Windows are the Weakest Link
Even double glazed windows are colder than the surrounding walls. Where are code compliant wall today will have an R-value of about 2.0, a typical double glazed window will have an R-value of just 0.26. For an older house with no wall insulation, the wall will have an R-value of about 1.5 and a window with a single pane of clear glass has an R-value of about 0.15.
So assume that a window will lose about 10 times more heat than an equal area of surrounding wall.
Don’t forget the Draughts
The drastic difference in R-value is just the conductive heat loss, but that’s only half the story. Windows are a great source of ventilation (controlled air flow) but are usually a really good source of infiltration (uncontrolled air ‘leakiness’).
Draughts are best dealt with on a case-by-case basis. The materials are relatively inexpensive, but good sealing of old windows requires some time and attention to detail. You’ll need:
- A stack of various sized foam or rubber draught strip
- Cleaning solution such as mineral turpentine and a clean cloth
- Selected window latches (if required) to suit the frame and window type
Retrofitting Glazing Options
The range of options for improving windows includes:
- Thin plastic film
- Thermal curtains
- Magnetic fitted acrylic
- Secondary glazing
- Retrofit Double glazing
Cost Benefit
See this report by Nick Smith, architecture masters student at the Victoria University of Wellington for a good analysis of the best bang for buck.
Health and Comfort
Having warm windows is not just about saving money. Cold surfaces around the house make you feel could. Condensation also harbours mould.
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