Methodology and data

In short

For a house with 25 m² of single-glazed aluminium windows heated with a heat pump in the mornings and evenings, the model puts the yearly saving from a low-E retrofit at $90–$261 across our 15 cities, and the payback at 58–166 years. DIY secondary glazing pays back in 12–36 years. Heated all day with plug-in heaters, savings are about nine times larger. The sections below explain how those numbers are made.

The heat-loss model

Heat lost through a window in a year, in kWh, is its area in m² times the year’s heating degree-hours divided by its whole-window R-value, divided by 1,000. A degree-hour is one hour with the outdoor air one degree below the 18 °C indoor target.

We count degree-hours two ways for each city: for the hours most homes are heated (7–9 am and 5–11 pm, eight hours a day), and for all 24 hours. The saving from an upgrade is the difference in heat lost between the old and new window, divided by the heater’s efficiency (3.0 for a heat pump, a typical seasonal figure; 1.0 for a resistive electric heater), times the city’s electricity price. Payback is the installed price divided by the yearly saving, with no discounting and no change in power prices.

The model ignores solar gain through north-facing glass, which slightly overstates savings, and reduced draughts after an upgrade, which understates them.

The condensation index

The inner surface temperature of a window is taken as the room temperature minus the temperature difference across the window times the inside surface resistance (0.13 m²K/W) divided by the window’s R-value. With the room at 18 °C and 65% relative humidity, the dew point is 11.3 °C (Magnus formula). For each hour of 2021–2025 we test whether the surface would fall below the dew point, and report the average hours a year. It is a comparison between windows and cities, not a forecast for a particular room.

The Healthy Homes heating calculation

On the Healthy Homes page we apply the formula in Schedule 2 of the Residential Tenancies (Healthy Homes Standards) Regulations 2019 to one example living room: 25 m² of floor, 2.4 m stud, two external walls, 6 m² of windows, ceiling 2.9, underfloor 1.3, walls 0.5. The R-values for the ceiling, floor and walls are our assumptions for an older insulated house.

Data

• Temperatures. ERA5 reanalysis, hourly 2 m air temperature for 2021–2025 at each city’s coordinates, from the Copernicus Climate Change Service, retrieved through the Open-Meteo historical weather API.
• Electricity prices. Ministry of Business, Innovation and Employment, Quarterly Survey of Domestic Electricity Prices, 15 August 2026: average price per kWh including GST for each city.
• Climate zones and window R-values. MBIE, Acceptable Solution H1/AS1 sixth edition (27 November 2025), Table C.1.1.2 (climate zones by territorial authority) and Table D.1.1.1 (construction R-values of windows for housing); fifth edition for the schedule-method window minimums.
• Single-glazing and secondary-glazing R-values. BRANZ’s comparison of typical window R-values (single glazing: aluminium R0.15, timber R0.19); hot-box tests of secondary glazing reported by Victoria University of Wellington and BRANZ (film R0.35, magnetic acrylic R0.36, aluminium-framed R0.34, aluminium-framed low-E R0.57).
• Timber windows. BRANZ Bulletin 659, Upgrading the thermal performance of timber windows (February 2021).
• Prices. Installed price ranges published by New Zealand glaziers and cost guides in 2026 (retrofit, replacement by frame type, secondary glazing, extras), checked against prices owners reported on Geekzone forums. We use mid-points.
• Building consent. Building Act 2004, Schedule 1, exemptions 1 and 8, version as at 8 October 2026.
• Healthy Homes. Residential Tenancies (Healthy Homes Standards) Regulations 2019, version as at 25 September 2025; Tenancy Services guidance on the heating, ventilation and draught-stopping standards.
• Grants and loans. EECA, Warmer Kiwi Homes programme and home energy loans list (as at June 2026); EECA window insulation and draught-stopping guidance.
• Owners’ experiences. 43 Geekzone threads, 1,388 posts, 2010–2026, each coded by topic.

Official pages: MBIE energy efficiency (H1) documents and Tenancy Services Healthy Homes standards.

Updates

We update power prices each quarter when MBIE publishes its survey, and check prices, grants and rules at the same time. The date at the top of each page is the date of the last check.