Retrofit vs full-frame double glazing in a NZ villa

If you own a pre-1940 NZ villa with original timber joinery, you have two genuinely different choices when you decide to double glaze. You can keep the existing timber sashes and have the installer route a sealed insulating glass unit (an IGU) into the existing rebate — that's the retrofit. Or you can rip the whole window assembly out, frame and all, and replace it with new joinery and new glass — that's full-frame. The choice has very different cost, aesthetic, and thermal implications, and the right answer for a villa is almost always one of them.

What retrofit actually means

The installer removes each sash, takes it to a workshop, routs out the existing single-pane rebate to fit a 14–20mm sealed double-glazed unit, drops the IGU in with new beading, re-hangs the sash. The frame stays. The sash itself is altered but recognisable. A two-bedroom villa typically takes 2–4 weeks of workshop time, but onsite the install is usually a single day per room.

What full-frame replacement means

The whole window assembly comes out — frame, sashes, sills, the lot. New joinery (usually aluminium, occasionally new timber) goes in with double glazing pre-fitted. The old window's gone. The new one is more thermally efficient than retrofit (because you've also replaced the frame, which is a significant heat-loss path) but the original character is gone with it.

The thermal difference

A retrofit IGU into a timber sash will give you a centre-of-glass U-value around 1.6–2.0 W/m²K with a good warm-edge spacer and argon fill. The whole-window U-value (which includes the timber frame's heat loss) is typically 2.4–2.8. A full-frame thermally-broken aluminium replacement sits around 2.0–2.4 whole-window. The original single glazing was around 5.5–5.8 — so either approach roughly halves the heat loss. The difference between the two is real but second-order; the gap from single to either kind of double is the big move.

Where the heritage argument matters

A villa's character is largely in the proportions and the joinery profile of the windows. Replacing original kauri sashes with aluminium fundamentally changes the look — the frame depth is wrong, the muntin patterns disappear, the sill profile is different. Retrofit keeps all of that. If you're inside a Council heritage zone (parts of Mt Eden, Thorndon, central Dunedin and central Christchurch all have these), retrofit is often required rather than optional.

When full-frame is the right call

• The original sashes are too far gone — rotten sills, splitting timber, loose joints. Retrofit needs sound sashes to work.
• The window openings are non-standard sizes you want to bring to standard for furniture or curtains.
• You're already doing a major renovation that includes recladding or replacing internal linings — the marginal cost of full-frame at that point is much lower.
• You're in a 60s/70s timber house where the joinery profile isn't worth preserving anyway.

What we'd recommend asking each installer

• U-value of the proposed IGU (centre-of-glass and whole-window if known)
• Spacer type — warm-edge plastic-foam composite, not aluminium
• Gas fill — argon, not air, for the marginal extra cost
• Low-E coating — soft-coat low-E on surface 3 (the inside of the outer pane) for NZ climates
• Whether weather-sealing of the joinery is included or quoted separately
• Manufacturer's IGU warranty (10-year minimum is standard)