How to Stop Window Condensation in NZ Homes: Winter 2026 Fixes That Don't Blow Your Power Bill

If you're reading this in late April, you're probably about to lose the war with your windows. May is when condensation starts in most NZ homes, and this year there's an added twist — power prices are going up the same week. Contact lifts daily charges from 1 May, Genesis raises retail prices an average of 12.8% from 16 May, and Meridian has warned customers to expect up to 7%. Cranking the heater to dry the place out has just got more expensive.

Good news: most of what works costs nothing. This guide walks through what's actually causing the fog on your windows, the four-step diagnostic to figure out where the moisture is coming from, and a tiered list of fixes from $0 to $8,000 — so you can spend exactly as much (or as little) as your home actually needs.

At a Glance

The two ironclad rules: find the moisture source before you spend a dollar, and a drier house heats more cheaply than a damp one — which matters more this winter than last.

Why Your Windows Fog Up (The Physics in Plain English)

Air holds water as invisible vapour. The warmer the air, the more vapour it can hold. When warm, moist indoor air touches a cold surface (your single-glazed window at 6am), the air right next to that surface cools down. Cold air can't hold as much moisture, so the excess condenses out as visible water droplets.

The temperature at which that happens is called the dew point. Two things change it: how much moisture is in your air (humidity), and how cold the surface is. Lower the humidity or warm the surface, and the condensation goes away.

That's why there are really only three things you can do:

1. Stop putting so much moisture into the air in the first place
2. Get the moisture out faster (ventilation)
3. Stop the surface getting so cold (insulation, double glazing, curtains)

Most homeowners reach straight for option 3 (heating harder, buying thermal curtains) when option 1 and 2 are usually cheaper and faster. With Genesis and Contact putting prices up in May, getting that order right matters more than usual.

The Four-Step Moisture Diagnostic

Before you buy anything, spend ten minutes working out where your moisture is actually coming from. The average NZ household pumps out 8–10 litres of water vapour a day from cooking, showering, breathing, and drying clothes. Yours might be more.

Step 1 — Cooking. Do you boil pasta with the lid off? Run the kettle without venting? Slow-cook with the rangehood off? Each open pot is a couple of hundred millilitres of steam straight into the air.

Step 2 — Showering. Two daily showers in a household of three is roughly 2 litres of moisture per day, all dumped into a small tiled room. If your bathroom door gets opened straight after the shower without the fan running, that moisture migrates to the coldest room in the house — usually the main bedroom, which is exactly where you'll see the worst window condensation.

Step 3 — Laundry. A single load of wet washing dried indoors releases about 2 litres into the air. If you're drying overnight in winter with the windows closed, you've effectively filled a tray of water and left it evaporating in the lounge.

Step 4 — Unflued gas. Portable LPG heaters and unflued gas cooktops release roughly 1 litre of water vapour for every litre of gas burned. If you've still got an unflued gas heater going in winter, it is a moisture machine. Get rid of it.

If steps 1–4 produce a clear culprit, fix that first before spending money on a system. A $30 lid for the pasta pot does more than a $500 dehumidifier most days.

Free Fixes (Do These Tonight)

These cost nothing and will reduce indoor humidity by 5–15%. For a lot of homes, that's the difference between condensation and clear glass.

• Open windows for 10–15 minutes each morning — yes, even in winter. Cold outdoor air is drier than warm indoor air. Swap them and you remove moisture without having to reheat the whole house. Do it before you put the heat on, not after.
• Run the rangehood every single time you cook — not just when you remember. Recirculating-only rangehoods (the ones that vent back into the kitchen through a charcoal filter) are nearly useless for moisture; if yours doesn't actually vent outside, you're effectively cooking with no extraction.
• Bathroom fan on at the start of the shower, off 10 minutes after — get the moisture out at source. A fan running for 30 minutes uses about 1 cent of electricity. A wet bedroom across the hallway costs you a lot more.
• Stop drying laundry indoors — or if you must, do it in one closed room with the window cracked and the extractor fan on. Never overnight.
• Lids on pots, pan covers when you can — halves the steam from boiling and simmering.
• Don't block trickle vents — those little slots above your windows are there for a reason. Curtains, dust, and "I sealed those up to keep warm" all defeat them.

If you've tried all six of these for two weeks and your windows still pour with water every morning, the moisture is coming from somewhere structural and you need to spend a bit.

The $200–$800 Tier: Better Extraction

The biggest single upgrade for most homes is a properly sized, properly vented bathroom and kitchen extractor.

Bathroom extractor fan: A modern unit ducted directly outside (not into the roof space) costs $200–$500 supplied and installed. Run-on timers and humidity sensors are worth the extra $50 — they keep the fan going for 10–15 minutes after you've left the room, which is exactly when most of the steam is still in the air.

