Condensation on Windows: What Your Home Is Telling You

Most people treat the winter morning wipe-down as a household chore, somewhere between emptying the dishwasher and putting the bins out. It is actually a measurement. Your windows report on the air in your home every cold morning, for free.

Why the water appears on the glass first

Air holds water vapour, and warm air holds more of it than cold air. Cool a parcel of air far enough and it reaches a point where it cannot hold what it is carrying. That temperature is the dew point, and the vapour comes out as liquid water on whatever cold thing it touched.

In a Brisbane bedroom on a July morning, the room air might be sitting at 20 degrees and carrying plenty of moisture. The single pane of glass, with outdoor air pressed against the other side of it, might be sitting at 12. The aluminium frame is often colder still, which is why the frame and the bottom of the sill are usually wetter than the middle of the pane. Nothing has gone wrong. The window is simply the first place in the room cold enough to make the moisture visible, and that visibility is the useful part. It is one of the earliest signals in the whole damp and mould picture in Brisbane homes.

Glass is not the problem. It is the first surface cold enough to show what the air in the room is already carrying.

This is not a window fault

People often ring a glazier. Occasionally that is warranted, if a double-glazed unit has failed and the fogging sits sealed between the panes where no cloth can reach it. Almost always it is not. The glass is behaving exactly as physics requires.

There is an uncomfortable wrinkle here. Homes that have been sealed up and made more airtight, with new windows, weather seals and draught stopping, frequently get worse condensation rather than better. The leaky old house was ventilating itself by accident, all day, through every gap. Take the gaps away without adding deliberate ventilation and the moisture generated inside has nowhere to go. This is one reason the problem is not confined to older housing stock. Queenslanders carry their own risk pattern, and new builds have theirs, including condensation forming inside steel-framed wall cavities where nothing at all is visible until it is advanced.

The glass is only where you can see it

This is the part that matters most, and the part that gets missed. Condensation is not happening because the window is special. It is happening because a surface in that room dropped below the dew point of the air around it. Every external wall in that room is colder than the internal walls. Some spots on those walls are colder again, and the air over them is barely moving.

So while you are wiping the glass, the same process is running in places nobody is looking:

• The external wall behind a wardrobe or a tall bookcase pushed flat against it, where no air moves at all
• The back of a bedhead against an external wall, in a bedroom with the door and window shut overnight
• Room corners where 2 external walls meet, which lose heat faster than the flat wall between them
• Inside built-in robes built on an external wall, behind a full row of hanging clothes
• Behind long curtains that hang past the sill, trapping a pocket of cold, still, damp air against the wall
• Under and behind furniture on the south side of the house, where the wall gets no sun all winter

Those locations are worse than the window in every respect. Darker. Stiller. And unlike glass, plasterboard, timber and fabric are porous, so growth can extend below the surface rather than sitting on top of it where a cloth can reach. Housekeeping genuinely matters here, not because dust causes mould, but because dust and clutter feed it, block the airflow that would otherwise dry those surfaces, and hide the early signs until they are no longer early.

Do not rely on your nose. Most significant mould problems have no smell at all, and where there is a smell, nose blindness sets in within a few days of living with it. Absence of odour is not reassurance. Colour is no better as a guide: the same species can appear in different colours on different materials in the same room, and some moulds are effectively transparent.

Most of the water is coming from inside the house

Brisbane's outdoor humidity gets the blame, and it does contribute. But a closed-up house on a dry winter morning can still stream with condensation, because the occupants are the humidifier.

• A hot shower puts a substantial volume of water vapour into the air within minutes, and most of it leaves the bathroom the moment the door opens
• Cooking, especially anything boiling or simmering uncovered, releases water continuously
• Drying laundry indoors on an airer moves every litre of water in that load into your air, slowly, over a day or 2
• Unflued gas heaters produce water vapour as a combustion product, directly into the room being heated
• 2 adults sleeping in a closed bedroom overnight add a meaningful amount of moisture simply by breathing
• Aquariums, indoor plants, wet towels on rails and drying pet bedding all add to the total

That is why condensation is often worst in bedrooms rather than bathrooms. The bathroom has an extraction fan and hard tiled surfaces you can squeegee. The bedroom has soft furnishings, a closed door, 8 hours of breathing and a wardrobe pressed against the coldest wall in the house.

