Guide · Energy Efficient Rooflights
Condensation, ventilation and rooflights
Condensation on a rooflight is a meeting between two numbers: the amount of water vapour in the room air, and the temperature of the coldest surface that air can reach. When the surface falls below the dew point of the air touching it, water comes out of the air and lands on the glass. That is all it is, and it means there are only ever two levers. Make the surface warmer, or make the air drier.
Almost every complaint about a “sweating” rooflight is the second lever being ignored. The glass gets blamed because the glass is where the water is visible, but the same air is depositing moisture in every cold corner of the house. This page covers the arithmetic, the ventilation rates that actually shift it, and the specific reasons a rooflight shows the problem earlier than a wall window does.
Dew point, and the temperature the glass has to stay above
Air at a given temperature can hold a maximum amount of water vapour. Cool it and that maximum falls, and at some point the air is holding exactly as much as it can. That temperature is the dew point.
The numbers are worth memorising because they settle most arguments. A room at 20 degrees and 50 per cent relative humidity has a dew point of about 9.3 degrees, so any surface below 9.3 degrees will wet. The same room at 60 per cent humidity has a dew point of about 12 degrees. At 70 per cent it is about 14.4 degrees, which is a temperature that the inner face of a poorly specified rooflight will reach on a cold night without difficulty. Ten percentage points of humidity moves the threshold by roughly two and a half degrees, which is more than any realistic glazing upgrade delivers.
Relative humidity, and why each room reads differently
Relative humidity is a percentage of a moving target, which is why the same house can read 45 per cent in the hall and 75 per cent in the kitchen with the same amount of water in the air.
Cool the air and the relative humidity rises without a single gram of water being added. This is exactly what happens in a room where the heating goes off at eleven at night: the absolute moisture content is unchanged, the air temperature falls from 20 to 15, and the relative humidity climbs from 55 to nearly 80 per cent. The rooflight, which is the coldest surface in the room and cooling faster than everything else, is the first place that shows it. Condensation appearing overnight and gone by ten in the morning is this pattern and nothing more sinister.
Grams per hour, and the daily vapour budget
The volumes surprise people. Standard figures used in building physics put an adult at rest at around 40 grams of water vapour an hour, and at around 55 grams when active.
Cooking a main meal releases something in the order of a kilogram of water. A shower releases a few hundred grams. Washing clothes releases a few hundred more, and drying a load indoors on an airer releases between one and a half and three kilograms, all of it into the room air. A household of four generates somewhere in the region of ten to fifteen litres of water a day in normal use. That water leaves the house through ventilation or it condenses on something, and the rooflight is a strong candidate for something.
A load of washing dried on an indoor airer puts more water into a room than any other single household activity.
Where the coldest surface in a room usually is
Warm air rises, so the air at ceiling level in a heated room is typically one to three degrees warmer than the air at seated head height, and considerably warmer than the air at floor level. That warmer air also carries the same absolute moisture, so its dew point is unchanged.
Now put the coldest surface in the room directly into that warm layer. A rooflight sits at the top of the space, where the air is warmest and where the moisture concentration is at its highest because the buoyant plume from a hob or a bathroom carries moisture upwards. A wall window at 1.5 metres sits in cooler, drier air. The rooflight is therefore doing something a wall window never does: presenting a cold surface to the wettest air in the house.
Surface temperature factor, and the number specifiers use
Rather than argue about dew points, designers use a dimensionless figure called the temperature factor, written fRsi. It is the internal surface temperature minus the external air temperature, divided by the internal air temperature minus the external air temperature.
The critical value used for dwellings in the United Kingdom is 0.75. A junction achieving fRsi of 0.75 or better keeps its internal surface warm enough to avoid persistent mould under normal domestic moisture loads. Below that figure the detail is a risk regardless of how well the occupants behave. It is a useful concept for a rooflight because it applies to the whole assembly rather than to the glass alone, and the weakest fRsi in a rooflight installation is almost never the middle of the pane.
Mould does not need liquid water
The threshold people assume is 100 per cent relative humidity at the surface, because that is where visible water appears. Mould has a much lower bar.
Common domestic moulds germinate and grow at a surface relative humidity around 80 per cent sustained over days, which corresponds to a surface several degrees above the dew point. This is why black spotting appears along the bottom edge of a rooflight reveal, or in the corner where the lining meets the frame, in situations where nobody has ever seen a drop of water. The visible condensation is a symptom that arrives late. The surface has been damp enough to support growth for some time before that.
Why a rooflight shows it earlier than a wall window
Four things stack up, and they are cumulative rather than alternative.
The unit sits in the warmest and wettest air in the room. It faces the night sky, so it loses heat by radiation to a sky temperature that on a clear winter night can be ten or fifteen degrees below the air temperature, which chills the outer pane and pulls the inner pane down with it. Water that forms on it cannot run away down a vertical face, so it sits, spreads and eventually drips. And on a flat or shallow unit, the reveal below it forms a pocket where warm moist air collects and does not circulate. The last of those is the one most often designed in by accident.
Ventilation rates, in litres per second
Part F of the Building Regulations sets the rates, and they are worth knowing because they are the ones an extract fan is meant to hit rather than the ones it usually does.
| Room | Intermittent extract rate |
|---|---|
| Kitchen, fan sited above the hob | 30 litres per second |
| Kitchen, fan sited elsewhere in the room | 60 litres per second |
| Utility room | 30 litres per second |
| Bathroom or shower room | 15 litres per second |
| Separate WC | 6 litres per second |
Those are the rates at the point of extract, in service, through the duct actually installed. A fan rated at 15 litres per second on the box, connected to four metres of flexible duct with two tight bends, is delivering a fraction of that.
