Is double glazing worth it in New Zealand?

What we found

Our calculations, data as of 23 October 2026

1. Of 60 combinations of city and heating habit, retrofit low-E double glazing pays back within 25 years in 21, new uPVC windows in 14, and DIY secondary glazing in 55.
2. Every case where double glazing pays back in under 25 years is a home heated all day, or heated with plug-in heaters, or both. In Dunedin a house heated around the clock with plug-in heaters gets its retrofit back in 6 years; with a heat pump used mornings and evenings it takes 58.
3. The cheapest way to cut window heat loss is DIY secondary glazing at about $3,800 for 25 m². It pays back in 12–36 years with a heat pump used part of the day, the fastest of all seven options in every city.
4. If you have neither ceiling nor underfloor insulation, start there. EECA puts the average insulation job at about $4,300 and the saving at up to $340 a year. With a Warmer Kiwi Homes grant of 50–90%, your share is $430–2,150, paid back in roughly one to six years: far quicker than any glazing.

Payback in Wellington, fastest first

DIY magnetic secondary glazing	26 years
Installed low-E secondary glazing	60 years
Retrofit double glazing, clear	over 100 years
Retrofit double glazing, low-E argon	over 100 years
New uPVC, low-E	over 100 years
New thermally broken aluminium, low-E	over 100 years
New timber, low-E	over 100 years

Our model: 25 m² of single-glazed aluminium windows, heat pump, heated 7–9 am and 5–11 pm, Wellington power at 37.2 c/kWh.

Payback depends on how you heat

Windows only save the heat you would otherwise have paid for. A home heated for eight hours a day has far less to save than one heated around the clock, and a home heated with plug-in heaters pays three times as much for each unit of heat as one heated by a heat pump. We ran the low-E retrofit, $15,000 for 25 m² of single-glazed aluminium windows, through four heating patterns in each city. Payback in years:

The left column is how most New Zealand households heat. There, retrofit double glazing takes more than half a century to repay itself on power savings in every city. In the right column, a home heated all day with plug-in heaters, the payback falls to well under ten years in Dunedin, Invercargill and Queenstown.

DIY secondary glazing pays back fastest

Secondary glazing, a clear acrylic or glass sheet fixed inside the existing window, roughly doubles a single-glazed window’s R-value for a fraction of the price of a retrofit. The same four heating patterns, in years:

It has drawbacks: the panels must come off to open the window, condensation can form between the panes if the seal is poor, and it looks like what it is. As a cheap trial in the rooms you heat most, it is hard to beat. See secondary glazing.

New windows: the slowest to pay back

New uPVC or thermally broken aluminium joinery costs two to three times as much as a retrofit. Its extra R-value adds only a little to the saving, so on power alone it is the slowest option:

New joinery makes sense for other reasons: frames that are rotten or leaking, a renovation that is replacing the cladding anyway, or noise. If the frames are sound, a retrofit or secondary glazing gets most of the thermal benefit for much less.

The return that does not show on the power bill

Single glass in winter is a cold surface. With the room at 18 °C, its inner face falls well below the air temperature, which chills people sitting near it and condenses moisture on it. Our condensation index counts the hours each year when single glass in an aluminium frame sits below the dew point of a room at 18 °C and 65% humidity: about 550 in Auckland, 1,160 in Wellington and 4,220 in Dunedin. A clear retrofit cuts these to 0, 0 and 770. Less condensation means less mould on sills and curtains, and that, more than the power bill, is why many owners say the upgrade was worth it. See window condensation.

Before you spend on windows

• Insulation first. If the ceiling or underfloor is uninsulated, those losses are larger than the windows’. Owner-occupiers of pre-2008 homes may qualify for a Warmer Kiwi Homes grant; see grants and finance.
• Heat efficiently. Swapping plug-in heaters for a heat pump cuts the cost of every unit of heat by about two-thirds, more than any window upgrade.
• Curtains. Lined curtains that close tight to the wall or a pelmet reduce night-time losses for little money.
• Then the windows, starting with the rooms you heat and the ones that run wet.

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: Heat loss by citySecondary glazingRetrofit costCondensationGrants and finance