Condensation, ventilation and rooflights

The moisture balance that decides everything downstream

A dwelling is a box with water going in and water coming out. Water goes in from the occupants and their activities. Water comes out almost entirely in the air that leaves the building, whether through a fan, an open window, a background ventilator or an unintended gap. A small amount is absorbed and released by furnishings and finishes, which buffers short spikes and changes nothing over a week.

If generation exceeds removal, the indoor humidity climbs until it settles at a new higher level. That level is a property of the whole house, not of the room the water was produced in, because moisture-laden air moves between rooms freely through doors, stairwells and gaps in the construction. A rooflight in a hallway can run wet because somebody dries washing in a bedroom two doors away.

Moisture excess, and the number that drives the problem

Relative humidity is a poor way to think about this, because it changes with temperature even when no water has been added or removed. The useful quantity is the mass of water carried per kilogram of dry air, written in grams per kilogram.

What matters is not the indoor figure on its own but the difference between indoors and outdoors, called the moisture excess. That difference is produced entirely by what happens inside the building and is removed entirely by ventilation. Work it back: air holds about 1.2 kilograms per cubic metre, so a moisture excess of 3 grams per kilogram means each cubic metre of air leaving the house carries about 3.6 grams of water away with it. Removing 10 litres a day at that excess requires around 2,800 cubic metres of air, which is 116 cubic metres an hour, or about 32 litres per second running continuously. That figure is not a coincidence: it is very close to what the ventilation guidance asks a family home to achieve.

Air change rate, and the three ways it gets written down

Ventilation is quoted in three different units and confusion between them is common.

Air changes per hour, written ach, is the volume of the space replaced per hour. Litres per second is an absolute flow rate and is how fans and ventilators are rated. Litres per second per square metre of floor area is how a whole-dwelling requirement is normally set, because it scales with the size of the home rather than its shape. To convert, multiply litres per second by 3.6 to get cubic metres an hour, then divide by the room volume for air changes per hour. A background rate of about 0.5 ach across a dwelling is the traditional rule of thumb for moisture control, and a purge is ten to twenty times that for a few minutes.

What the guidance asks a dwelling to achieve

The ventilation part of the Building Regulations sets a whole-dwelling rate that rises with the number of bedrooms, from roughly 19 litres per second for a one bedroom home to over 40 for a five bedroom one, and never less than 0.3 litres per second for every square metre of internal floor area. That rate is what carries the daily bucket of water out.

It also sets purge ventilation, which for a habitable room is an openable area of at least one twentieth of the floor area, and extract at source in the wet rooms. Adding a rooflight to a habitable room engages both: a unit with an opening sash contributes to the purge requirement, and one with a ventilation flap in the head contributes to background ventilation. A fixed unit contributes daylight and nothing to either, which is worth knowing before choosing one for a room that was already marginal.

Background ventilators, equivalent area, and the vent nobody opens

Background ventilation is the small, steady, controllable air exchange that runs all the time, and it is the part of the system that has quietly disappeared from most houses.

Background ventilators are sized by equivalent area in square millimetres, which is not the same as the physical hole: it is the area of a sharp-edged orifice that would pass the same air at the same pressure difference, and it accounts for the resistance of the grille, the insect mesh and the airflow path. Current guidance sets a required equivalent area room by room, and for a habitable room in a reasonably airtight dwelling that commonly falls in the region of 5,000 to 8,000 square millimetres. Ventilators are meant to sit high in the room, typically at least 1.7 metres above floor level, so the incoming cool air mixes before it reaches anyone sitting down. A roof window ventilation flap sits at the head of the unit, which is the highest point in the room and thermally the right place for it.

The universal problem is that these are closable and they get closed in November. A ventilator that is shut is a ventilator that is not there, and the humidity climb through a winter is gradual enough that nobody connects the two. Humidity-sensitive ventilators, which open and close on a hygroscopic element with no power and no controls, solve this by taking the decision away from the occupant.

Airtightness, and why a tighter house makes the glass wetter

Air permeability is measured by pressurising the dwelling to 50 pascals and recording the leakage in cubic metres per hour per square metre of envelope. An untouched Victorian terrace with suspended floors and open chimneys might be 15 to 25. A well-built current house is 3 to 5. Deep retrofit and high-performance construction go below 1.

Here is the part that catches people out. The leaky house ventilated itself accidentally, all winter, at enormous heating cost, and its rooflights ran clear because the moisture excess never had a chance to build. Improve the envelope without installing the deliberate ventilation that is supposed to accompany it and you have removed the removal mechanism while leaving the generation untouched. The moisture excess rises, the dew point rises with it, and the coldest surface in the house, which is now the glass overhead, becomes the place it shows. It reads as a fault in the new rooflight. It is the arithmetic of the whole dwelling arriving at its weakest point.

