Low-E, argon and frames: which double-glazing extras are worth paying for

What we found

Our calculations, data as of 23 October 2026

1. The first step does most of the work. Plain clear double glazing in an aluminium frame saves about $221 a year in Dunedin, 54% of the $412 saved by the best window in our comparison, high-performance low-E in timber.
2. In a new window, the thermal break is the most valuable single upgrade. Entry low-E glass in a thermally broken aluminium frame rates R0.39, against R0.30 in a plain frame, worth more a year than the low-E coating itself.
3. Low-E pays in warm frames. EECA says a low-E coating can cut window heat loss by up to 30% and argon by 5–20%; in a plain aluminium frame, the Building Code’s table shows the whole window improving far less, from R0.26 to R0.30.
4. Returns fall at the top. Moving from entry to high-performance low-E in uPVC adds R-value from R0.50 to R0.63 but only a few tens of dollars a year in Dunedin, less in warmer cities.

What each step is worth

R-values from the Building Code’s window table (H1/AS1), ranked from the plainest double glazing to the best we tested:

Only the last three rows reach the Building Code’s minimum for new houses and consented work, R0.46 in zones 1–4. Upgrading existing windows without a consent does not have to meet it. Each step saves less than the one before. The jump from single glazing to any double glazing is the big one; after that, each improvement trims a shrinking remainder. If budget is tight, glaze more rooms with standard units rather than fewer rooms with premium ones.

Low-E coatings

A low-emissivity coating is a microscopically thin metal oxide layer on one glass surface inside the unit. It reflects long-wave heat back into the room while letting light through. EECA puts the reduction in heat loss at up to 30% compared with regular glazing. Coatings vary: glass makers sell several grades, and the Building Code’s table spans glass Ug values from 2.9 (clear) down to 1.1 for the best double-pane units. Ask for the Ug or the whole-window R-value, not just “low-E”.

Low-E belongs inside a sealed unit. On a single pane it can over-cool the glass and make condensation worse; see villas and timber windows.

Argon and the gap

Argon is a heavier, slower-moving gas than air. EECA says filling the cavity with it cuts window heat loss by 5–20% compared with air, and that gaps of 12–16 mm give the best insulation. In BRANZ’s comparison of glazing alone, argon lifts a standard 4-12-4 unit from R0.37 to R0.39 and a low-E unit from R0.53 to R0.62: small on clear glass, larger with low-E.

Spacers

The spacer holds the two panes apart at the edge. Aluminium spacers conduct heat and make the glass edge the coldest strip on the pane, where condensation starts. EECA notes that plastic or stainless-steel spacers insulate better. “Warm-edge” spacers cost little extra and are worth asking for.

Frames

The frame decides how good the whole window can be. Plain aluminium conducts heat straight through; thermally broken aluminium has an insulating strip between its inner and outer halves; uPVC and timber are poor conductors. With entry low-E glass, the Code’s table gives R0.30, R0.39, R0.50 and R0.56 respectively. Frames also decide where condensation lands: plain aluminium frames run wet even with good glass.

Triple glazing

The Code’s table lists triple-pane units only in thermally broken aluminium, uPVC and timber frames, reaching about R0.62, R0.91 and R1.01 at best. Units are heavy and usually unsuitable for retrofitting into existing sashes. In most New Zealand climates the extra over good double glazing saves little on heating; it makes most sense in zones 5 and 6 on new builds.

Safety and noise

Laminated glass, two sheets bonded with a plastic interlayer, holds together when broken and cuts noise; toughened glass breaks into small pieces. The Building Code requires safety glass in places where people could fall against or walk into glass, such as doors and low windows. Laminated glass is also an upgrade for security and for street noise, often at modest extra cost.

This week: Palmerston North

Single glazing in an aluminium frame runs below the dew point about 2,430 hours a year in Palmerston North (H1 zone 3); clear double glazing cuts that to 350. A low-E retrofit of 25 m² saves $190 a year heated mornings and evenings with a heat pump, or $569 heated all day. Fastest payback here: DIY magnetic secondary glazing, 17 years.

Cheap fix. EECA estimates that closing curtains before dark saves $80–90 a year. Lined, full-length curtains with a pelmet work best. Read more

Read next: H1 zones and R-valuesReplacement costRetrofit costCondensationVillas and timber windows