Whether you’re building new or making improvements to your existing home, on this page you’ll find low cost and big-ticket investment options for improving the insulation of your windows.

Insulating your windows makes a big difference

Up to 30% of a home’s heating energy is lost through windows – making them the biggest source of heat loss in an otherwise well insulated home. Double glazing can reduce this to 20% or less.1 

Quick wins like curtains and window film can get the most out of your existing windows. If you're building new or replacing your windows, double glazing can make a big difference. It’s a big investment, but can significantly improve the comfort, health and energy efficiency of your home. 

On this page

Key features of good double glazing

Double glazing your windows and doors will help reduce heat loss, condensation on the glass, and heating and cooling costs — working best in combination with good home insulation.

  • Inert gas

    Compared to air, filling the gap between panes with an insulating gas like argon can reduce window heat loss by 5-20%.2

  • Spacers

    These sit around the edge of the glass to keep the panes apart. Plastic or stainless steel spacers provide better insulation than aluminium.

  • Larger gap size

    Gaps between 12mm and 16mm give the best insulation performance.

  • Low-E film

    Low-emissivity (low-E) film has an invisible coating that lets light and warmth in but helps keep heat from escaping.

  • Quality frames

    Plastic (uPVC) or wood frames lose less heat and are less likely than aluminium to attract condensation.

  • Insulating glass units

    Two panes of glass are sealed together with a gap between them. Together, the panes form an ‘insulating glass unit’.

Is low-E window film important?  

Low-E film is a micro-thin composite metal coating fitted to the inside of windows. It is often offered as an extra feature when you order your windows – and can reduce heat loss by up to 30% compared to regular glazing.3

The film has great benefits: 

  • Virtually transparent and tint-free. 

  • Can be applied to single or double-glazed window glass. 

  • Deflected heat and UV light coming in from the sun can reduce overheating and fading of furnishings. 

It’s best used if your home isn’t prone to condensation on the windows. The film increases the occurence of condensation because it reflects heat from the glass, making the glass colder. This condensation may hinder the performance of the film. 

You don’t need to double glaze all of your windows at once. Start with the rooms you use the most, or the ones that are tricky to keep warm – this gives you the biggest improvement with the money you’re spending, in terms of cost savings and comfort.

Gareth Gretton — Home energy efficiency expert, EECA

Get more out of your existing windows 

You don’t need to be building from scratch to make smart glazing choices. Upgrading your windows as part of a renovation project is one of the easiest and most effective ways to boost the warmth, comfort and energy efficiency of your home. It can also help cut down on heating costs — without major changes to the rest of your home.  

Retrofitting double glazing 

You can replace your existing windows with brand-new double-glazed units, or opt for a simpler option like adding secondary glazing to your existing frames. The right option depends on the type and condition of your windows, and your budget.  

There are two main options for retrofitting double glazing:

  • Replacement windows — This removes the entire unit, including the frames. They are replaced with brand-new double-glazed windows, with high quality frames and a sealed gap between the panes. This option is best if your existing frames are at the end of their life. 
  • Secondary glazing — This adds an extra pane of glass to your existing window, while keeping the existing frame. The additional pane creates a sealed, insulating gap. This is a more cost-effective option but relies on the condition of your existing windows being up to scratch. 

If you’re not sure which option is best for you, get advice from a professional. It often pays to get multiple quotes to compare your options.

Keep the heat in with curtains and blinds  

Blinds and curtains can reduce the amount of heat loss through single glazed windows, and in combination with draughtproofing can minimise the impact of other issues with older windows.  Closing your curtains before it gets dark can save $80-$90 per year in energy costs.4

The best types of curtains are: 

  • Double-layered with thermal linings 
  • Fit tightly against the wall or window frame 
  • Full length (touching the floor) and cover the full width of the window 
  • Have pelmets fitted above them.  

If you want to install blinds – the warmest option is ‘honeycomb’ blinds, which have thin tubes that open when you close the blind, creating air pockets that provide insulation. 

New curtains can be an investment - check if there is a curtain bank near you, where you may be able to source them for free. 

Find a curtain bank(external link)

Window film kits are a quick win

Shrink wrap secondary glazing kits are a low-cost, DIY option that can reduce heat loss by almost as much as double glazing in some cases, as well as reducing condensation. They involve sticking an insulation film to the frames of wooden windows and using a hairdryer to shrink the film to fit. This creates a layer of air in front of the glass, like double glazing.

Window film provides a great solution if you are living in a rental home, on a limited budget, or need a short-term fix.

Tips on window insulation kits 

Insulation kits are available from most hardware stores or can be purchased online. The film is designed to last one winter season but could last for several years.  

The measurement between the inside surface of the window frame and the glass should be at least 5mm. Frames need to be clean, dry, leak-free, and the paint in good condition (inside and out). Follow the installation instructions for the best results.  

Things to consider before you buy: 

  • The film won’t stick to aluminium windows. 
  • Don’t use where people or pets could damage the film – piercing it will stop it from working. 
  • The tape that sticks the film to the window frame can leave a stain if you leave it more than one season. 
  • Window film is transparent but may make your view less crystal clear.

Build quality windows into your home design

Most new houses and additions now require double glazing to comply with the New Zealand Building Code.  

If you’re building a new home, you’ve got a unique chance to think about your home design with energy efficiency in mind. That includes thinking carefully about the size, type, and placement of your windows.

Other ways to improve your home

All the individual building blocks that provide insulation in your home (and make up the thermal envelope) need to be working together to form a ‘seal’ around your home — without any gaps.

  • Reduce moisture in your home

    The key to keeping dampness out of your home is to keep fresh air moving through throughout the house and stopping moisture at the source.

  • Insulate your home

    Insulation works best when it forms a continuous thermal envelope around the home — including windows, ceilings, walls and floors.

  • Reduce draughts in your home

    Airtightness and insulation work together to keep your home comfortable, dry and healthy during the winter months — so it pays to seal up any sneaky draughts.