Options · secondary glazing
Secondary glazing in New Zealand: cost, savings and the catch
A second pane fixed inside the old window, magnetic acrylic you fit yourself or an installed aluminium-framed unit. It is the cheapest way to more than double a single-glazed window’s R-value, and the one owners most often get wrong.
What would your windows save?
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What we found
Our calculations, data as of 23 October 2026
- DIY magnetic acrylic secondary glazing costs about $3,800 for 25 m² and pays back in 12–36 years with a heat pump used mornings and evenings, the fastest of all seven options in every one of our 15 cities.
- In university hot-box tests on a single-glazed aluminium window, magnetic acrylic lifted the R-value from R0.15 to R0.36, and an aluminium-framed low-E secondary unit to R0.57, better than a low-E retrofit in the same frame (R0.30).
- The room-side pane stays warm enough to run dry almost all year: our condensation index falls from 2,980 hours a year to 0 in Christchurch. The catch is the old pane behind it, which can still fog if moist room air leaks into the gap.
- Installed low-E secondary glazing, at about $11,300, pays back in 29–85 years: slower than DIY, faster than any retrofit or new window.
| 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.
Two kinds of secondary glazing
DIY magnetic acrylic. A sheet of 3–4 mm clear acrylic is cut to size, edged with flexible magnetic strip and pressed onto a steel strip fixed around the window reveal. Kits and pre-cut panels are sold online; materials run roughly $100–250 per m² depending on panel size and trim. The panel comes off in seconds to open the window.
Installed secondary units. A glazier fits a slim aluminium frame inside the reveal, holding glass or acrylic, often low-E coated, sometimes as sliding or hinged sashes so the window behind can still be opened. Installed prices run about $300–600 per m².
R-values: what a second pane does
Hot-box tests at Victoria University of Wellington on a single-glazed aluminium window measured these whole-window R-values:
- Original single-glazed window: R0.15
- Thin plastic film kit: R0.35
- Magnetic acrylic: R0.36
- Aluminium-framed secondary glazing: R0.34
- Aluminium-framed low-E secondary glazing: R0.57
The still air between the panes does most of the work. The gap is usually much wider than in a factory-sealed unit, and the original frame is shielded from the warm room, which is why even a plain acrylic sheet rivals a clear retrofit IGU in an aluminium frame (R0.26).
Savings and payback by city
With a heat pump used mornings and evenings and 25 m² of single-glazed aluminium windows:
| City | DIY: saves/yr | DIY: payback | Installed low-E: saves/yr | Installed: payback |
|---|---|---|---|---|
| Dunedin | $304 | 12 | $385 | 29 |
| Queenstown | $295 | 13 | $373 | 30 |
| Invercargill | $267 | 14 | $337 | 34 |
| Palmerston North | $222 | 17 | $280 | 40 |
| Taupō | $217 | 18 | $274 | 41 |
| Christchurch | $217 | 18 | $274 | 41 |
| Rotorua | $211 | 18 | $267 | 42 |
| Nelson | $172 | 22 | $218 | 52 |
| Tauranga | $163 | 23 | $206 | 55 |
| New Plymouth | $160 | 24 | $202 | 56 |
| Napier | $149 | 26 | $188 | 60 |
| Wellington | $149 | 26 | $188 | 60 |
| Hamilton | $148 | 26 | $187 | 60 |
| Whangārei | $138 | 28 | $174 | 65 |
| Auckland | $105 | 36 | $133 | 85 |
Heated all day, or with plug-in heaters, the paybacks fall to about a third. In a Dunedin house heated around the clock with plug-in heaters, DIY panels costing $3,800 save $2,728 a year.
Condensation: better, with one catch
Hours a year the room-side glass sits below the dew point of a room at 18 °C and 65% humidity:
| City | Single glazing | DIY secondary | Installed low-E secondary | Retrofit low-E |
|---|---|---|---|---|
| Auckland | 550 | 0 | 0 | 0 |
| Wellington | 1,160 | 0 | 0 | 0 |
| Christchurch | 2,980 | 0 | 0 | 120 |
| Dunedin | 4,220 | 30 | 0 | 270 |
| Queenstown | 4,420 | 90 | 0 | 590 |
The room-side pane is warm, but the old pane behind it is now colder than before, because the room no longer heats it. If moist room air leaks into the gap, it condenses there, where you cannot wipe it. Good practice:
- Seal the inner pane tightly to the room; a gap that leaks room air is the cause of fogging.
- Leave the outer window to breathe slightly to the outside, which is how most old windows behave anyway.
- Fit panels when the window and air are dry, and check the gap on the first cold mornings.
Noise
The wide air gap of secondary glazing, often 50–100 mm, is better at blocking traffic noise than the narrow cavity of a standard IGU. If noise matters as much as heat, an installed secondary unit with laminated glass or thick acrylic is usually more effective per dollar than new double-glazed windows.
Drawbacks
- Opening windows means removing panels or sliding a second sash. In a rental, tenants need to be shown how.
- Acrylic scratches. Clean with a soft cloth and plain water or a plastic-safe cleaner.
- It looks like an add-on. Installed units with powder-coated frames are much less visible than DIY strips.
- The window still needs to open for ventilation. Rooms that rely on window opening to vent moisture need a plan for how that will happen.
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