Purge, and why ten minutes beats an hour

Purge ventilation is a short, large exchange rather than a small continuous one, and it is the most efficient moisture removal available in a house because of a property of building fabric that is easy to overlook.

Air has almost no heat capacity. The plaster, the floor, the furniture and the masonry hold hundreds of times more heat than the air in the room. Open a rooflight and a window fully for ten minutes on a cold morning and the air is completely exchanged, taking its water with it, while the surfaces barely cool. Close up, and the fabric reheats the fresh air within minutes at very little energy cost. Leaving a single window on the latch for four hours does the opposite: it exchanges less air in total and it chills the surfaces, which is both colder and less effective. The stack effect makes a rooflight the best purge opening in any room, because warm wet air is already collected at the ceiling waiting to leave.

The heat cost of ventilating, and why it is smaller than it feels

Householders resist background ventilation because it feels like throwing heat away. The arithmetic puts it in proportion.

Heating outside air to room temperature costs about 0.33 watt hours for every cubic metre and every degree of temperature rise. A 90 cubic metre house at half an air change per hour is 45 cubic metres an hour. On a January day 15 degrees colder outside than in, that is 0.33 multiplied by 45 multiplied by 15, or around 223 watts. Meaningful, and considerably less than the cost of running a dehumidifier and a heater to fight the consequences of not ventilating, and far less than the cost of mould at the reveal junctions. Uncontrolled leakage is the expensive version of the same thing, because it is largest on the windiest, coldest nights when heating is already working hardest.

Loft conversions, and the ventilation that has to be redesigned

A loft conversion changes the ventilation of the whole house, not just the new room, and rooflights are usually the only openings the new space gets.

Three things shift at once. The stairwell now runs from the ground floor to the ridge, so the stack pressure across the building increases and warm wet air from the kitchen and bathrooms is drawn upward far more strongly than before. The new room is at the top of that column, so it receives the most humid air in the house. And any positive input ventilation unit that was sitting in the roof space has lost its plenum and needs relocating or replacing with a different system.

What follows is that a loft rooflight needs an opening sash rather than a fixed pane wherever it is the only ventilation the room has, and that on the trussed estates at Quedgeley, Brockworth and Bishops Cleeve, where the narrow 550mm wide units are what fits between trusses, two openings in adjacent bays give both the purge area and the cross-flow that one unit cannot. Where the unit sits high over a stairwell and no handle can be reached, a rain-sensing electric opener with a humidity sensor is the arrangement that gets used, described on electric rooflight installation.

Severn Vale mist, and why cold saturated air still dries a house

Gloucester, Longford, Tewkesbury and the vale villages sit low beside a tidal river on ground that holds mist, and outdoor relative humidity through a still winter night is frequently at or near 100 per cent. The reasonable conclusion is that opening anything will make the house wetter. That conclusion is wrong, and the reason is the difference between relative and absolute humidity.

Saturated outside air at 4 degrees carries about 5 grams of water per kilogram, because cold air cannot hold much. Indoor air at 20 degrees and 60 per cent relative humidity carries about 8.8 grams per kilogram. Bring the outside air in and warm it to 20 degrees and its relative humidity falls to about 34 per cent, because its capacity has tripled while its water content has not changed. It arrives thirsty. Even river mist at saturation is drier in absolute terms than the air in a Severn Vale kitchen, and ventilating with it removes water from the house.

The exception is a mild damp spell, when outside air at 14 degrees and 95 per cent carries around 9.5 grams per kilogram and ventilating gains nothing. Those are also the days when surfaces are warm and condensation is not forming, so the strategy holds through the season that matters.

Reading the house before changing the glass

A digital thermometer and hygrometer costs very little and settles most of these arguments in a fortnight. Leave one in the room with the rooflight and a second one outside in the shade, and record both morning and evening.

What the readings tell you: indoor relative humidity consistently above 60 per cent at 20 degrees means the moisture balance is the problem and no glazing specification will fix it. Indoor humidity at 45 to 55 per cent with condensation still forming means the surface temperature is the problem, which points at the glass edge, the spacer, the frame or the insulation continuity at the reveal. A large moisture excess against the outdoor reading points at generation and extraction. A small excess with high indoor humidity means the outside air is genuinely wet and the timing of purge matters more than its duration.

Where the water is inside the sealed unit rather than on it, none of the above applies and the unit has reached the end of its life, which is set out on replacing a failed or misted rooflight. Where the damp is in the plaster at the reveal corners rather than on the glass, it is a junction temperature question covered on thermal bridging at the rooflight perimeter, and where warm air is getting into the construction itself the answer sits with vapour control layers sealed to the frame. Send photographs and a fortnight of readings to info@severnskylightsgloucester.co.uk and the reply will say which of the three it is.