Window Condensation Checklist: What's Normal and What Needs Fixing

You wake up, pull back the curtain and find the bedroom window wearing a coat of water. By lunchtime it has gone, only to return the next morning. Before you start ringing glaziers, it helps to know that condensation is not automatically a fault. Quite often it is the window telling you something useful about the air in your home — and where the moisture sits tells you most of what you need to know.

There are three places condensation turns up, and each one means something different. Work out which you have first; the fix, or the lack of one, follows from there.

External condensation: usually a good sign

Water beading on the outside of the glass on a cool, clear morning is normal, and on a modern double- or triple-glazed unit it is actually a sign the glazing is doing its job. Because the outer pane is well insulated from the warmth inside, it stays close to the outdoor air temperature. When that surface drops below the dew point, moisture from the air settles on it.

You will see it most in spring and autumn, on north- and east-facing windows, and in sheltered spots where the air is still. It usually clears within an hour or two of sunrise. There is nothing to repair and nothing to spend. If it bothers you, a quick wipe with a squeegee is the whole remedy.

The one thing to watch: if external water is running down the glass and pooling on the frame or sill, check that the drainage slots — the small openings in the bottom of the frame, sometimes called weep holes — are clear of dirt, paint and insect debris. Blocked drainage is a genuine problem; a misty pane on a sunny morning is not.

Internal condensation: the one that points to moisture

Water on the room side of the glass, or trickling down the frame onto the sill, is a ventilation and moisture story rather than a window fault. Warm, moist air meets the coldest surface in the room, and that surface is usually the glass.

Everyday life produces a surprising amount of water vapour: showering, cooking, boiling kettles, houseplants, drying laundry on a rack, and simply breathing. A sealed-up modern home holds onto all of it, because there are fewer draughty gaps for it to escape through.

It is worth tackling when you notice puddles on the sill each morning, black mould in the corners of frames or on the surrounding wall, a musty smell, or curtains and blinds that feel damp. Left alone, that moisture works its way into plaster and timber, which is far more expensive to sort than a change in habits.

Between the panes: a failed sealed unit

If the misting will not wipe off, it is inside the sealed unit — the gap between the two panes. The seal has failed or the desiccant that keeps the cavity dry is spent. You cannot clean it, ventilate it or heat it away, and no dehumidifier will touch it. It does not usually mean the whole window has to go, but the sealed unit will need replacing, and the frame and hinges are worth checking at the same time. If your windows are still within their guarantee period, dig out the paperwork before booking anyone.

A five-minute check to identify what you have

• Wipe it. If the moisture comes off with a cloth, it is surface condensation, inside or out. If it will not budge, it is inside the unit.
• Note the timing. Outside in the morning and gone by ten? Normal. Inside every single morning? Moisture and ventilation. Visible all day, every day, in the same patch? Seal failure.
• Look below the glass. Water pooling on the sill, mould in the frame corners or damp plaster beneath suggests a cold spot as well as moist air.
• Check the pattern. A single hazy patch that never shifts points to the unit. An even film across the whole pane points to the room.
• Test the air. A cheap hygrometer gives you a rough reading of indoor humidity, so you can see whether your changes are making a difference.

What actually fixes internal condensation

Work in this order: cut the moisture at source, then ventilate, then heat. Water you never put into the air cannot land on your glass.

1. Ventilate with intent. Opening a window wide for five minutes shifts more moist air than leaving it ajar for an hour. Do it after showers and cooking, and once in the morning in bedrooms.
2. Use the extractor fans. Run kitchen and bathroom fans while you cook or shower, and for ten to fifteen minutes afterwards. Clean the grilles and check the fan is actually pulling air — a sheet of tissue held near it should be drawn in.
3. Keep trickle vents open. Background ventilation does the quiet work. If yours were painted shut or sealed up by a previous owner, free them.
4. Rethink laundry. Drying indoors releases a lot of water. Where you can, use a vented or condenser tumble dryer, or dry clothes in the bathroom with the door shut and the fan running.
5. Hold some background heat. Gentle, steady warmth suits most homes better than short, intense bursts. Warm air holds moisture; cold surfaces condense it.
6. Close internal doors. Keep steam from the kitchen and bathroom out of bedrooms, where it will find the coldest glass in the house.
7. Keep radiators clear. Curtains draped over a radiator send warmth straight out of the window and leave the glass cold.

If you are still mopping sills after a fortnight of this, a dehumidifier will take the edge off, and a window vacuum makes short work of morning films without soaking the frame. Don't be tempted to seal up an older window with silicone to stop draughts: if it was providing the only ventilation, you will simply move the moisture elsewhere.

When to call in a glazier

Misting between the panes is the clear case for a glazier, along with cracked glass, water coming in during rain, or a window that no longer closes properly. Persistent damp and mould that returns despite good ventilation deserves a proper damp survey rather than guesswork, because the cause may be a leak, blocked guttering or a cold bridge in the wall. If anyone in the home has asthma or another respiratory condition, treat repeated mould as a priority and take professional advice.

Keeping on top of it

Twice a year, take ten minutes to clear the drainage slots, wipe down sills and frames, check trickle vents are open and free of dust, and look for any pane that has started to cloud. Most condensation in British homes is a sign of decent insulation paired with too little air movement — cheap to fix, and easy to ignore until the mould appears. External droplets on a cold morning: leave them. Internal film: ventilate and dry less laundry indoors. Mist that will not wipe away: book the glazier and check the guarantee.

Photo: Dimhou / Pixabay