Windows are an essential part of any home, but they account for considerable heat loss in winter. Secondary and double-glazing help reduce heat loss. Other benefits include noise reduction, improved airtightness and reduced condensation.
Window performance relies on a number of factors such as orientation, frame type, size, glazing elements and fabric window treatments. Window treatments are covered in Information Sheet 13, Curtains and Blinds.
North-facing windows can contribute significantly to the ‘passive’ heating of a home in winter. On sunny days they allow free solar heating for six to eight hours, but for the rest of the day and night they account for heat loss.
South-facing windows receive no direct sunlight in winter, and therefore only cause heat loss from a home in winter.
East- and west-facing windows may provide a small amount of ‘passive’ heating in winter, but are likely to be heat losers for 22 or more hours each day.
Large west-facing windows have a tendency to overheat homes in summer and should be avoided or addressed with shade trees or pulling curtains or blinds in the late afternoon.
The window frame material has a big influence on overall performance. (See Table 1)
Aluminium windows are great for low maintenance, but not for thermal performance.
Thermally-broken aluminium frames reduce heat loss further.
PVC frames offer increased performance combined with low maintenance,
Wooden windows offer the best thermal performance, however they generally cost more and have increased maintenance requirements.
With small windows the heat loss from the frame has a larger effect on window performance, but with large windows the glazing has a larger impact on performance.
The performance boost from secondary and double glazing comes from the layer of still air trapped between the glass/film and the properties of the glazing material.
High performance glass is one area where you can substantially boost performance relative to cost. Low E coatings let the sun’s heat through the glass but act like a mirror to prevent it from leaving the room. This probably has the single most significant impact on the thermal performance of a window.
Laminated glass substantially reduces noise. Talk to your window supplier for advice on options.
Double-glazed windows have a gap between the panes of between 6 mm and 18 mm. The optimal size is 12-14mm for a standard double-glazing, and 16mm for ones with high performance glazing such as low-e coatings.
The air between the panes can be replaced with special gases, which are better insulators than air.
Spacer bars hold the panes of glass apart. Thermal spacer bars further improve performance over standard aluminium spacer bars.
Retrofitting existing windows
Single-glazed windows can let out heat 10 times more readily than insulated walls.
If you have single-glazed windows, there are three main options:
1) Secondary glazing: This might include a window insulation kit, acrylic sheets, or adding a second layer of glass. Research shows this approach is just as effective as double-glazing and is almost invisible if well-applied. It is most effective for retrofitting wooden frames, but can also reduce condensation on aluminium-framed windows.
Critical for secondary glazing is that the seals must be absolutely right or condensation can occur between the panes. In some cases opening such windows can be awkward.
2) Retrofit double glazing: This is where the existing glass is removed and a new double glazed unit is retrofitted into the existing frame. It is usually cheaper than replacing the complete window.
3) Replacement windows: Special consideration should be given to retrofitting
timber windows rather than replacing them with new aluminium frames, because of the better thermal performance of timber frames. (See Table 1).
If you’re in the market to buy new windows, consider both the frame and glazing performance as discussed above.
The R-values in table 1 give a guide to thermal performance: the bigger the number, the greater the effectiveness. Note how the performance improves with better window frames and different glazing elements.
To put things in perspective, even the best glazing available in New Zealand (R0.6) falls short of a standard insulated wall (R 1.9).
Some companies offer a Window Energy Efficiency Rating System (WEERS) evaluation so you can compare performance gains for different options.
Table 1: Comparison of Types of glazing
|Single glazing||Standard double
|Double glazing with
Low E glass
|Double glazing with Low E glass
|Aluminium frames||R 0.15||R 0.26||R 0.31||R 0.32|
|R 0.17||R 0.31||R 0.39||R 0.41|
|Timber/uPVC||R 0.19||R 0.36||R 0.47||R 0.50|
* Stops transfer of heat and cold via window frames
Note: Figures from NZS 4218:2009. Figures for double glazing based on 10mm space between panes
Rules of Thumb
- Ask for WEERS ratings to compare performance.
- Aim for the highest specification your budget can accommodate.
- When planning a new home, consider window placement carefully. See Information Sheet #4, Passive Solar Design.