Replacing a single pane in a double-glazed unit (also called an IGU — Insulated Glass Unit) is not a straightforward DIY task. Unlike a single-glazed window where you simply swap out one sheet of glass, a double-glazed unit is a factory-sealed assembly, and once that seal is broken, the whole unit needs to be replaced — not just one pane. Here's what you need to know before reaching for the putty knife.
How a Double-Glazed Unit Actually Works
A double-glazed unit consists of two panes of glass separated by a spacer bar filled with desiccant (a moisture-absorbing material), with the cavity between the panes filled with either dry air or an inert gas such as argon. The entire assembly is then hermetically sealed at the factory.
This sealed construction is what gives double glazing its thermal and acoustic performance. When a pane cracks or the seal fails (you'll notice fogging or condensation between the panes), the integrity of the whole unit is compromised — not just one piece of glass.
Why You Can't Simply Replace One Pane
- The seal cannot be re-created at home. Achieving a proper hermetic seal requires specialist equipment, specific sealants, and controlled conditions. A DIY attempt will almost certainly result in moisture ingress and fogging between the panes.
- Gas fill is lost. If your unit was argon-filled, opening it releases the gas. Refilling requires specialist equipment.
- Structural and weathertightness risks. A poorly sealed unit can allow water into the frame, potentially breaching the NZ Building Code clause E2 (External Moisture). This can lead to costly damage over time.
- Safety glass requirements. In many locations — near doors, low to the floor, in bathrooms — NZS 4223 requires safety (toughened or laminated) glass. Using the wrong glass type is both a safety and compliance risk. Check with MBIE Building Performance if you're unsure what's required in your situation.
What You Can Do Yourself
While replacing the IGU itself is best left to a professional glazier, there are a few things a homeowner can reasonably do:
- Assess the damage. Determine whether it's a cracked pane or a failed seal (internal fogging). This helps when getting quotes.
- Temporary protection. If a pane is broken, you can carefully remove loose shards (wear heavy gloves and eye protection) and cover the opening with heavy-duty polythene taped securely — as a short-term measure only.
- Measure up. You can measure the existing unit's overall dimensions (width × height × total thickness) to give to your glazier, though they will always measure themselves before ordering.
- Check your window frames. While the glazier is coming, it's worth inspecting the frame, seals, and flashings for any deterioration.
Safety note: Glass edges are extremely sharp. Always wear cut-resistant gloves and safety glasses when handling broken glass. Dispose of glass safely — wrap it in newspaper and place it in a rigid container before putting it in the bin.
What a Glazier Will Do
A qualified glazier will:
- Remove the damaged IGU from the frame without damaging the frame or surrounding joinery.
- Measure precisely and order a replacement IGU from a glass manufacturer — this typically takes a few days to a week.
- Install the new unit with the correct setting blocks, edge clearances, and glazing tape or sealant per manufacturer specifications.
- Ensure the replacement meets any safety glazing requirements under NZS 4223.
As a rough guide, replacement IGU costs vary considerably depending on size, glass type (standard, low-e, laminated, toughened), and your location. Expect to pay for both the unit itself and the glazier's labour — get at least two or three quotes.
Does This Require a Building Consent?
In most cases, like-for-like replacement of a broken IGU in an existing frame is considered maintenance and falls under Schedule 1 of the Building Act as exempt work. However, if you're changing the window size, frame, or type of glazing significantly, a consent may be required. Always confirm with your local council before proceeding with anything beyond a straightforward replacement. You can find general guidance at MBIE Building Performance.
Healthy Homes and Rental Properties
If you're a landlord, note that the Healthy Homes Standards include requirements around draught stopping and, in some cases, glazing. A failed double-glazed unit that allows cold air or moisture ingress could be relevant to your compliance obligations — another reason to get it fixed promptly by a professional.
When to Call a Professional
Call a qualified glazier if:
- Any pane is cracked, broken, or showing internal fogging/condensation.
- You need to ensure the replacement glass meets safety glazing standards.
- The window is in a high-risk location (near a door, stairwell, bathroom, or low to the floor).
- You're a landlord with Healthy Homes obligations.
- You're unsure about any aspect of the job.
A skilled glazier will get the job done safely, to code, and with a proper warranty on the new unit. You can find qualified local glaziers on tradedirectory.co.nz.
Window & Glass Association