Kitchen rangehood: If yours is recirculating, replacing it with a ducted unit is the highest-impact $400–$800 you'll spend on moisture. Make sure the duct goes the shortest possible path to the outside — long, kinked ductwork kills extraction.

Plug-in dehumidifier: A 12L–20L unit ($250–$700) is genuinely useful in one problem room — drying clothes overnight, reducing wardrobe condensation, or knocking the worst off a bedroom in mid-winter. Running 8 hours a day at the new May electricity rates costs roughly $0.40–$0.70 per day. They are not, however, a whole-house solution; you'll just be moving moisture from one room to another while paying to run a fan.

The $3,000–$10,000 Tier: Mechanical Ventilation

If your home is consistently above 65% relative humidity in winter, you have visible mould returning year after year, or a family member's asthma flares every May, mechanical ventilation starts to earn its keep. This is where the system choice matters — and where the existing article on positive pressure vs balanced ventilation goes deep on the trade-offs.

Quick summary of what to expect in 2026:

• Positive pressure ($3,000–$5,000) suits older, draughty homes with a clean, dry roof space. The catch: if your roof space isn't dry, the system can make things worse. Insist on a pre-install moisture check.
• Balanced / HRV ($5,000–$10,000) suits newer, sealed homes and chronic moisture cases. Heat recovery means the fresh air arrives pre-warmed, which becomes more valuable every time the gentailers put prices up.

For the full diagnostic on which system suits which home, plus brand-by-brand comparison and the controversies around positive pressure, see our complete home ventilation guide.

How a Drier House Heats More Cheaply

Here's the bit most articles skip — and the part that matters most given the May 2026 price hikes.

Heating damp air takes more energy than heating dry air. There are two reasons:

1. Latent heat load: when you heat moist air, some of the energy goes into raising the temperature of the water vapour itself, not the air around it. Drier air heats faster for the same number of watts.
2. Perceived temperature: a room at 18°C and 50% relative humidity feels noticeably warmer than the same room at 18°C and 75% humidity. So you can run your heat pump at a lower setpoint and still feel comfortable. Drop the thermostat by 1°C and you'll cut your heat pump's running cost by roughly 8–10%.

In practical terms: if you're paying $250 a month to heat a damp house this winter, getting the humidity down to 50–55% could shave $20–$30 a month off the bill while making the place feel warmer. Over a four-month winter, that's $80–$120 — not life-changing, but enough to cover the cost of two new extractor fans within a single season.

Healthy Homes Standards and WKH

If you're a renter or a landlord, the Healthy Homes Standards require every habitable room to have a window that opens, plus mechanical extraction in any kitchen and bathroom installed after 1 July 2019. If your bathroom fan vents into the roof space rather than outside, it doesn't count — that's the most common compliance gap.

For owner-occupiers thinking about insulation as part of the moisture fix, the Warmer Kiwi Homes programme covers up to 80–90% of insulation costs for eligible households (the income and Deprivation Zone thresholds were widened in early 2026). Insulation alone won't fix a moisture problem, but it warms cold surfaces — which raises your dew point margin and reduces condensation. Check your eligibility before you book any insulation work.

The Order of Operations

If you're starting from scratch, do them in this order:

1. Diagnose — work through the four moisture sources above. Most homes have one dominant culprit.
2. Habits first — two weeks of disciplined window-opening, lid-on cooking, and proper extractor use. Free.
3. Extraction upgrades — bathroom fan, ducted rangehood. $400–$1,500 total for most homes.
4. Insulation if you're still cold — ceilings and underfloor make the biggest difference. WKH-eligible households often pay $300–$800 out of pocket. See our underfloor vs ceiling insulation breakdown for where to start.
5. Mechanical ventilation last — if humidity is still chronically high after steps 1–4, that's when a $3,000–$8,000 system pays off.

Spending in the wrong order — installing HRV before fixing a recirculating rangehood, for instance — is one of the most common ways NZ homeowners waste money on moisture problems.

Get Quotes Without Wasting Time

If you've done the habits and extraction tier and you're still seeing condensation in May, it's worth getting two or three ventilation quotes. Get installers to walk through your home, look at your roof space, and explain why they're recommending what they're recommending. Be wary of any quote that doesn't include a moisture inspection of the roof cavity — that's the single biggest red flag for a positive pressure install.

Find ventilation installers in your area for a few quotes, or take our free assessment if you're not sure whether ventilation is even the right starting point for your home.

Winter's coming, and so are higher power bills. The cheapest way through both is a drier house — and most of that, you can do this weekend with nothing but the windows you already own.