Why Condensation Forms Inside Your Windows, and How to Stop It

Condensation on the room side of the glass is not a fault in the window. It is a measurement: the pane is running colder than the dew point of the air in front of it. New glazing changes the first by a few degrees and the second not at all, which is why replacing windows sometimes cures the problem, sometimes moves it somewhere worse, and sometimes does nothing.

Six things that look like condensation

Start here rather than with the physics, because these six have almost nothing in common.

1. Room side of the glass, worst first thing, gone by lunchtime. Ordinary surface condensation. The rest of this article is about this.
2. Trapped between the two panes of a sealed unit, and you cannot wipe it off. The edge seal has failed and the desiccant inside the spacer bar is saturated. The unit has to come out and be remade.
3. On the outside face of double glazing on a clear autumn morning. Not a fault. The outer pane has radiated its heat to a clear night sky and fallen below the dew point of the outdoor air, which can only happen when very little warmth is coming through to keep it up.
4. On the frame and the reveal, while the glass stays dry. Your glazing is now better than your joinery or your wall. Common where sealed units have been dropped into old timber, and on a deep reveal with nothing behind the plaster.
5. One room only, all year round, including July. Look for a leak: a blocked gutter, a cracked hopper head, penetrating or rising damp. Water from outside does not come and go with the weather the way condensation does, and it usually leaves a tidemark.
6. Worst along the bottom of the pane, running onto the timber. Ordinary condensation with consequences. Cold air sinks down the inside face of the glass, so the lowest few inches are the coldest part of it, and everything that runs down stands in the putty line. That is the mechanism that rots sash bottom rails from the inside out, and it is why wet rot in sash windows so often starts at the glazing line rather than the cill.

Only one and six respond to anything below. If you have two or five, stop reading and go and look at your gutters.

What the glass is doing at five in the morning

Air holds water vapour, and the amount it can hold climbs steeply with temperature. Cool a parcel of air far enough and the surplus comes out as liquid on the nearest cold surface. That threshold is the dew point, and it is set entirely by how much moisture the air is already carrying.

At a comfortable 20°C indoors, it runs like this:

Any surface colder than that figure will run with water. Nothing else has to go wrong.

Now the glass temperature, and this is where quotes confuse people, because two different U-values circulate for the same single-glazed sash and almost nobody says which is which.

• Through the glass alone, roughly 5.4 W/m²K. This is the figure that sets the surface temperature, because it is the glass that runs with water.
• The window as a whole, roughly 4.8 W/m²K. Lower, because timber insulates far better than glass, so averaging the frame in pulls the number down. This is the figure for a heat-loss calculation, and the one on an energy quote.

Both describe the same window honestly, and setting a glass-only number on one quote against a whole-window number on another is the commonest way glazing comparisons go astray; window U-values explained unpicks it.

For condensation it is the glazing figure that counts. With 20°C in the room and 5°C outside, an ordinary London winter night, the inner face of the glass sits at about:

Read the two tables together and the thing becomes obvious. Single glazing at 9.5°C is below the dew point of any room above about 50% relative humidity, so it streams. Secondary or double glazing at 16 to 17°C stays dry until the room is close to 80%. That is why new glazing appears to fix condensation, and why it stops fixing it in a house that is genuinely too damp.

There is a sting in that. When the glass stops being the coldest surface, the moisture does not go anywhere. It finds the next coldest thing: the reveal, the corner behind a wardrobe on an external wall, the cold bridge where a floor joist runs into the brickwork. Double glaze a damp house and you often trade a wet window for mouldy corners, which are harder to see coming.

Where ten litres of water a day comes from

A household of four typically puts 10 to 14 litres of water vapour into the air every day. The approximate figures the trade works to:

• Two people asleep for eight hours: around 0.5 to 0.75 litres
• An active adult: roughly 0.2 litres an hour
• Cooking across a day: 2 to 3 litres
• A bath or a shower: 0.5 to 1 litre each
• A load of washing, in the machine: around 0.5 litres
• Drying one load of washing indoors: 4 to 5 litres
• A portable bottled-gas or paraffin heater: about a litre of water for every litre of fuel burned

That drying figure is not a misprint, and it is why the airing rack in the spare bedroom is so often the entire answer. One load dried on radiators in a closed room puts more water into the house than the whole family does breathing. An unvented tumble dryer does the same, and so does a bathroom door left open after a shower, which walks the steam down the landing and lays it on the coldest window in the house.

Reading the room instead of guessing

A digital hygrometer costs less than a tube of decent sealant and will tell you more than a survey. Leave it in the worst room for a fortnight and read it at seven in the morning with the door still shut, when humidity is highest and surfaces coldest.

• Below 55% at that hour and the room is fine. If the glass is still wet it is unusually cold, and glazing or shutters are a fair conversation.
• 55 to 65% is borderline. Ventilation and habits will almost certainly clear it.
• Persistently above 65% and you have a moisture problem no window will solve. Fix that first or you will pay twice.

Run a second one in a room that stays dry. A wide gap between the two points at the wet room, a narrow one at the whole house.

