Calculator · 15 cities
Double glazing in New Zealand: what it costs and what it saves in your city
Pick your town and your windows. We work out the heat your glass lets out from five years of hourly temperatures, price it at your city’s power rate, and rank every upgrade by how fast it pays for itself.
What would your windows save?
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What we found
Our calculations, data as of 23 October 2026
- Heated with a heat pump in the mornings and evenings, as many homes are, retrofitting low-E double glazing into a house with 25 m² of single-glazed aluminium windows saves about $90 a year in Auckland and $261 in Dunedin. On a $15,000 job, that is a payback of 58–166 years.
- DIY magnetic secondary glazing pays back fastest in every one of the 15 cities, in 12–36 years, because it costs about a quarter as much and still more than doubles the window’s R-value.
- Power savings are not the main return. Single glass in an aluminium frame sits below the dew point of an 18 °C room for about 550 hours a year in Auckland and 4,220 in Dunedin. Retrofit double glazing cuts that to 0 and 770 hours.
- Glazing pays back in the cold south when a home is heated all day with plug-in heaters: 6 years for the low-E retrofit in Dunedin and 6 in Queenstown.
| Dunedin | $261 |
| Queenstown | $253 |
| Invercargill | $229 |
| Palmerston North | $190 |
| Taupō | $186 |
| Christchurch | $186 |
| Rotorua | $181 |
| Nelson | $148 |
Our model: 25 m² of single-glazed aluminium windows, heat pump, heated 7–9 am and 5–11 pm to 18 °C, electricity prices from MBIE's August 2026 survey.
How the calculator works
Windows lose heat in proportion to their area, to how cold it is outside, and inversely to their R-value. We took five years of hourly temperatures for each city (2021–2025, ERA5 reanalysis) and added up how far each hour fell below 18 °C while a home is usually heated: 7–9 am and 5–11 pm. That gives 12,366 degree-hours a year for Wellington and 22,107 for Dunedin. Divide by the window’s R-value, multiply by the area, and you have the heat that leaves through the glass.
The saving is the heat a better window keeps in, divided by how efficiently you make heat (a heat pump delivers about three units of heat per unit of electricity, a plug-in heater one), priced at your city’s average power rate from MBIE’s August 2026 survey. Prices for each option are mid-points of the installed ranges New Zealand glaziers published in 2026, including GST.
Every option, side by side
For a Wellington home with 25 m² of single-glazed aluminium windows and a heat pump used mornings and evenings:
| Option | Window R | Cost | Saving a year | Payback, years |
|---|---|---|---|---|
| DIY magnetic secondary glazing | R0.36 | $3,800 | $149 | 26 |
| Installed low-E secondary glazing | R0.57 | $11,300 | $188 | 60 |
| Retrofit double glazing, clear | R0.26 | $12,500 | $108 | over 100 |
| Retrofit double glazing, low-E argon | R0.30 | $15,000 | $128 | over 100 |
| New uPVC, low-E | R0.50 | $31,300 | $179 | over 100 |
| New thermally broken aluminium, low-E | R0.39 | $28,800 | $157 | over 100 |
| New timber, low-E | R0.56 | $42,500 | $187 | over 100 |
Secondary glazing is cheap because it leaves the old window alone. And in a plain aluminium frame, low-E glass barely beats clear glass: the frame conducts so much heat that the whole window only rises from R0.26 to R0.30. In timber frames the same glass upgrade takes it from R0.44 to R0.56.
Why your heating habits matter more than your windows
The same upgrade saves three times as much when the house is heated all day, and nine times as much when it is heated all day with plug-in heaters. A household that runs a heat pump for a few hours each evening has little heat to save:
| City | Heat pump, part-day | Heat pump, all day | Plug-in, all day |
|---|---|---|---|
| Auckland | $90 | $269 | $806 |
| Hamilton | $127 | $392 | $1,175 |
| Wellington | $128 | $379 | $1,138 |
| Christchurch | $186 | $557 | $1,670 |
| Dunedin | $261 | $780 | $2,339 |
| Queenstown | $253 | $805 | $2,416 |
If you rarely heat, double glazing will not cut your bill much, but the room will feel warmer near the windows and the glass will stay drier. That is a comfort decision, and the calculator’s condensation figure is the one to watch.
What the calculator leaves out
- Sun. North-facing glass gains heat on clear winter days. We ignore it, which slightly flatters the savings.
- Draughts. Old sashes leak air. A retrofit or new window usually seals better too, a gain we do not count.
- Frames. Aluminium frames still run wet even when the glass is double glazed. Thermally broken aluminium, uPVC and timber frames stay drier.
- Noise and security. Laminated or thicker glass can be specified for both. Neither is in the price.
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