Cheaper than double glazing: curtains, film and draught seals

What we found

Our calculations, data as of 23 October 2026

1. EECA estimates that closing curtains before it gets dark saves $80–90 a year in energy, and sealing draughts around the average home about $100.
2. Shrink-wrap film kits more than double a single-glazed window’s R-value in laboratory tests. On 25 m² of single-glazed timber windows heated mornings and evenings with a heat pump, our model puts the saving at about $103 a year in Wellington and $211 in Dunedin.
3. The film will not stick to aluminium frames, EECA warns, so it suits timber windows. On aluminium windows, magnetic acrylic secondary glazing does a similar job.
4. Curtains work best double-layered with a thermal lining, full length, tight to the wall and topped with a pelmet, so warm room air cannot circulate down behind them against the cold glass.

Curtains and blinds

At night a single-glazed window is a large cold panel. A curtain traps a layer of air in front of it, but only if that air is still. Curtains that stop short of the floor, hang away from the wall or have an open top let warm room air flow down behind them, cool against the glass and spill out at the bottom, which is why a curtain can feel like it is “breathing” cold air. EECA’s checklist for curtains that work:

• double-layered, with a thermal lining;
• tight against the wall or window frame;
• full length, touching the floor, and covering the full width of the window;
• a pelmet fitted above.

Honeycomb (cellular) blinds are the warmest blinds, because their cells trap air. Close everything before dark; EECA puts the saving from that habit alone at $80–90 a year.

Shrink-film kits

A film kit is a sheet of clear plastic taped to the inside of the window frame and shrunk tight with a hair dryer. It creates a still air gap in front of the glass, like secondary glazing. In hot-box tests at Victoria University of Wellington, film lifted a single-glazed window from R0.15 to R0.35, close to magnetic acrylic.

EECA’s cautions: the film will not stick to aluminium windows; it needs at least 5 mm between the inner face of the frame and the glass; frames must be clean, dry and sound; the tape can stain if left on for more than a season; and pets or children can pierce it. It is designed to last one winter, sometimes several. For renters with timber windows it is the cheapest window upgrade there is.

Draught seals

Gaps around sashes, doors and old aluminium seals let cold air in and warm air out. EECA’s estimate for sealing draughts around the average home is about $100 a year. For windows: replace perished rubber seals in aluminium joinery (they are sold by size), fit compressible seals to hinged timber sashes and brush-pile seals to sliding and double-hung sashes, and get a builder to fix sashes that no longer fit their frames. Do not seal kitchen and bathroom vents or block the openings you need for ventilation.

Where they fit

Curtains, film and seals are not alternatives to a heat pump or insulation; they complement them. Against double glazing they compare well on cost but not on condensation: a curtain hides the cold glass and can make condensation behind it worse, while film and secondary glazing keep the room-side surface warm. If condensation is the problem, look at secondary glazing or double glazing and at ventilation.

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 glazingIs it worth it?CondensationGrants and financeHealthy Homes and windows