The four things you can actually change

There are only four, and they are nothing like equal.

Almost everybody starts at the bottom of that table. Almost everybody should start at the top.

Make less of it

Lids on pans. The extract fan on before the shower rather than after, and left running a quarter of an hour afterwards. Washing dried outside, in a vented dryer, or in one room with the door shut and the window open. Bathroom and kitchen doors closed while in use, so the steam is taken out where it is made instead of distributed round the house. In a great many houses that is the whole fix.

Get it out of the building

Part F of the Building Regulations asks for extract ventilation in the wet rooms, kitchens, bathrooms, utility rooms and WCs, and background ventilation in habitable rooms. The intermittent extract rates are modest: 15 litres a second for a bathroom, 30 for a utility room, 30 for a kitchen with a hood ducted outside and 60 for one without. A cheap axial fan on a long flexible duct often fails to reach them.

In a period house, four things:

1. Check the fans actually extract. Hold a sheet of tissue against the grille. If it does not stick, the fan is moving no air, and one discharging into a blocked duct or a loft void is worse than none at all.
2. Run them long enough. Fifteen to twenty minutes after use, on a run-on timer or a humidity sensor rather than the light switch.
3. Purge briefly and hard. Five minutes with both sashes open changes the air in a bedroom without cooling the structure. An hour on the latch chills the walls and costs more.
4. Use the sash as designed. A box sash open an inch top and bottom sets up a real convective exchange, warm moist air out at the top and cooler air in at the bottom, which a casement cannot do at all.

If whole windows are being replaced, Approved Document F expects the replacement to provide background ventilation, in practice trickle vents, with equivalent areas set out by room type; where a room already has a wall ventilator meeting the figure, nothing further is needed. Nor can you sign it away, since a homeowner disclaimer declining ventilators is specifically not accepted as a route to compliance. Repairing and overhauling the sashes you already have does not trigger any of it.

Warm the surfaces, not just the air

Intermittent heating is a condensation generator. A room heated hard for three hours in the evening and left cold overnight has warm, moisture-laden air meeting cold plaster all night. The same energy spread over longer at a lower flow temperature keeps surfaces above the dew point instead.

Thermostatic valves, used to hold unoccupied rooms at 15 or 16°C rather than switching them off, are usually the cheapest structural change available in an old house. The government's guidance to rented-housing providers on damp and mould is blunt on the underlying point: it puts the cause on building deficiencies, inadequate ventilation, inadequate heating and poor energy efficiency rather than on the way people live.

Warm the glass

By now it should be clear this is the last lever rather than the first. Secondary glazing lifts the inner surface temperature about as far as a slim sealed unit does and costs less to fit, and it has one particular advantage here. With the secondary panel sealed on the room side and the original sash left slightly permeable to outside air, moist room air never reaches the cold outer pane. Get it the wrong way round, a well-sealed original sash and a leaky secondary panel, and you get condensation in the cavity where you cannot wipe it. What each glazing route achieves is compared in our guide to insulating sash windows.

Does draught-proofing make it worse?

It depends on the order you do things in.

Done badly, yes. Sealing every gap in a house with no working extract ventilation is exactly how a merely chilly house becomes a mouldy one.

Done properly, no, and it usually helps. Sash draught-proofing seals the uncontrolled leakage paths, the meeting rails, the pulley stiles and the gap behind the staff bead, which are the ones that make a room cold without ventilating it usefully, leaving you with ventilation you can control. A colder room condenses more rather than less, so lifting the room temperature moves every surface further from its dew point.

Seal the leaks, then make certain there is a deliberate way out for moist air. Our draught-proofing service works to that sequence, and the DIY route, with the places it comes unstuck, is in how to draught proof a sash window.

When it has already turned into mould

Black speckling on the putty line and bottom rail means the timber underneath has been wet repeatedly, not once. Wipe it off with a proprietary fungicidal wash rather than bleach, which takes the colour out and leaves the spores.

Then find out what the timber is doing. Press a bradawl into the bottom rail either side of the glazing line, and into the nose of the cill. If it sinks more than a few millimetres under light thumb pressure you are past cleaning and into joinery, and the sooner it is dealt with the smaller the splice. Why mould keeps returning to the same corner is covered in black mould around windows, and the failure at the base of the opening in our window cill guide.

The order to do it in

Buy the hygrometer before anything else. Stop drying washing in the room that streams. Check the fans move air, and put them on a timer if they do not run long enough. Open both sashes an inch top and bottom for five minutes each morning. Give it a month.

If the seven o'clock reading comes down and the glass stays clear, you are finished and it cost nothing. If it stays above 65% with the door shut, there is a defect in the fabric, and it is worth having someone look at the building rather than the windows.

And if the readings drop and the glass stays dry but the metre nearest the window is still too cold to sit in, that is a heat problem rather than a moisture one, and the right point to talk about draught-proofing, shutters and glazing, in that order. Our sash window repair work across the London boroughs usually starts with exactly that conversation, and it is very often the cheap end of the list that settles it.