Retrofit alternatives to double glazing

If you have an older home, you may be able to upgrade your windows without completely replacing them with new double glazed units. This will depend on the type and condition of your existing single glazed windows. Secondary glazing works by forming a sealed, insulating air gap on the inside of existing, single glazed windows. To work properly there should be at least 1 cm between the existing and secondary glass.

Options for secondary glazing

There are a number of options available in New Zealand, including:

  • installing a new double glazed glass unit into the existing sash
  • replacing existing sashes with new, double glazed sashes
  • installing separate aluminium framed secondary glazing on the inside of the existing glazing - typically installed as sliding panes in frames fixed to the internal window sill, jamb and head reveals
  • DIY window insulation kits - heat-shrinkable thin plastic film that is attached to the window sash or frame using double sided, adhesive tape. This can be an alternative option in rented houses
  • low-E window film - a thin film stuck to the window glass that reflects radiant heat
  • acrylic sheet magnetically attached to the inside of the existing window sash or frame.

Not all of these options are suitable for every glazing situation so check with a professional before purchasing. 

 DIY window insulation kits

 Low-E window film

Checklist for secondary glazing

Secondary glazing systems are effective but can come at a significant cost (with the exception of inexpensive DIY window insulation kits). Before you invest in secondary glazing make sure:

  • ceiling and underfloor insulation is up to standard
  • you have a clean and affordable heating system
  • you compare it against the cost of installing new double glazing
  • windows (including frames) are still in good condition and that they can be expected to last at least as long as the secondary glazing
  • you shop around and ask for independently verified performance guarantees and warranties.

Secondary glazing: how well does it work? - article on the Build website