How to Stop Condensation Building Up on Your Windows in Winter

Condensation on windows in winter is one of the most common complaints from NZ homeowners, and it's more than just an annoyance — left unchecked, it can lead to mould, rot, and health issues. The good news is that most condensation problems can be significantly reduced with a combination of better ventilation, moisture control, and a few affordable upgrades.

Why Does Condensation Form on Windows?

Condensation happens when warm, moist indoor air meets a cold surface — like a single-glazed window on a frosty Southland morning. The air can no longer hold its moisture, and water droplets form on the glass. New Zealand homes, particularly older villas and bungalows, are often poorly insulated and ventilated, making this problem worse. The BRANZ research organisation notes that NZ homes tend to have higher indoor humidity than many comparable countries, partly due to our mild, damp climate and building traditions.

Step 1 – Reduce Moisture at the Source

The single most effective thing you can do is reduce how much moisture you're putting into the air in the first place.

• Cooking: Always use your rangehood and keep lids on pots.
• Showering: Run the bathroom extractor fan during and for at least 15 minutes after every shower.
• Drying clothes: Avoid drying laundry indoors if at all possible. If you must, open a window or use a vented dryer.
• Gas heaters: Unflued LPG and gas heaters release significant moisture (and carbon monoxide) into the air. Consider switching to a heat pump or electric panel heater.
• Subfloor moisture: If your home has a suspended timber floor, a damp subfloor can push moisture up through the house. Check that your subfloor has adequate ventilation and consider a polythene vapour barrier.

Step 2 – Ventilate Your Home Properly

Ventilation is your best friend in the fight against condensation. Fresh air dilutes moist indoor air and carries it outside.

• Open windows briefly each morning (even 10 minutes helps) to flush out overnight moisture.
• Make sure extractor fans in bathrooms and kitchens are working properly and vented to the outside — not just into the ceiling cavity.
• Consider a heat recovery ventilator (HRV) or positive pressure ventilation system if condensation is severe throughout the house. These systems bring in fresh air while retaining warmth. Check Energywise for guidance on whole-house ventilation options.

Step 3 – Keep Your Home Warmer

A warmer window surface means less condensation. Keeping your home consistently heated (rather than letting it get very cold overnight) reduces the temperature difference between the glass and the air.

• Insulation is the most cost-effective long-term fix. Ceiling and underfloor insulation keeps the whole building warmer. The Healthy Homes Standards require rental properties to meet minimum insulation levels — owner-occupiers should aim for the same.
• Curtains and blinds trap a layer of air near the glass, but they can also trap moisture between the curtain and the window. Make sure curtains don't seal tightly at the bottom, and open them during the day to let air circulate.

Step 4 – Upgrade Your Glazing

Single-glazed aluminium-framed windows are the worst offenders because both the glass and the frame get very cold. Upgrading to double glazing dramatically raises the inner glass surface temperature, greatly reducing condensation.

• Double glazing retrofit: Many NZ companies offer secondary glazing or full double-glazed unit replacements. This is a significant investment but makes a real difference.
• Secondary glazing film: Affordable DIY window insulation film kits are available at Bunnings and Mitre 10. They create an insulating air gap and can reduce condensation noticeably.
• Thermal curtain liners are a budget-friendly option that adds some insulation value.

Note on building consent: Replacing windows with a different size or type may require a building consent. Replacing like-for-like is generally a Schedule 1 exemption, but always confirm with your local council before starting work. See MBIE Building Performance for guidance.

Step 5 – Deal With Existing Mould Promptly

If condensation has already caused mould on window frames or walls, clean it up before it spreads.

• Use a solution of water and household bleach (1 part bleach to 9 parts water) or a commercial mould remover on hard surfaces.
• For painted surfaces, Resene offers mould-resistant paints and a wash product called Resene Moss & Mould Killer that can help treat affected areas before repainting.
• Soft furnishings or curtains with mould growth may need to be replaced.

Quick Comparison: Condensation Solutions

When to Call a Professional

Some condensation fixes are straightforward DIY jobs, but others need a qualified tradesperson:

• Extractor fan installation or rewiring is restricted electrical work and must be carried out by a registered electrician. Check the Electrical Workers Registration Board (EWRB) to verify your electrician's registration.
• Gas heater removal or replacement must be done by a licensed gasfitter registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB).
• Window replacement involving structural changes requires an LBP (Licensed Building Practitioner).
• Persistent mould or moisture issues that don't respond to ventilation improvements may indicate a deeper problem — a building inspector or experienced handyman can assess subfloor drainage, wall cavities, and flashings.

For general condensation-busting work like installing ventilation systems, fitting secondary glazing, or improving insulation, a skilled local handyman can save you time and get the job done right. You can find qualified tradespeople in your area on tradedirectory.co.nz.

Sources & Further Reading

• BRANZ – Moisture and condensation in homes
• Energywise – Ventilation and heating advice
• Tenancy Services – Healthy Homes Standards
• MBIE Building Performance – Building consent guidance
• WorkSafe NZ – Safe use of household chemicals
• Resene – Mould-resistant paints and treatments