What to actually do about it

1. 1Wipe it the same morning, not that afternoon. Given the right conditions mould can begin growing in around 48 hours, and drying a wet surface within hours rather than days is the single most useful action anyone takes against it. Use a dry cloth, wring it into the sink, and do the frame and the sill as well as the pane.
2. 2Ventilate hard and briefly rather than gently and all day. Opening 2 windows on opposite sides of the house for 5 to 10 minutes moves the moisture-laden air out and lets drier air in, without stripping the heat out of the walls. Cross-ventilation beats 1 window on its own by a wide margin.
3. 3Extract at the source. Run the bathroom fan during the shower and for at least 20 minutes afterwards with the door shut, so it pulls from the bathroom rather than the hallway. Check that your rangehood actually vents outside rather than recirculating through a charcoal filter, because a recirculating hood returns every gram of that water vapour to the kitchen.
4. 4Move the laundry outside, or contain it. If drying indoors is unavoidable, put it in 1 room with the door closed and either a window open or a dehumidifier running. Do not spread it through the living areas.
5. 5Pull the furniture off the external walls. A gap of 5 to 10 cm behind a wardrobe, a bedhead or a lounge lets air circulate and lets the wall dry. It costs nothing and it removes the exact conditions that make those spots worse than the window.
6. 6Heat more evenly rather than in bursts. A room heated hard for 2 hours at night and then left to go cold gives you a large temperature swing and plenty of condensation at the bottom of it. Gentler, steadier heat produces less.
7. 7Measure. A hygrometer takes the guesswork out entirely. Put 1 in the worst room and 1 in a room you consider fine, then watch what actually happens across a day.

Dehumidifiers help, and in a genuinely damp room they help a lot. They also need maintaining. The tank, the filter and the coils all need regular cleaning, because a neglected dehumidifier sitting in a warm room with a reservoir of standing water becomes a moisture and microbial problem in its own right. Which type suits your home depends on the room, the temperature and how you intend to use it, so there is no single right answer to hand out.

When wiping it up is no longer the answer

Persistent condensation is worth taking further, particularly condensation appearing on wall surfaces rather than only on glass. So are stained or blackened timber sills, paint bubbling or lifting near the floor or the window reveal, a room that feels different from the rest of the house, or clothes at the back of a robe that feel faintly cool and damp when everything else is dry.

At that point the field of mould and moisture assessment exists for a reason. An assessment involves an inspector taking moisture readings in building materials, using thermal imaging to find cold and wet areas behind linings, and where appropriate collecting air or surface samples for laboratory analysis. The laboratory is the only place identification actually happens, which is also why "black mould" is a colour rather than a species.

What Beyond Clean Living does is narrower and more practical. As part of a clean, surface mould is removed where it is safe and appropriate to do so, physically rather than chemically, because physical removal is the goal and killing it is not. On non-porous surfaces a cloth, detergent and genuine effort will take it off. On porous materials the growth extends below the surface, and the right approach depends on the material and how far it has gone: sometimes the material has to come out. Bleach is a poor choice on both counts, because it strips the colour while leaving growth in porous materials, and it is mostly water, so it adds moisture to the exact place that already has too much. During a whole-home walkthrough and risk review Belinda looks at where moisture is being generated, where it is settling, and which parts of the house are quietly heading the wrong way, then says plainly what needs a specialist. Full remediation is specialist work.

A reasonable target

A healthy home sits at roughly 40 to 65 percent relative humidity, dries any water event within hours rather than days, and carries an indoor fungal profile that broadly resembles the outdoor air around it. You cannot eliminate the 3 things mould needs, which are moisture, food and time. You can influence all 3: less dust and clutter, fewer soft furnishings holding damp, and a much shorter gap between something getting wet and something being dry. Dormant spores and dead growth still count, so whether what you find is alive is not the measure of whether you have a problem.

The condensation on your glass is the cheapest diagnostic your house will ever hand you. Read it before you wipe it.