Double glazing a villa or bungalow: options for timber windows

What we found

Our calculations, data as of 23 October 2026

1. A low-E retrofit into existing timber sashes rates about R0.56 in the Building Code’s table, better than new double-glazed thermally broken aluminium with the same glass (R0.39) and far better than a retrofit into plain aluminium (R0.30).
2. Because the frame is warm, low-E glass is worth its premium in timber: it lifts the window from R0.44 to R0.56. In plain aluminium the same upgrade only goes from R0.26 to R0.30.
3. Timber retrofits cost more, about $750 per m² against $600 for aluminium, because every sash comes out and is routed by hand. For 25 m² in Dunedin, heated mornings and evenings with a heat pump, the low-E timber retrofit saves about $272 a year.
4. Not every sash can take a double-glazed unit. Thin meeting rails on double-hung windows and split-rail awning sashes often cannot be routed deep enough, and new sashes or secondary glazing are the answer there.

Why timber changes the comparison

Aluminium is a good conductor and timber a good insulator. BRANZ points out that a small single-glazed timber window has almost the same R-value as a double-glazed aluminium window without a thermal break. In timber, most of the heat goes out through the glass, so upgrading the glass gives a large gain. Ripping out sound timber joinery to fit double-glazed aluminium can leave you with a window that loses more heat through its frame.

What the options cost in timber frames, by window area:

Retrofitting IGUs into timber sashes

The glazier removes each sash, takes out the old glass and routes the glazing rebate deeper to take the insulated glass unit, sloping the bottom rebate outward so water drains. The unit sits on setting blocks, is sealed with glazing tape or silicone and is held with timber beads, the bottom one with drainage slots. Putty is not used: linseed-oil putty attacks the units’ edge seals. Proprietary systems follow the same principles.

BRANZ notes that a building consent is not needed for retrofitting IGUs into existing sashes; the exemption comes from Schedule 1 of the Building Act. The work must still comply with the Building Code.

Check before you commit:

• Rot and loose joints. Small areas of rot can be cut out and spliced or filled first; a sash with widespread rot or open joints should be replaced, not retrofitted.
• Double-hung sashes. Meeting rails are often too thin to rebate. The sash weights also need increasing to balance the heavier glass.
• Hinged windows. Weight can overload hinges and stays; plan to replace hardware.
• Split-rail awning windows. Usually too thin for an IGU; the sash must be replaced.
• Leadlights. A leadlight panel can be sealed between two panes of clear glass. The thermal gain is modest, but the leadlight is protected.

Draught seals: the cheapest gain

Old timber windows leak air at the sashes. Compressible seals for hinged and awning sashes, and brush-pile seals for double-hung and sliding sashes, which handle gaps of 3–15 mm, cut draughts for a fraction of the cost of glazing. Double-hung sashes have to come out to fit the seals. Tighten loose catches and hinges first.

Secondary glazing for heritage windows

Where the original glass or sash profiles should be kept, secondary glazing adds a pane on the room side and leaves the window untouched. On a single-glazed timber window it reaches about R0.40 with magnetic acrylic and R0.61 with installed low-E units. See secondary glazing.

Avoid single low-E glass

Replacing ordinary single glass with a single pane of low-E glass was once offered as a cheap upgrade. BRANZ reports that the coating can over-cool the pane, so condensation covers the low-E surface and the window ends up performing worse than clear single glass. Use low-E only inside a double-glazed unit.

Heritage and character areas

If the house is heritage-listed or in a character overlay, the district plan may control changes to street-facing windows. A retrofit that keeps the sashes and profiles is usually the easiest option to get approved. Check with the council’s planning team before work starts; see building consent for windows.

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: Secondary glazingBuilding consentRetrofit costCondensationOwners' experiences