Roof windows in a bedroom and blackout

How dark a bedroom actually has to be

There is a useful number. Sleep research generally puts the threshold at which light starts to interfere with sleep at somewhere around 5 to 10 lux at the eye, roughly the level of a dim corridor. Full daylight outdoors on an overcast morning is 1,000 to 10,000 lux, and direct summer sun is far higher again.

So the job of a darkened bedroom is to knock three orders of magnitude off what arrives. That is achievable, and an opaque blind in properly overlapping channels does most of it. What it will not do is reach zero, because running clearances exist and a perimeter outline of light is normal. The practical target is a room where you cannot read a book at six in the morning, not a room with no photons in it.

Sunrise over Gloucestershire, and the hour the room lights up

At Gloucester’s latitude, around 51.9 degrees north, the sun rises at approximately 04:53 British Summer Time at the solstice and sets around 21:32. Civil twilight starts earlier still, so the sky is genuinely bright by half past four in the morning for roughly eight weeks either side of midsummer.

That is the case the specification is written against. In December the sun does not clear the horizon until nearly eight, and a bedroom with no blind at all is perfectly comfortable. The five months from April to August are what decide whether a room works, which is why a blind bought in October rarely gets specified properly. It also explains why east-facing loft slopes generate more complaints than any other orientation in the county.

Which slope the bedroom sits on, and what the morning does

Orientation changes the problem more than any product does.

An east-facing slope takes the sun directly at first light, at a low angle, straight through glass that is tilted towards it. This is the hardest case, and it is the one where a blackout specification is not optional. A south-facing slope gets less light at dawn and far more at midday, so the issue shifts from waking early to overheating in the afternoon. A west-facing slope is the summer evening problem: a child’s bedroom that has to be dark at half past seven in July while the sun is still full on the glass. A north-facing slope never takes direct sun at all in this country except for a brief early and late clip in high summer, and a room on one is usually the easiest bedroom to darken. Whether north light is worth having is discussed on the north facing slope page.

The reveal is half the darkening, and it goes in first

The plasterboard box between the frame and the room is the thing that decides whether a blackout blind can do its job, because the blind’s side channels have to sit clear of it and its head box has to have somewhere to live.

On a bedroom the reveal is set out with the manufacturer’s published clearance built in on all four faces, and the head lining in particular is held back so a cassette can sit across the top of the frame rather than fouling the plaster. Where the reveal has already been taken hard to the frame, the lining gets cut back and re-skimmed, which is a plasterer and a decorator on top of the blind. It costs nothing to allow for at the point the boards go up, and it is the single most common reason a loft bedroom stays light. The geometry of the box itself is on the reveals and splaying page.

Splayed sides in a room that has to go dark

Splaying the two side linings outward widens the beam of daylight and makes a room brighter, which is the right thing to do in almost every room in a conversion. In a bedroom it has one consequence worth knowing about.

A splayed side reveal means the plaster falls away from the blind channel at an increasing angle, so any light that does escape past the channel is thrown into the room rather than running down a flat face beside it. In practice the effect is small and the extra daylight during the day is worth more than the marginal loss at dawn. Where a room is genuinely required to go dark, for a shift worker or a very young child, a square reveal with the linings brought parallel to the channels is the tighter arrangement, and it is a deliberate trade of daytime brightness for night-time darkness.

A nursery, a shift worker and a teenager are three specifications

The word blackout covers a range of requirements, and they are not the same job.

A nursery needs the room dark for daytime naps as well as at night, and it needs the hardware out of reach of small hands, which usually means a powered blind with the control mounted low on the wall or held by an adult. A shift worker needs the room to hold at sleeping darkness through the middle of a bright summer day for years on end, which is the case where an external roller shutter earns its cost, because it also cuts the heat that would otherwise build up behind an internal fabric. A teenager generally needs the room dark until nine on a Saturday and does not want to operate anything complicated, which a well-channelled internal blackout roller handles perfectly.

A guest room is the fourth case and the one people over-specify. It is used a dozen nights a year, often not in June, and a good dimming roller is usually enough.

Heat in a bedroom under the roof, and the night that never cools

Darkness and heat pull in opposite directions, and this is where bedrooms go wrong. An internal blackout blind is an opaque fabric sitting behind glass that is taking direct sun. The energy has already passed the glazing, so the fabric absorbs it, warms up, and radiates that heat into the room. A dark blind in full sun can be genuinely hot to the hand.

So a room that is dark all day is often hotter at bedtime than one that is not. The answers are all on the outside of the glass: a solar control coating on the outer pane, which cuts incoming solar energy by roughly half and does nothing to the view; an external awning across the glass; or an external shutter, which does both jobs at once. Getting the glass specification right at order stage is far cheaper than adding external hardware later, and the numbers behind it are on the energy efficient rooflights page.

Night ventilation with the room dark

A loft bedroom that has absorbed heat through the roof all day needs to dump it overnight, and the only mechanism that moves enough air is opening the window.

The useful arrangement is a window that can be left in a ventilation position with the blind still down, which most roller and pleated blinds allow because the bottom bar runs in channels and does not flap. A background ventilation flap in the head of the frame gives continuous air change with the sash locked shut, which matters more for a bedroom than for any other room because it runs all night with the door closed. And an insect screen sharing the same channels makes an open window tolerable in the Severn Vale in August, where a lit room with the sash open collects everything on the wing. The flap and filter detail is on the ventilation flaps and filters page.

