Flat rooflights over a bathroom or wet room

The moisture a bathroom actually puts into the air

A five-minute shower releases roughly half a litre of water into the room as vapour. A bath releases a similar amount more slowly. Add towels drying, and in a lot of houses a washing machine somewhere nearby, and the room can run at 70 to 90 per cent relative humidity for an hour or more after use.

The number that matters from all of that is the dew point, which is the temperature at which that air has to give the water back up as liquid. Air at 22 degrees and 70 per cent relative humidity has a dew point of about 16.3 degrees. Any surface in the room colder than 16.3 degrees will be wet. Not might be. Will be.

So the whole specification comes down to two moves: get the vapour out fast, and keep every surface above that figure.

Extraction is the first specification, not the second

If the extract in the room is inadequate, no rooflight specification will save you, and a new rooflight will simply become the place where the existing problem becomes visible. It is the coldest surface, so it condenses first, and the owner concludes the new unit is at fault.

We say this before quoting, every time, because it is the single most useful thing we can tell somebody planning a bathroom rooflight. A humidity-sensing extract fan with a run-on timer, ducted in insulated rigid duct by the shortest route to outside, is worth more to the room than an upgrade from double to triple glazing.

An opening rooflight helps and is worth having, but it is a summer device. Nobody opens a rooflight in February, and February is when the condensation appears.

The extract rates Approved Document F asks for

Approved Document F sets minimum intermittent extract rates for dwellings, and they are worth knowing because a lot of existing fans do not meet them.

Those are rates at the terminal, in use, not the figure on the fan box. A fan rated at 15 litres per second delivering through four metres of concertinaed flexible duct and two bends will not achieve 15 litres per second. Rigid duct, insulated where it crosses a cold void, and as few bends as possible.

Where the room is on continuous extract as part of a whole-house system, the high rate for a bathroom is lower, at 8 litres per second, because it is running all the time.

Where the fan and the rooflight sit relative to each other

Put the extract terminal on the opposite side of the ceiling from the rooflight, not next to it, and put it as close to the shower or bath as the ceiling allows.

The reasoning is about air movement. You want the moist plume drawn away from the glass and out, which means the extract sits over the source and the replacement air comes in under the door on the far side. A terminal fitted right beside the rooflight pulls the plume across the glass on its way to the fan, which is the opposite of what you want, and it also tends to put a ducting run straight through the insulation next to the kerb.

Where the duct has to cross the roof void near the kerb, it gets insulated and sealed at both ends. An uninsulated duct in a cold void condenses inside itself and drips back down the run.

Surface temperature, and the arithmetic that decides the outcome

You can work out roughly how cold the inside face of a rooflight will get, and it is a genuinely useful sum.

Take the room temperature, subtract the U-value multiplied by the temperature difference multiplied by 0.13, which is the standard internal surface resistance for glazing. At 21 degrees inside and 0 outside, a flush unit with a whole-unit U-value of 1.2 gives an inner surface of about 17.7 degrees. That is above the 16.3 degree dew point calculated earlier, so the glass stays clear.

Run the same sum on a single skin dome at a U-value of 5.0 and the inner face lands around 7 degrees. That surface is nine degrees below the dew point and it will run with water every time the shower is used. This is arithmetic, not opinion, and it is why we do not fit single skin units over bathrooms.

Why the frame matters more than the glass here

The glass is rarely the coldest part. The frame is, and so is the junction between frame and kerb, because that is where a thermal bridge can run right around the perimeter.

Building researchers use a temperature factor, fRsi, to describe how cold the worst point of a junction gets relative to the room. For dwellings the accepted minimum is 0.75 at a window or rooflight junction. Below that, the corner runs wet in normal winter conditions even when the glass in the middle is dry.

In practice that means a polyamide thermal break through the aluminium frame, a warm edge spacer rather than aluminium in the glazing unit, and continuous insulation on the outside face of the kerb carried up to meet the frame. Miss the kerb insulation and the mould appears in the corners of the reveal, not on the glass, which is exactly the pattern we get called to look at.

Vapour control in a room that generates vapour

The vapour control layer is the sheet on the warm side of the roof insulation, and in a bathroom it is doing harder work than anywhere else in the house.

It has to be continuous, and it has to be sealed to the rooflight frame all the way round with the correct tape. A vapour control layer that stops short of the frame is a hole in the ceiling directly above the wettest room in the building, and moist air will find it. Where the ceiling below is plasterboard, every downlight, extract terminal and cable penetration through it is another route, and they all need collaring.

Where the roof is a cold build-up with a ventilated void, a bathroom underneath is the worst case, and we usually recommend converting that section to a warm build-up while the covering is open rather than trying to ventilate around a new kerb.

Privacy overhead, and where you are actually overlooked

Most people worry about this more than they need to, and then discover the one case that matters.

You are not overlooked from ground level, and you are rarely overlooked from a house at the same height, because the sight line into a horizontal pane from a distance is very shallow. What does overlook a flat roof is an upstairs window in a taller neighbouring building, a first floor window of your own house looking down onto a single storey rear extension, and anything on a slope above you. In a lot of Oxford terraces the rear extension roof is directly under the neighbour’s bathroom or bedroom window, and that is a genuine sight line.

