Windows are one of the weaker links in the thermal envelope of a house. Heat is conducted rapidly across the glass and frame, causing warmth to be lost.

The rate that heat is transferred (Thermal Resistance or R-value) of windows ranges from R0.1 to about R0.5 – compared to an R-value of R3.6 insulation that you might install in your ceiling or R2.2 in your walls. Most double glazing on a new house will have an R-value of R0.26. Because of the transfer of heat through windows, uncovered glass can be very cold and uncomfortable to be near in cold weather, and can make the house cold in general. This would indicate that windows will require special attention when improving the thermal performance of a house.

Thankfully windows are usually accessible and there are different ways of reducing this heat loss. Curtains are usually the first and best choice for reducing heat loss and making a house comfortable. Although new curtains can be expensive, with some initiative they can often be purchased or improved on a limited budget.

If purchasing curtains from a curtain shop, be sure to ask for one or more backing layers and avoid pole-and eye curtain rails, or any type of curtain tracks or rails that leave a large gap at the top of the curtain. There are curtain tracks available that have no gap at all, which make them as effective as pelmets – a system that has become unfashionable but which works very well. If your budget is smaller there can be good deals found on internet auction sites such as Trade-me and op-shops. Older curtains can be usually be improved by the addition of thermal backings. Insulation works by trapping layers of air. As still air is a poor conductor of heat, this slows the rate of heat loss from the house. A double-layered curtain works the same way, by trapping a layer of warm air between the layers. Old curtains make good backings. For those with a community cervices card, some areas have a ‘Curtain bank’ which can provide free curtains for a living area or child’s bedroom.

Even with good double glazing, curtains or blinds are still recommended to attain maximum thermal performance and comfort from your home.

Do your curtains meet the three criteria for good performance?

When hanging curtains, there are a few things needed to have them working effectively. Aim to have plenty of ruffles when the curtains are closed, and to have curtains that fit well all around the frame – especially the curtain track. It is important to avoid the ‘reverse chimney effect,’ as this will negate the efficacy of your curtains. The way that the reverse chimney effect works is that if there is a gap around the curtain, the cold surface of the glass will cool the air that is in contact with it. The air that has been cooled by the glass will sink, drawing more warm air from the top part of the room through the gap at the top of the curtain or curtain track. This creates a cycle, with the warm air in the room being cooled by the glass, and coming out the bottom as a cold draught. This cycle actually creates that draught.

  1. At the top, there should be no gap between the curtain and the frame or wall
  2. At the bottom, the curtain should be in contact with the window sill and ideally reach the floor
  3. There should be two or more layers of curtains.
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