Rain noise directly over a bed

Rain on horizontal glass is louder than rain on a tile roof, and a bedroom is the room where that registers. A single toughened outer pane on a shallow slope drums audibly in heavy rain, and hail on the same pane is genuinely loud.

Two things reduce it. A laminated outer pane, typically 6.8mm with a polyvinyl butyral interlayer, damps the vibration in the glass itself and takes a noticeable amount off the peak. Asymmetric panes, where the inner and outer sheets are different thicknesses, stop the two resonating together. Pitch helps too: at 45 degrees the rain strikes at a glancing angle and much of the energy goes into sliding water rather than into the pane, which is why a steep Forest of Dean bedroom is quieter under the same storm than a 22 degree vale lean-to. Where quiet is a stated priority, it goes on the order alongside the solar control coating rather than being discovered afterwards.

Where the bed goes, relative to the window

The furniture layout and the window position are one decision, and doing them together avoids most of the complaints.

A bed directly under a roof window puts the sleeper’s face beneath the glass, which is the worst position for early light, for the heat coming off an internal blind, and for the drips of condensation that a cold morning occasionally produces on an unheated reveal. It is also directly under the sash and its handle, so opening the window means reaching over a pillow.

Offsetting the bed by half a metre, or setting the window over a walkway or a run of wardrobes instead, changes the room out of all proportion to the effort. Where the bed has to go under the glass because the room shape allows nothing else, the specification tightens: opaque blind, closed cassette, solar control glass, laminated inner pane, and a background vent that works with the sash shut.

Escape from a bedroom in the roof

Where the bedroom is part of a loft conversion reached by a new staircase, one window in it normally has to serve as the escape opening, and that fixes both the size and the height at which it sits. The clear openable area has to be at least 0.33 square metres with no dimension under 450mm, and the bottom of that opening between 800 and 1,100mm above the finished floor.

Two consequences follow for a bedroom specifically. A fixed unit cannot be the escape window, so if a room has only one roof window it must open. And the escape sash has to be operable in the dark by somebody who has just woken, so anything that locks it, restricts it or requires a key needs thinking about carefully. The full requirement is on the escape openings and the rules page, and the hardware side on handles, catches and child safety.

Two windows in a bedroom, and lighting it evenly

A single window in the middle of a slope lights one bright zone and leaves the corners of the room dim, which reads as a darker room than the numbers suggest. Two smaller units spread across the slope give a more even wash and cost very little more when they go in on one scaffold.

For a bedroom there is a second argument. Only one of the pair has to be the escape unit, so the other can be a fixed light with a slimmer sightline, or a unit set where the view is rather than where the regulations want the cill. It does mean two blinds, two sets of channels and two reveals built to clearance, so the darkening cost doubles even though the window cost does not. Setting out multiple units is covered on two rooflights side by side and pairs and triples on one slope.

Powered operation above a bed, and the control that gets used

A window or blind that is hard to reach from the floor is a window or blind that stops being operated, and in a bedroom that means the room stops being ventilated.

Where the sash handle is above about 2.3 metres, which happens quickly on a steep slope or with a long unit, the practical answer is powered opening with a rain sensor so the window can be left open on a summer night without anybody worrying about a shower at three in the morning. Powered blinds follow the same logic. Solar-powered versions charge from a small cell on the sash and need no cable, which suits a finished room, though on a north slope or under a Stroud valley hillside the cell can struggle through the winter and a mains spur run while the reveal is open is the better decision. The whole control question is on the electric rooflight installation page.

Condensation over a sleeping room

Two adults asleep in a closed bedroom put something in the order of half a litre of water into the air overnight through breathing alone. The coldest surface in a loft bedroom is the glass overhead, so that is where the moisture lands, and on a clear February morning a roof window can run with condensation while every other surface in the room is dry.

Three things control it, and none of them is a defect in the window. Background ventilation through the frame flap, left open, so the moist air has somewhere to go. A horizontal cill lining below the window with a heat source under it, so warm air washes up the face of the glass. And insulation carried tight and continuous around the frame with the vapour control layer taped to it, because the black spotting that appears in the top corners of a reveal in February is a cold bridge rather than a leak. A closed bedroom door and a closed vent flap together are what turn a normal amount of moisture into a wet sill every morning.

Loft bedrooms in this county’s roof stock

The room the specification lands in varies more than people expect across thirty miles of Gloucestershire.

On the trussed estates at Quedgeley, Bishops Cleeve and Ashchurch, the bedroom sits between trusses at 600mm centres, so the windows are narrow 550mm units and a pair in adjacent bays is the normal arrangement. Two narrow blinds is more perimeter to seal than one wide one, and the blackout specification has to be tight on both.

On interwar hipped rear slopes at Longlevens, Hucclecote and Charlton Kings, a purlin at mid height usually restricts the window to a shorter unit set high, which puts the cill above 1,000mm and generally above a bed head. Those rooms darken well.

In Cotswold attics at Painswick or Northleach the roof is 300mm thick with a very deep reveal, and the depth itself shades the glass at high sun angles while making the blind channels a long way inside the room. A conservation unit in a stone slate roof also has a slimmer aperture, so there is less glass to darken in the first place.

In Stroud valley cottages the bedroom is often on a slope facing a hillside that keeps direct sun off it until mid morning, and the practical problem shifts from light to damp air and cold outer glass.