Window insulation
A lot of your home's heat can escape through your windows, especially if you already have well insulated ceilings, floors and walls. One way to minimise this heat loss is by providing insulation for your windows.
There are three main types of window insulation:
Retrofit alternatives to double glazing
Curtains or blinds
Most houses have some form of curtains or blinds on the windows. For these to be effective they need to trap a layer of air behind them and have enough thickness to trap some air inside them, too.
Blinds are usually very thin and often don’t really seal off the air behind them. Generally they will provide very limited insulation unless they are specifically designed thermal blinds.
To provide good insulation curtains should be made from thick, thermally backed material and preferably be double layered. Your curtains should be a tight fit against the wall and either be floor to ceiling or have pelmets on them. This is to prevent a cooling cycle forming. This is where warm air behind your curtains gets cooled by the window, falls out of the bottom of the curtain and sucks more hot air in at the top.
Close-fitting thermal curtains that cover the entire width of the window, fall to the floor, and have pelmets that are installed tight against the wall can reduce the heat loss through single-glazed windows by about 60%. This applies only with the curtains drawn, and curtains are therefore not a substitute for double-glazed windows.[1]
For double glazing, good curtains can reduce heat loss through windows by 40-50%.
Having your curtains open during the day in winter and closing them just before it gets dark will help keep your rooms warm.
Double glazing
Double glazing is a way of creating an insulating layer of air or special gas between two panes of glass. It lets in as much sunlight as single-glazed windows, but holds in the heat much better.
A well designed double-glazed window with a wooden, PVC or thermally broken aluminium frame:
- Can halve the heat loss through the window
- Significantly improves thermal comfort
- Reduces external noise
- Reduces condensation.
Double glazing is now standard in many new houses but it's also worth retrofitting double glazing any time that you are renovating. If you need to prioritise, install double glazing to the main heated areas of the house and where there are large windows.
Some double-glazing is better than others. For best performance, look for the following things:
- Frames that are thermally broken, or made from an insulating material such as u-PVC or wood. These will perform better thermally than windows with standard aluminium frames. It can reduce window heat loss by between 20% (thermally broken aluminium frames) and 40% (PVC or wooden frames), compared to double-glazing in standard aluminium frames.
- Low-emissivity (low-e) glass. This allows light and heat in, but reflects escaping heat back to the inside. Low-e glass cuts window heat loss by about 20% to 30%, compared to double-glazing without low-e.
- Multiple layers of good seals to keep draughts, moisture and noise out. The joint between the glazing unit and the frame also needs to be well-sealed.
- Spacers made of plastic or stainless steel to separate the glass panes (instead of aluminium) to reduce heat loss and condensation at the glass edge.
- An inert gas filling (such as argon) between the glass layers. This acts as a better insulator than air, reducing window heat loss by about 3% to 9%, compared to double-glazing with air filling.
Close-fitting thermal curtains will reduce heat loss through double-glazed windows even further.
When choosing skylights it is particularly critical to choose ones with good thermal performance characteristics to avoid high heat losses on cold winter nights and overheating in summer. Apart from the features listed above, effective shading to block out the sun on hot summer days and to improve insulation on cold winter nights is highly recommended.
Skylights that open can be effective for summer cooling through ventilation, so make sure that your skylight can open and is not a fixed model. If your skylight is installed through a roof space, make sure the lightshaft between the roof and the ceiling is well insulated.
Retrofit alternatives to double glazing
If you have got an older home (depending on the type and condition of your existing single-glazed windows) you may be able to upgrade their thermal performance without having to completely replace them with new double glazed windows. There are a number of alternative window retrofit 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
- Acrylic sheet magnetically attached to the existing window sash or frame
- Other proprietary secondary glazing systems.
All the above options work by forming a sealed, insulating air gap on the inside of existing, single glazed windows. To be thermally effective the air gap should have a minimum thickness of 10mm.
Not all of the above options are suitable for every situation and cost and performance can vary significantly. Some secondary glazing options may lead to condensation forming inside the air gap if it is not sealed effectively. Shop around and ask for independently verified performance guarantees and warranties.
For research results on the performance of secondary glazing options, see the following article in the BRANZ Build magazine (issue 116).
Take action
Hang curtains and blinds
Good, thermal backed and lined curtains can significantly reduce heat loss through windows.
Curtains should be bigger than the window frame and close fitted with no gaps for warm air to escape. They should either be ceiling to floor, or have a pelmet to reduce air circulating behind the curtains. Use thermal or close-woven materials and linings.
Draught proof your windows
Draughts and air leakage can increase your heating costs and make your home uncomfortable. DIY weather-stripping products are available from most hardware stores.
Double glazing
Install double glazing where possible or explore the cost and suitability of the above-mentioned retrofit alternatives for your situation.
More information
- For a checklist for regular maintenance of windows and doors, see the Consumer Build website.
Independent and unbiased research, testing, consultancy and information resource for the building and construction industry is available at:
Notes
[1] Lloyd, B., Bishop, T., Callau, M.: Retrofit alternatives for State Houses in Cold Regions of New Zealand - Report Nº 2", Physics Department, University of Otago, Dunedin, New Zealand, September 2007.
NZS 4218:2004 "Energy Efficiency - Small Building Envelope", Standards New Zealand, 2004