Background ventilation, and equivalent area
Extract removes moisture only if replacement air can get in, and background ventilators are how that happens without opening a window in February.
Trickle ventilators are specified by equivalent area in square millimetres, which is the area of a sharp edged orifice that would pass the same airflow, and it is a smaller number than the physical opening. Habitable rooms in a dwelling generally want 8,000 square millimetres each, with wet rooms at 4,000. Where an extension has replaced the windows that used to provide this, the moisture balance of the whole ground floor can change without anyone connecting the two events. A rooflight with a ventilation flap in the frame contributes here, and on a roof window that flap is part of the head of the frame rather than an accessory.
Continuous extract, and why intermittent fans underperform
An intermittent fan runs while somebody is in the room and for an overrun of a few minutes afterwards, which means it deals with the shower and not with the towels, the damp floor and the wet walls that are still evaporating an hour later.
A continuous mechanical extract system runs permanently at a low trickle rate and boosts on humidity or occupancy. It moves a similar total volume of air over a day at a fraction of the noise, and it holds the room humidity down between events rather than chasing it afterwards. In a bathroom or wet room with a rooflight over it, this is the single most effective change available, and it costs less than upgrading the glass.
Heat recovery ventilation under a rooflight
Where a house is airtight enough that background ventilators are not doing much, mechanical ventilation with heat recovery extracts from the wet rooms and supplies fresh air to the living spaces, passing both through a heat exchanger.
Two details matter for rooflights. First, the supply terminal should not be aimed at the glass, because a jet of cool incoming air across a pane drops its surface temperature and creates condensation that would not otherwise have formed. Second, the extract terminal in a room with a rooflight is best placed at high level near the unit, since that is where the moisture laden air collects. Getting these two the wrong way round is a common and entirely avoidable outcome.
Air movement across the glass, and what blocks it
A surface with air moving over it stays warmer than a surface in still air, because the convective film resistance falls. The practical consequence is that anything which stops air circulating across a rooflight makes condensation more likely.
A deep square reveal is the usual culprit: it creates a still pocket of air directly under the glass which cools, stratifies and wets. A splayed reveal, opening outwards towards the room, lets warm room air wash across the underside of the unit and is worth a degree or more of surface temperature. That is the same detail that improves the light distribution, which is set out on the page about reveals and why splaying them matters. Two benefits from one plastering decision is unusually good value.
Blinds, and the cold layer behind them
An integrated blind closed against a rooflight on a January night isolates the pane from the room. The air trapped between blind and glass is no longer being warmed by the room, so it cools towards the glass temperature, and its relative humidity climbs.
The result is condensation behind a closed blind, on a unit that was dry with the blind open. It is not a fault in the blind or the glass. Where a blackout blind is genuinely needed, in a bedroom under a roof window for instance, the answer is to raise it in the morning and to make sure the room has background ventilation, rather than to assume the unit is underperforming.
Moisture inside the roof build-up
Everything above concerns water forming on a visible surface. Water can also form inside the construction, where warm moist air from the room finds its way into a cold part of the build-up and condenses there against the underside of the deck or on the cold side of the insulation.
This is interstitial condensation and it is a different problem with a different solution, which is continuity of the vapour control layer and its junction with the rooflight frame. That subject is covered properly under vapour control layers sealed to the frame. The one thing worth knowing here is that a rooflight opening cuts through every layer of a roof, so it is where continuity is most often lost.
Water inside the sealed unit
Condensation between the panes is not a ventilation matter at all and it cannot be improved by anything the occupant does.
It means the perimeter seal of the unit has failed and the desiccant inside the spacer bar has taken up as much moisture as it can hold. Once that happens the cavity breathes moist air in and out with every temperature cycle, and the misting appears and clears on the same daily rhythm. The unit has reached the end of its working life and wants replacing. The signs, and how to distinguish them from surface condensation, are set out under misted sealed units and why they go.
Making the surface warmer rather than the air drier
The second lever is the specification, and it works by raising the temperature of the inner pane so that it stays above the dew point of the air in the room.
A double glazed unit with a low emissivity coating and argon holds its inner pane around 15 to 16 degrees with the room at 20 and the outside at zero. Triple glazing pushes that towards 17 or 18, which buys real headroom against a humid kitchen. The edge of the pane behaves differently from the middle, and is where the wetting starts, which is dealt with under warm edge spacers and where condensation starts. The wider trade off between glass performance and cost sits on the energy efficient rooflights hub.
A winter routine that actually works
Deal with moisture at source and the glass looks after itself. Lids on pans while cooking, and the extract on before the hob rather than after. Bathroom doors closed during a shower and the fan left to overrun. Laundry dried outside or in a vented or condensing dryer, not on an airer in the room with the rooflight over it.
Then keep the heating steady rather than sharply intermittent, because a room that swings from 21 degrees to 14 overnight drives its relative humidity up by twenty points in the process. And leave the trickle ventilators open through the winter. They are there precisely for the months when nobody wants to open a window, and closing them in October is the most common reason a house that was dry last year is not dry this year.
Keep reading
More on energy efficient rooflights
Solar gain, overheating and shading
Read the guide →Triple glazing in a rooflight and when it pays
Read the guide →Warm-edge spacers and where condensation starts
Read the guide →Low-emissivity coatings and argon fills
Read the guide →Thermal bridging at the rooflight perimeter
Read the guide →
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