Retrofit double glazing in New Zealand
Keep your frames, change the glass. The cheapest honest route to double glazing in a house that already stands.
Retrofit double glazing fits a sealed double-glazed unit into the window frames you already have. It costs $450–800 per m² of glass (CheckMyBuilder, checked 15 Sep 2026) — roughly half the $800–1,600 per m² of full replacement — and in an existing house of up to two storeys it is usually exempt from building consent.
How the work is done
The glazier removes the sash or the glass, prepares the existing rebate so it will take a thicker sealed unit, fits the unit with new beading, and refits the sash. Most of the work is off site, in a workshop. Your joinery, your sill profiles and your glazing bars all stay where they are.
What a sealed unit is
Two panes of glass held apart by a spacer bar and closed with an edge seal, so the cavity between them stays dry and still. That still cavity is the insulation. EECA notes that cavities of 12–16mm perform best, that a low-E coating can improve performance by up to 30%, and that argon fill adds 5–20%.
Which frames suit it
- Timber sashes, if the timber is sound. $800–1,200 per window (Wellington Glass, Feb 2026). More workshop time than aluminium, which is why they cost more. Timber window retrofit.
- Aluminium joinery, if it still slides and seals. $550–850 per window (Wellington Glass). The most common retrofit in New Zealand. Aluminium window retrofit.
- Not suitable: rotten timber, corroded or distorted aluminium, and frames too shallow to hold a thicker unit. In those cases replacement is the honest answer, not a retrofit that will not last.
What the measure checks before you commit
- Rebate depth: whether the frame will physically hold the unit.
- Frame condition: rot, corrosion, movement, failed seals.
- Hardware: whether hinges, stays and sash cords will carry the extra weight, and what it costs to sort them if not.
- Anything that takes the job outside the consent exemption, such as strengthening a lintel.
- The exact glass sizes, which is what turns a range into a fixed price.
When replacement is the better call
If the joinery has failed, if you want a thermally broken frame, or if you are changing the opening, retrofit is the wrong tool. Thermally broken aluminium adds 25–40% to replacement joinery (Superior Renovations, Sept 2026), and it is the only way to fix the frame as a heat path.
| Rate | Range | Source |
|---|---|---|
| Retrofit into your existing frames | $450–$800 per m² of glass | CheckMyBuilder |
| Aluminium window, retrofit | $550–$850 per window | Wellington Glass |
| Timber window, retrofit | $800–$1,200 per window | Wellington Glass |
A sealed double-glazed unit in cross-section. The spec of your units — glass type, cavity, spacer — is set at the quote, not here.
Retrofit double glazing questions
What is retrofit double glazing?
A sealed double-glazed unit fitted into the window frame you already have. The glazier takes out the single pane, prepares the rebate to take a thicker unit, and refits it. The frame stays.
How much does retrofit double glazing cost in NZ?
$450–800 per m² of glass (CheckMyBuilder, checked 15 Sep 2026; retroGLAZE's own calculator publishes the same band). Per window, Wellington Glass publishes $550–850 for aluminium and $800–1,200 for timber.
Which frames can be retrofitted?
Sound timber sashes and most aluminium joinery. The frame has to be deep enough to take a thicker unit, and the sash has to still open, close and seal once it is heavier. Rot, corrosion or a failed seal usually rules it out.
Do I need a building consent for a retrofit?
Usually not. Schedule 1 Exemption 8 covers window work in an existing dwelling of up to two storeys, provided the window hasn't failed its 15-year durability requirement. Exempt work still has to meet the Building Code. Full conditions.
How long does it take?
It depends on your glazier's workshop, and we won't invent a number. The measure is where you get a real programme, in writing, with your quote.
Is retrofit as warm as replacement?
The glass is the same idea; the frame is not. Retrofit leaves your existing frame, so a plain aluminium frame stays a plain aluminium frame. Only replacement can give you thermally broken joinery.
What about condensation?
A warmer inside pane condenses much less. EECA lists reduced condensation and mould among the benefits of window insulation. It doesn't remove the moisture from the room. More on that.