Why Your Windows Fog Up and How to Stop It
Wake up on a cold Waikato morning and the windows are usually the first thing you notice: streaked with water, pooling on the sill. Most of us just wipe it away and get on with the day. But that fogged-up glass is telling you something about how your home is performing. Left unchecked, condensation feeds mould, damages frames and sills, and makes a home feel colder than it actually is. Here's what's actually happening on that glass, and what stops it for good.
Condensation forms when warm, moist air meets a surface cold enough to cool it below its dew point, and glass is usually the coldest surface in any room. Showering, cooking, drying washing indoors, even breathing, all add moisture to the air, and in winter that warm indoor air is constantly meeting cold glass and frames. BRANZ research found mould in 48% of owner occupied NZ homes, with 32% reporting dampness and 75% experiencing winter condensation, driven by inadequate moisture removal, insufficient heating and poor building design. Bedrooms fare worst: three quarters of measurements showed indoor humidity above 60% in winter, largely because doors stay shut overnight while people breathe out moisture for eight hours straight.
Why single glazing and older aluminium frames make it worse
Single glazed windows condensate more because there's almost nothing between the warm air inside and the cold air outside, so the inner surface drops close to outdoor temperature and hits dew point easily. Standard aluminium frames without a thermal break make it worse again, since aluminium conducts cold just as efficiently as heat. Double glazing helps because the sealed gap between panes acts as an insulating buffer, keeping the inner pane closer to room temperature, part of why we think double glazing is a smart choice for New Zealand homes. Thermally broken frames take that further, using an insulating barrier so the frame itself stops acting as a bridge for cold. More on that in our piece on what thermally broken windows are and whether they cost more.
Worth knowing: double glazing won't eliminate condensation completely. A touch of moisture on the outer face of the glass on a cold, still morning is normal, and actually a sign the insulation is doing its job.
When does condensation turn into a mould problem?
Persistent condensation matters because it's the trigger for mould, and mould is a health issue as much as a maintenance one. Moisture sitting on a sill or frame soaks into paint, timber and plasterboard, giving mould spores exactly what they need, and from there it spreads to curtains, carpets and wall linings. Damp, mouldy housing is linked to higher rates of asthma and respiratory illness, particularly in children, which is why the government's Healthy Homes ventilation standard now sets specific requirements for rentals: every living room, dining room, kitchen and bedroom needs openable windows, doors or skylights totalling at least 5% of floor area, and kitchens and bathrooms need extractor fans that vent outside, not just recirculate.
What we do differently to keep glass and frames dry
Our double glazing and thermally broken aluminium joinery is built to keep glass and frame surfaces closer to room temperature, the single biggest factor in whether condensation forms at all. Installation matters just as much as the product: a poorly sealed unit lets cold air in around the frame and creates cold spots exactly where condensation likes to start, which is why we walk homeowners through our installation process from site measure to final seal rather than cutting corners. Getting the joinery right handles the building side; moisture management inside the home does the rest, covered in our guide to improving a home's thermal efficiency this winter.
A few habits make a genuine difference alongside better glazing:
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Run kitchen and bathroom extractor fans during and after cooking or showering, and make sure they vent outside, not into the roof cavity
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Open windows for ten to fifteen minutes a day, even in winter, to let moist air out and dry air in
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Dry washing outside or somewhere ventilated. An indoor clothes airer can add litres of moisture to the air over a day
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Keep the home consistently heated rather than letting it swing cold overnight
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Wipe down condensation before it has time to soak into sills or frames
None of these replace good glazing, but they take the load off it, and they work best alongside joinery that isn't fighting a losing battle against a cold frame or a single pane.
Is condensation between double glazed panes normal?
No. Condensation on the outer or inner face is normal in the right conditions, but moisture between the two panes means a failed seal. That's a warranty and repair issue, worth getting checked rather than living with.
Does double glazing stop condensation completely?
It significantly reduces it, but won't eliminate it outright if a home has poor ventilation, high indoor humidity or inconsistent heating. Those factors still decide how much moisture the air is carrying in the first place.
Condensation is a symptom, not the problem. Fixing it means working both sides: joinery that keeps glass and frames warmer, and a home ventilated and heated well enough that the air isn't overloaded with moisture. We've seen what a difference thermally broken frames and properly sealed double glazing make to homes that used to wake up dripping every winter morning. Pair that with a few sensible habits around ventilation and heating, and the problem is usually gone for good. If foggy windows and damp sills have become a regular feature of your winter, get in touch for an obligation-free assessment. We'll walk through your home's specific windows and frames and talk through what an upgrade would involve.
