Costs · new windows
New double-glazed windows: replacement cost and which frame to choose
Replacing the whole window, frame and all, costs two to three times as much as a retrofit. What you get for the extra money is mostly in the frame: a warmer, drier edge and a seal that does not leak air.
What would your windows save?
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What we found
Our calculations, data as of 23 October 2026
- For 25 m² of windows, new double-glazed joinery costs about $28,800 in thermally broken aluminium, $31,300 in uPVC and $42,500 in timber, against $15,000 for a low-E retrofit into existing aluminium frames.
- The frame decides the R-value. With the same entry-level low-E glass, the Building Code’s own table rates a window at R0.30 in plain aluminium, R0.39 thermally broken, R0.50 in uPVC and R0.56 in timber.
- Plain aluminium cannot meet today’s new-build minimum of R0.46 even with the best double-pane glass in the table (R0.39). Thermally broken aluminium needs high-performance low-E glass to reach it.
- On heating savings alone, new uPVC windows take 86–248 years to pay back with a heat pump used part of the day. Replace windows when the frames are failing, not for the power bill.
| New timber, low-E | $42,500 |
| New uPVC, low-E | $31,300 |
| New thermally broken aluminium, low-E | $28,800 |
| Retrofit double glazing, low-E argon | $15,000 |
| Retrofit double glazing, clear | $12,500 |
| Installed low-E secondary glazing | $11,300 |
| DIY magnetic secondary glazing | $3,800 |
Mid-points of 2026 installed price ranges including GST. New joinery prices include removal of the old windows; scaffolding and cladding repairs are extra.
What new joinery costs
Published 2026 ranges for supplied and installed double-glazed windows run about $800–1,500 per m² for thermally broken aluminium, $900–1,600 for uPVC and $1,200–2,200 for timber. We use the mid-points. For the same window area:
| Option | 10 m² | 15 m² | 25 m² | 35 m² |
|---|---|---|---|---|
| DIY magnetic secondary glazing | $1,500 | $2,300 | $3,800 | $5,300 |
| Installed low-E secondary glazing | $4,500 | $6,800 | $11,300 | $15,800 |
| Retrofit double glazing, clear | $5,000 | $7,500 | $12,500 | $17,500 |
| Retrofit double glazing, low-E argon | $6,000 | $9,000 | $15,000 | $21,000 |
| New thermally broken aluminium, low-E | $11,500 | $17,300 | $28,800 | $40,300 |
| New uPVC, low-E | $12,500 | $18,800 | $31,300 | $43,800 |
| New timber, low-E | $17,000 | $25,500 | $42,500 | $59,500 |
Large sliders and bifold doors cost more per m², and every opening needs flashings, reveals and trims made good, which is why per-window quotes for full replacement often start around $1,500–2,500.
Frame and glass: the Building Code’s table
The Building Code’s energy-efficiency acceptable solution, H1/AS1, includes a table of whole-window R-values for typical New Zealand house windows, by glass performance (Ug, lower is better) and frame. The double-pane rows:
| Glass (Ug) | Aluminium | Thermally broken aluminium | uPVC | Timber |
|---|---|---|---|---|
| Clear glass, Ug 2.9 | R0.24 | R0.30 | R0.37 | R0.41 |
| Clear, wide cavity or argon, Ug 2.6 | R0.26 | R0.32 | R0.40 | R0.44 |
| Entry low-E, argon, Ug 1.9 | R0.30 | R0.39 | R0.50 | R0.56 |
| Mid low-E, argon, Ug 1.6 | R0.33 | R0.42 | R0.56 | R0.63 |
| High-performance low-E, argon, Ug 1.3 | R0.35 | R0.46 | R0.63 | R0.71 |
| Better high-performance low-E, Ug 1.1 | R0.37 | R0.50 | R0.69 | R0.77 |
| Top double-pane low-E, Ug 1.0 | R0.39 | R0.52 | R0.73 | R0.81 |
Two things follow. First, a better frame does more than better glass: entry low-E glass in a timber or uPVC frame beats the top glass in plain aluminium. Second, if you are replacing frames anyway, the step from plain to thermally broken aluminium is the cheapest large gain in the table.
For a new house or an addition, windows must meet R0.46 in climate zones 1–4 and R0.50 in zones 5 and 6 under the schedule method of the fifth edition, which can be used for consent applications made until 26 November 2026. The sixth edition, in force since 27 November 2025, drops the schedule method and uses a whole-house calculation instead. Replacing windows in an existing house does not have to reach those figures, but they show what the market now builds to.
Frames and condensation
Aluminium conducts heat about a thousand times better than timber, so a plain aluminium frame is the coldest surface in a heated room and the first to run with water. Retrofit glass fixes the pane but not the frame. If condensation pooling on sills is your main complaint, a frame with a thermal break, or uPVC or timber, is the fix, and that is one of the best reasons to replace rather than retrofit. See window condensation.
When replacement is the right call
- The frames are rotten, distorted or leaking, so retrofit glass would be fitted into a failing window.
- You are re-cladding or renovating and the openings are being opened up anyway.
- You want a different opening type, such as tilt-and-turn, or windows that seal far better against draughts and noise.
- Condensation on the frames, not the glass, is the problem.
If the frames are sound, a retrofit or secondary glazing delivers most of the comfort for much less.
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