Stand at the window that concerns you and look. Thirty seconds settles it, and it settles it better than any assumption made from the garden.

Obscured glass against a blind in a humid room

Where privacy is needed, obscured glass is the low-maintenance answer and a blind is the flexible one.

An acid-etched or sandblasted inner pane obscures the view completely, keeps almost all the light, has nothing to operate and nothing to go wrong. It is permanent, which is its only drawback: you lose the view of sky as well.

Fabric blinds in a bathroom are the option we are least keen on. Fabric in a room that runs at 80 per cent humidity holds moisture, and blinds mounted inside a rooflight reveal sit in the dampest air in the room. If a blind is wanted, an integral one within the sealed cavity of the glazing unit is far better, because it is sealed away from the room air entirely and operated magnetically or electrically from outside the unit.

Opening gear that survives the humidity

If the rooflight opens, the gear lives in wet air for the life of the unit, so specify accordingly.

Electric chain or spindle actuators with a sealed motor housing are the sensible choice. The motor sits within the frame, the cable is a low voltage run, and there is nothing for the user to touch. Manual pole-operated units work but the spindle mechanism is more exposed and the pole has to live somewhere in a small room.

Rain sensors are worth specifying on any opening unit and are close to essential over a bathroom, because a rooflight left open on a mild morning and forgotten is how water gets onto a tiled floor. The sensor closes the unit within a few seconds of getting wet. Set the actuator so it also closes on a timer, and the room looks after itself.

Electrical work, zones and who signs it off

Electrical work in a bathroom is notifiable under Part P of the Building Regulations, and bathrooms are divided into zones with a required protection rating in each.

A rooflight actuator sits in the ceiling, which is outside the defined shower and bath zones in almost every room, but the wiring, the switch position and the transformer all have to be considered by whoever is doing the electrical work. We coordinate the power supply and the switch location with your electrician rather than doing that work ourselves. Our scope is the rooflight, the kerb and the weathering.

Practically, it means a fused spur and a low voltage run to the unit are pulled in before the ceiling closes up. Retrofitting a cable to an actuator through a finished plasterboard ceiling is possible and unpleasant.

Wet rooms: the ceiling is the surface at risk

A wet room is tanked at floor and walls, so the surfaces designed to get wet are protected. The ceiling is not tanked, and in an open wet room it takes far more airborne water than a bathroom ceiling ever does.

That changes three things. The board should be moisture-resistant plasterboard or a cement-based board rather than standard wallboard. The paint should be a moisture-resistant emulsion, and the reveal should be finished in the same specification as the ceiling rather than being treated as a decorative detail. And the extract rate should be at the higher end, since the plume rises straight to the ceiling with nothing to slow it.

A rooflight directly over the shower head itself is a specific question, and we deal with it separately on can a flat rooflight go over a shower.

The reveal, and why we splay it

The opening through the roof build-up creates a shaft, and how that shaft is shaped changes both the light and the condensation behaviour.

A square reveal makes a box. Warm room air does not circulate well up a straight shaft, so the air right against the glass stagnates, cools and condenses, and the corners are the coldest points of all.

A splayed reveal, flaring outwards into the room, does two useful things. It throws more light into the room from the same aperture, and it lets warm air wash up and across the glass, which keeps the surface temperature closer to room temperature. In a bathroom we splay wherever the depth of the build-up allows it, insulate the reveal continuously, and finish it in the same board and paint as the ceiling.

Heating, and keeping surfaces above the dew point

A cold bathroom condenses. A warm one does not, and this is the cheapest fix available to anybody reading this.

A heated towel rail on the hot water circuit is a towel dryer, not a heat source, and in a lot of bathrooms it is the only heat in the room. If the room drops to 15 degrees between uses, the glass drops with it, and warm moist air from a shower then meets a cold surface from a standing start.

Keep the room at 18 degrees or above through the heating season and the surface temperature arithmetic earlier in this page starts working in your favour. Underfloor heating in a bathroom or wet room does this well, because it holds a steady background temperature rather than swinging.

Bathrooms in a loft, where the roof is pitched

Loft bathrooms are common in Oxford, where terraces and semis get a rear dormer and an en suite goes into the slope. The physics is the same and the details differ.

On a pitched roof you are fitting a roof window rather than a flat unit, so the weathering is a flashing kit rather than a kerb, and the minimum pitch for the model is a hard limit. The insulation still has to be carried tight to the frame all round, the vapour control layer still has to be taped to it, and the reveal is still splayed, usually vertical at the head and horizontal at the sill so the air can move.

The extract is often harder, because the duct run to outside is longer. Plan it before the plasterboard goes on.

What we need to know before quoting

Send us these and you will get a useful range rather than a guess.

• The room: bathroom, en suite, wet room, and how often the shower runs.
• The existing extract, if any, and where the fan discharges.
• Whether the roof is a warm or cold build-up, and what the covering is.
• The depth from the ceiling to the top of the roof, which sets the reveal.
• Whether you want it to open, and whether there is power at the ceiling.
• Any window in a neighbouring building that looks down onto the roof.

We will give a range by email from that, and a fixed price after a survey. The roof does not get left open overnight, and the workmanship carries a ten-year guarantee. Call 01865 704245 or email info@heritageskylightsoxford.co.uk.