Window condensation: how many hours your glass runs wet, and what stops it

What we found

Our calculations, data as of 23 October 2026

1. With a room at 18 °C and 65% humidity, single glass in an aluminium frame sits below the dew point for about 2,980 hours a year in Christchurch, 1,160 in Wellington and 550 in Auckland.
2. A clear double-glazed retrofit cuts Christchurch to 430 hours and Wellington and Auckland to 0. In the coldest cities some wet hours remain: 1,450 in Queenstown.
3. The aluminium frame does not improve with the glass. Retrofit units leave a plain aluminium frame as the coldest surface in the room, so condensation moves from the pane to the frame and sill. Thermally broken, uPVC and timber frames fix that.
4. Glazing treats the symptom. Moisture comes from people, cooking, showers and washing dried indoors; extractor fans vented outside and drying clothes outdoors or in a vented dryer reduce condensation on any window.

Hours a year single glazing runs wet, most first

Queenstown	4,420 h
Dunedin	4,220 h
Invercargill	4,060 h
Christchurch	2,980 h
Taupō	2,700 h
Palmerston North	2,430 h
Rotorua	2,370 h
Nelson	1,960 h

Our condensation index: single-glazed aluminium window (R0.15), room at 18 °C and 65% relative humidity (dew point 11.3 °C), ERA5 hourly temperatures 2021–2025.

How we counted wet hours

The inside surface of a window sits somewhere between the room temperature and the outdoor temperature. How close it stays to the room depends on the window’s R-value: the higher the R-value, the warmer the inner surface. For each hour of 2021–2025 in each city, we worked out the inner surface temperature of a window with the room held at 18 °C and counted the hours when it fell below the room air’s dew point. That is our condensation index. It is a comparison tool, not a forecast: a bedroom with two people asleep and the heating off will be more humid and colder than our assumption.

Wet hours by city and window

Single glass in an aluminium frame is wet for a large part of every winter night in the South Island and the central North Island. The single-glazed timber window does noticeably better (R0.19 against R0.15), because the timber frame is warmer. Any double glazing cuts the index sharply, and in Auckland and Wellington takes it to zero.

The frame problem

The index uses the whole-window R-value. A plain aluminium frame is far colder than the glass in a double-glazed unit, so after a retrofit the frame is where water forms first. Owners on New Zealand forums report exactly this after cheap double glazing in plain aluminium: dry glass, wet frames. Thermally broken aluminium has an insulating strip between its inner and outer parts; uPVC and timber are poor conductors anyway. If your frames are aluminium and frame condensation bothers you, compare new windows with a retrofit.

Reduce the moisture as well

Condensation needs a cold surface and moist air. The cold surface is the window; the moist air comes from the household. These cut it on any window:

• Run an extractor fan vented outside while showering and cooking, and for a while afterwards.
• Dry washing outdoors or in a vented or heat-pump dryer, not on racks indoors.
• Avoid unflued gas heaters, which add water vapour with the heat.
• Open windows briefly on dry days, and heat consistently; a cold house holds moisture on every surface.

Rentals must meet the Healthy Homes ventilation standard: openable windows, doors or skylights of at least 5% of the floor area in living rooms, dining rooms, kitchens and bedrooms, and extractor fans vented outside in kitchens and bathrooms. See Healthy Homes and windows.

Secondary glazing and fogging between the panes

Secondary glazing keeps the room-side pane warm, but the old pane behind it gets colder. If room air leaks into the gap, it condenses there. The inner pane must seal tightly to the room. See secondary glazing.

This week: Palmerston North

Single glazing in an aluminium frame runs below the dew point about 2,430 hours a year in Palmerston North (H1 zone 3); clear double glazing cuts that to 350. A low-E retrofit of 25 m² saves $190 a year heated mornings and evenings with a heat pump, or $569 heated all day. Fastest payback here: DIY magnetic secondary glazing, 17 years.

Cheap fix. EECA estimates that closing curtains before dark saves $80–90 a year. Lined, full-length curtains with a pelmet work best. Read more

Read next: Secondary glazingReplacement costHealthy Homes and windowsIs it worth it?Heat loss by city