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Is it worth it?

CalculatorRetrofit, new windows or secondary glazing: installed prices, yearly heating savings and payback for 15 New Zealand cities, from hourly weather data and MBIE power prices.
Is it worth it?We ran 60 combinations of city and heating habit. Retrofit double glazing pays back within 25 years in 21 of them, all in homes heated all day or with plug-in heaters.
Heat loss by cityFive years of hourly temperatures for 15 New Zealand cities, turned into the heat that leaves through single glazing and what it costs at each city's power price.
CondensationSingle glazing runs below the dew point about 550 hours a year in Auckland and 4,220 in Dunedin. Our index for 15 cities, and what fixes it.

Retrofit costRetrofit double glazing costs about $600 per m² in aluminium frames and $750 in timber, low-E included. Prices for 10–35 m², the extras that change a quote, and what it saves.
Replacement costNew double-glazed windows cost about $1,150 per m² in thermally broken aluminium, $1,250 in uPVC, $1,700 in timber. Which frame to choose.
Secondary glazingDIY magnetic secondary glazing costs about $150 per m² and pays back in 12–36 years, faster than any double glazing. What to watch for.
Glass and framesWhat each step up the Building Code's window table is worth a year. Plain double glazing delivers 54% of the best window's saving; frames matter more than extras.
Curtains and alternativesClosing curtains before dark saves $80–90 a year (EECA), sealing draughts about $100. Shrink film on timber windows saves about $103 a year in Wellington.
Villas and timber windowsA low-E retrofit into timber sashes reaches R0.56 and keeps the joinery. What works on double-hung sashes, leadlights and heritage villas.

H1 zones and R-valuesNew Zealand's six H1 climate zones by district, the Code's window R-value table, and which upgrades reach the new-build minimum of R0.46–R0.50.
Building consentMost window work on a house of up to two storeys needs no building consent under exemption 8 of the Building Act. The exceptions: leaking windows and taller buildings.
Healthy Homes and windowsDouble glazing is not required in rentals. We ran the Healthy Homes heating formula in 15 cities: double glazing cuts the heater size needed by 11–12%.
Grants and financeNo government grant pays for double glazing in 2026. Warmer Kiwi Homes funds insulation and heat pumps; banks offer 0–1% home energy top-ups.

Comparing quotesOwners report quotes for the same house 50% or more apart. What to compare: glass Ug, whole-window R-value, frames, spacers, scaffolding and making good.
Owners' experiencesWe read 1,388 posts in 43 New Zealand forum threads from 2010 to 2026: the prices owners were quoted, what they regretted, and what they would do again.

MethodologyHow PaneWise calculates window heat loss, savings, payback and condensation for 15 New Zealand cities, and every source we use.
AboutPaneWise is an independent New Zealand site that calculates what double glazing costs and saves, city by city. We do not sell windows or take referral fees.