Is double glazing required in NZ rentals?

No. None of the five Healthy Homes standards requires double glazing, and an existing rental with single-glazed windows can comply with every one of them. Windows still matter in three ways: the glass changes how big the living room heater has to be, gaps around windows fall under draught stopping, and habitable rooms need windows or doors that open.

What the standards actually say about windows

• Heating: the area and thermal performance of the living room's windows feed into the calculation of the heating capacity that room needs.
• Ventilation: the living room, dining room, kitchen and bedrooms each need at least one window, door or skylight that opens to the outside and can be fixed open, with a total openable area of at least 5% of that room's floor area.
• Draught stopping: unreasonable gaps and holes, including around windows, must be blocked.
• Insulation: the standard covers ceilings and underfloor spaces. Windows are not part of it, and neither is wall insulation.

How glazing changes the heating calculation

The formula in the regulations adds up the heat the main living room loses through each part of its outer shell (ceiling, walls, floor, windows and skylights) using each element's area and its construction R-value. For windows, the frame and joinery count as part of the window. Single glazing has a lower R-value than double glazing, so a living room with a lot of single-glazed glass loses heat faster and the required heater capacity goes up. Tenancy Services' heating guidance says plainly that whether a home has insulation or double glazing makes a difference to the size of heater needed.

There is a second, less known link. The regulations use a gentler formula for a modern dwelling, meaning one that first received building consent on or after a date between 31 October 2007 and 30 September 2008 depending on its climate zone, and for certain larger apartment buildings. An older house also qualifies if insulation and glazing have been installed throughout its thermal envelope to at least the NZS 4218:2009 standard for its zone. That usually allows a smaller heater, but new windows alone will not get an older house there, because the walls, ceilings and floors have to meet it too.

The practical step is the same either way: run the official heating assessment tool with accurate information about the living room's glazing, keep the printout, and run it again if the windows change. Our heating standard page covers the rest of the requirement.

Draught stopping around windows

This is where old windows most often cause a genuine compliance problem. The draught stopping standard requires the property to be free of unreasonable gaps and holes that let draughts in or out, and the age and condition of the house cannot be used as a reason to leave them. The official guidance offers a rule of thumb: gaps wider than 3 mm usually need blocking, roughly the width of the edge of a $2 coin.

• Gaps that are needed for a window to work, such as the sliding parts of a sash window, do not need sealing. Anything beyond what the window needs to operate does.
• Intentional openings are fine: trickle vents, and drainage holes built into window joinery.
• Poorly fitting windows, broken catches or hinges that stop a window closing tightly, and cracked or broken glass all count as gaps to fix.
• To find them, run the back of a wet hand along the frame on a cold or windy day and feel for moving air.

The fixes are usually cheap: weatherstripping, new catches, a bead of sealant, a pane replaced. More detail is on our draught stopping page.

New builds and existing rentals follow different rules

The Building Code and the Healthy Homes standards are separate regimes. The Building Code applies to building work, and EECA notes that new houses and additions now need double glazing to comply with it. The Healthy Homes standards apply to a rental as it stands, and they set a minimum that existing housing can reach without reglazing. So a new build will almost certainly have double glazing, but that is because of how it was built, not because of any rental rule. If you are adding on or replacing joinery, check the current Building Code requirements with your council or window supplier before ordering, since that work is judged under the building rules rather than the rental ones.

When double glazing is still worth doing

• The heating calculation comes back demanding a notably large heater. Better glazing reduces the required capacity and the running cost.
• Condensation streams down the glass every winter morning. EECA notes double glazing helps reduce condensation on windows.
• The windows are being replaced anyway, so the marginal cost of better glass is at its lowest.
• The house is otherwise well insulated. EECA says up to 40% of a home's heating energy can be lost through windows, and windows are the biggest source of heat loss in well-insulated homes.

Full replacement is not the only option. EECA describes secondary glazing (an extra pane fitted inside the existing frame), retrofitting double-glazed units into existing frames, DIY window film kits and well-fitted thermal curtains. None of them is required. Costs vary widely with window size, frame condition and supplier, so get quotes for your own house.

If you do upgrade, keep the invoice with the glazing specification and re-run the heating tool. That record changes the heating maths in your favour and is exactly what a future compliance statement, insurance claim or sale will want to see.