Retrofitting Wellington windows with double glazing
Retrofit double glazing keeps the window frames you have and changes what goes in them, whether that's new glass in the old sashes, new timber sashes or an aluminium insert fitted inside the timber surround. If your frames are sound, you might not need new windows at all. In Wellington, also ask what wind zone the new glass is designed for. And when the frames can't be kept, double glazed windows can go in as new joinery instead.
Ask for a quote
Retrofit methods side by side
Three of these methods keep the frame, and the fourth replaces the whole window.
Whichever one you pick, the glass going in is a sealed unit. Two panes are held apart by a spacer bar around the edge, and the gap between them is sealed.
New glass in the existing sash
New timber sash in the existing frame
Aluminium insert in a timber surround
Full replacement
New glass in the existing sash
The frame and the sash stay, and the single pane comes out to make way for a double glazed unit. Aluminium and timber windows can both be done this way if the frame and sash are sound.
New timber sash in the existing frame
The timber frame stays, and new timber sashes with double glazed units are made to fit it. A villa or cottage keeps its timber windows, and any sashes that have rotted or been patched come out.
Aluminium insert in a timber surround
An aluminium window with its own double glazing is fitted inside the existing timber surround, so the timber around the opening stays and the window itself is aluminium.
Full replacement
Frame, sash and glass all come out, and a whole new window goes in. It's the route for a frame that can't be kept.
On prices a Wellington glazing firm published in February 2026, the three methods that keep the frame come to $550–1,480 per glass unit, incl GST, depending on the method and the frame.
Double glazing prices by job typeFor a listed heritage building in Wellington City, or a house in one of the city's heritage areas, read the heritage rules for timber windows before ordering, because the District Plan's simpler rule for new glass in existing frames names only clear glass, while most units used for retrofit are low-E.
Let us know you'd like to keep the frames, and whether they're aluminium or timber.
Ask for a quoteWhat low-E and argon add
A double glazed unit can be ordered with a low-E coating, argon between the panes, or both. EECA gives a figure for each.
In the table MBIE publishes for Building Code clause H1, a timber window rates R0.41 with standard double glazing and R0.71 with high-performance glass.
Ask for the coating and the gas to be named in each quote, so you can tell whether two quotes are for the same glass.
How much less heat a window loses, according to EECA
With a low-E coating up to 30%
With argon 5–20%
Asking which wind zone the glass is designed for
The Window & Glass Association NZ's guide to insert windows says retrofit and insert glass has to be designed for the site, wind loads included, and can't be of a lower standard than the glass it replaces. Keeping the old frames doesn't change that.
NZS 4223.4 sets the minimum glass thickness for a given wind load. So ask which wind zone the glass has been designed for, and get the answer written on the quote.
Tell us if the house sits on an exposed site, so it goes to a local installer with your enquiry.
Ask for a quoteDraughts come through gaps around the sash
When a southerly gets in around an old window, it's coming through the gaps, and a new glass unit on its own won't close them.
- Where the sashes run in the frame
- The meeting rail between the upper and lower sash
- The sill under the lower sash
Wellington Airport's glazing fact sheet names new seals as the way to close gaps around frames, so ask whether new seals are part of the quote and what will be done about the hardware on each sash that opens. If the quote covers only the glass, the gaps stay as they are.
Before you sign the job off, open and shut every sash, check each one closes firmly against the frame, and look at the finish around the glass.
When the frame is too far gone
Before you pay for new glass, make sure the frame around it is worth keeping.
Start with leaks. If your aluminium windows let water in during storms, have them looked at before you go ahead with double glazing older aluminium frames.
Rot is the other thing to look for. New timber sashes can go into a frame that's still sound, but if the frame itself has rotted, ask whether it can be repaired or the window should be replaced.
Windows in a one- or two-storey house can usually be replaced without a building consent, but a window that failed early, generally within 15 years, can need one. Before that job is booked, read up on the consent rules and wind zone labels for replacing windows.
Think a frame might be past saving? Say which window, and whether it leaks or has rotted.
Ask for a quoteQuestions on keeping your frames
Can existing windows be retrofitted with double glazing?
Yes, as long as the frames are worth keeping. Timber frames give you more ways to do it than aluminium ones, because they can also take new sashes or an aluminium insert.
What does double glazing existing windows cost?
One Wellington glazing firm prices this work per glass unit. On its list, a window's price depends on the method and how many glass units it holds. Compare the double glazing prices for each method before you look at quotes.
Is it worth retrofitting double glazing?
It depends on what you want it to fix and what the quote includes, so weigh up the downsides of double glazing before you decide.
What's the cheapest way to double glaze?
Keeping the sashes you have and changing only the glass. On one Wellington glazing firm's list, that's the lowest-priced job per glass unit in aluminium or timber frames, and new timber sashes and aluminium inserts cost more.
Can I do it myself, or buy the units supply-only?
Restricted building work doesn't list glazing, but whether replacing a window falls under it has no official answer, so treat it as a grey area. Consent or not, the Building Code still applies, and the glass still has to be right for your site's wind zone. The Window & Glass Association NZ's guide also says each window must carry a label for its wind zone. Buying the units supply-only doesn't change any of that.
If you pay someone for any part of it, ask whether a Licensed Building Practitioner is doing the work or supervising it.
How long is the wait before the work starts?
Between 2017 and 2024, people posting on forums reported waits of several months, for someone to measure up and then for the units to be made. Those posts are a few years old now, so ask for a timeframe in writing with the quote.
Ask which method each window will take
Start with your name and number on the form, and we'll get your enquiry to a local installer. When the quote comes, check that it names the glass, the wind zone it's designed for and whether new seals are included.