Double glazing

Windows are the weakest link when trying to retain heat in a house during winter. Up to 30% of heat is lost through windows[1] but reducing heat loss and minimising condensation by installing double glazing can be expensive with little relative gain. Read more about:

The costs and benefits of installing double glazing.

Actions to take before investing in double glazing

Some optimum situations where double glazing is a good idea.

How double glazing is installed, and basic information about different kinds of double glazing,

More information about double glazing

If you can't afford double glazing, read more about cheaper alternatives to reducing heat loss through windows.

Where should I spend my money?

Before you install double glazing make sure that the rest of your house is warm and energy efficient. To put things into perspective:

R values describe the resistance to heat flow and are used to rate insulation and building materials. The higher the R-value the better the insulation provided. A single glazed window has an R Value of approximately 0.15-0.19.

In comparison, current Building Code Standards require new homes to have the following approximate R-values:[2]

A double glazed wood-framed window with low-e glass increases the R value to just 0.48.[3][4]

Research conducted by the Building Research Association of New Zealand (BRANZ) found it could take between 10-20 years (for houses located in the South Island) to pay back the cost of installing double glazing through savings in energy costs.[5]

So before you spend a lot of money on double glazing, make sure that you have:

1. Insulated your ceiling and under the floor to Building Code Standards wherever possible

 

2. Installed efficient, effective heating in the rooms you use the most

 

3. Minimised your hot water costs as much as possible

 

4. Installed extraction fans in your bathroom and kitchen to ventilate humid air out of your house

 

5. Minimised significant draughts or air leaks (especially around windows).

 

6. Investigated the cost of installing bulk wall insulation in south facing, external walls compared to the cost of installing double glazing. This is especially relevant if the rooms you heat the most has more external wall area than the window area. Installing wall insulation will increase the R-Values of the external walls of up to R1.9-R2 (compared to double glazing of R0.48).

 

7. Considered heating other parts of your house efficiently, particularly if you are intending to stay in the house long term or use other parts of the house a lot.

Some people investigate double glazing because they are worried about condensation on their windows in the cooler months of the year. Double glazing will certainly help to reduce or eliminate condensation, but it is possible to manage or reduce dampness and condensation in other ways that may be more cost effective.

If you need any further advice about the steps outlined above, contact the Energy Advice Service.

Get the best out of double glazing 

If you have limited funds and need to priortise how you spend your money....

Insulated windows in rooms that are most often heated (usually living areas) will result in greater energy savings than insulated windows used in less frequently used and heated rooms. Consider prioritising double glazing the windows in the rooms that are heated the most, especially if:

• There are large ‘picture' windows which is a significant area of external ‘wall'
• There are windows in south-facing living areas
• You are home for most of the day

2. Some houses cannot be insulated due to the nature of their design (e.g. cathedral ceilings, skillion roofs, low pitched roofs, or built too low to the ground). Double glazing windows in houses like these may be a good ‘next best' step to improving the energy efficiency and comfort of your home.

3. Don't get rid of your curtains. Double glazing is not a replacement for curtains. They do a different job and at different times and complement each other. No matter how much you improve the window glazing, your windows will still be the weakest link. Curtains will build on the insulation value of the window as a whole, especially at night.

How is double glazing installed?

Double glazing can be retrofitted by installing the double glazing into the existing frames or by replacing the entire frame. Another option is 'secondary' double glazing whereby another frame and window pane (often made of acrylic) is installed over the existing frame and window, sometimes with magnetic seals. These can be removed and cleaned. It is important that the seals provide a very good fit.

What is double glazing made of?

The effectiveness of double glazing depends on the whole unit (e.g. type of frames, window pane material, and how the unit is retrofitted), not just the glazing material.

1. Frames can be made out of wood, PVC, aluminium, or ‘thermally broken' aluminium.

2. The effectiveness of double glazing can be improved by using low-e glass and/or argon gas between the panes.

Consider matching the frames with the existing character of the house.

A caution about installing double glazing into unbroken aluminium window frames

Aluminium is a metal - and metal conducts heat very effectively!  A thermally broken frame has a material installed in between the frame on the outside of the house and the frame on the inside of the house to prevent heat loss through the metal. Think twice about installing or retrofitting into unbroken aluminium frames - they could be responsible for relatively high heat losses, plus attract condensation and mould.

Where to go to for more information

1) Window Association of New Zealand

The Window Energy Rating scheme administered by WANZ outlines how different kinds of double glazing are rated for energy efficiency.

Check if double glazing retailers in your area are members of the Window Association of New Zealand, and ask if their system has a WERS rating!

2) Energy Efficiency Conservation Authority

[1] Department of Building and Housing, (2008)Your Guide to $marter Insulation. Pg 3

[2] Required R-values depend on location in New Zealand

[3] Ibid, pg 17

[4]Low -e glass has specially designed transparent coatings, often based on metallic oxides applied to one or more surfaces of insulated glass. This results in more efficient windows because radiant heat originating from indoors in winter is reflected back inside.

[5] BRANZ Build, Issue 40, February 2010 BRANZ.

[6] Call the Energy Advice Service for more information on how to reline existing curtains.