces a blind that does not fit the channels, which is the most common ordering mistake.

Where a window is very old, the blind range for it may have ended, and the sensible answer is sometimes to replace the unit rather than hunt for discontinued accessories. We will tell you which situation you are in.

What we set out for a bedroom at survey

For a bedroom we ask three things before specifying anything: which way the slope faces, where the bed will sit, and whether the room is used by somebody who wakes with the light. Those three answers set the glazing, the blind system and, quite often, the position of the window itself.

We then agree the cill line in the room with the household present, mark the rafters, and check that a blind and its channels will clear the reveal and the opening mechanism before anything is ordered. Sightlines are covered further on our page about cill height and the view out.

The unit is the small half of the job. The perimeter detailing decides whether it stays dry, and the roof is never left open overnight. You get a range by email first, then a fixed price after a survey, with the workmanship guaranteed for ten years. Call 01865 704245 or email info@heritageskylightsoxford.co.uk.