Retrofit vs Full Frame Double Glazing: Which Is Right for Your NZ Home?

Double glazing is one of the most effective upgrades for an older New Zealand home, since single glazing is usually the weakest point in the whole building for heat loss, condensation and noise. Once you decide to upgrade, the next question is which approach: retrofit the glass into your existing frames, or replace the whole window.

Both are genuine, established options, not a budget-versus-premium choice. Here's how to work out which one actually suits your home.

Retrofit vs full frame double glazing: what's the actual difference?

Full frame replacement removes the entire window unit, frame included, and installs new double-glazed aluminium or timber joinery in its place. It's the more disruptive and expensive option, but it resets the whole window, not just the glass.

Retrofit, sometimes called secondary glazing, keeps your existing frame in place and either swaps the single pane for a double-glazed unit or adds a secondary glazed panel alongside it. Established New Zealand systems in this space include RetroFit Double Glazing (a Metro Performance Glass subsidiary with close to 40 years in the market) and Magnetite, which uses a magnetic secondary pane rather than modifying the original sash.

When retrofit makes sense

• Your existing frames are sound: no rot, warping or failing seals
• You want to preserve original timber joinery on a character or heritage home
• You're working to a tighter budget, since retrofit is typically the cheaper of the two
• You want the job done with minimal disruption, often within a day or two per window

When full frame replacement is worth the extra cost

If your existing frames are damaged, warped, or simply not built to accept a retrofit unit, replacement is usually the only real option. It's also worth considering if you're already renovating and want new joinery to match, or if your current frames aren't thermally broken (meaning heat transfers straight through the metal frame itself, undermining the benefit of better glass).

Why the Building Code changed the maths on this

Since May 2023, Clause H1 of the Building Code has required a minimum window and door R-value of R0.46 in most of New Zealand, and R0.50 in the coldest zones, up from R0.37 previously. That's a meaningful jump, and it's why the gap between old single glazing and a modern double-glazed unit is bigger than it used to be.

Any installer working on residential glazing should also be building to NZS 4223, the relevant New Zealand standard: Part 1 covers glass selection and design, and Part 3 sets human-impact safety requirements for glazing within 2000mm of floor level, which matters for low windows and glazed doors in particular.

There's no government grant for this one

It's worth being clear on this upfront: unlike ceiling and underfloor insulation or heat pumps, which can be funded through EECA's Warmer Kiwi Homes programme, double glazing and window glazing work is explicitly excluded from that scheme. There is currently no EECA grant for double glazing in New Zealand, so budget for the full cost of whichever approach you choose.

Look for installers who are current members of WGANZ (the Window & Glass Association of New Zealand, formerly WANZ), the genuine industry body for glazing professionals, and get quotes for both approaches before deciding, since the better option depends on the specific condition of your frames, not a fixed rule. Our researched double glazing installer lists check for exactly this kind of verifiable credential before a business makes the Top 5.