Single glazing or double glazing – which is better?
The question of single or double glazing to the north is an interesting one. Naturally, double glazing provides better insulation, but it also reduces the amount of solar radiation (heat) coming in to the building, so there is a trade-off.
The amount of glazing in a Strine home is maximised on the north elevation, to achieve as close as possible to 100% of the north facade. This is most unusual when compared to standard housing designs. It means that there is a lot more glass in a Strine home. Maximising the glazing to the north takes full advantage of the solar heat gain in winter.
The figures below indicate that single glazing is better than double glazing on the north face of a well-designed passive solar house. They explain why in Strines three decades long experience that single glazing (of course with drapes for insulation) to the north works well in the Canberra region climate and doesn’t warrant the extra expense of double glazing. In fact, we always get comments that our homes are lovely and hot in the middle of a sunny winter’s afternoon! That’s when the high thermal mass of a Strine concrete home is essential to store all that free solar heat.
The Solar Heat Gain Coefficient (SHGC) for Canberra winter days is very significant because of the clear skies and sunny days. Around noon Canberra gets approximately 800 watts (that’s 0.8 of a kilowatt) per square metre of northern facade. This is almost equal to a little 1 bar, 1 kW, radiator per square metre, or 2 kW per sliding door on the north. A ‘typical’ Strine home has the equivalent of 50 square metres of north facing glass: that collects a peak of 40 kW of free heat energy coming in to the home on a clear winter’s day. Of course a Strine home incorporates an optimised eaves overhang and other shading devices (particularly pergolas) to keep the high summer sun from shining on these generous northern windows.
However, the heat loss factor (U value) for glass is very poor compared to an insulated wall. Single glazed windows can be 10 to 20 times worse than an insulated wall . Heavy drapes or blinds (close fitting with a box pelmet or closed top and lined or made of an insulating fabric or cellular air traps ) can reduce the heat loss through single glazed windows by more than 50%. Thermal double glazing (different to acoustic double glazing) has a similar effect as heavy drapes and can also reduce heat loss by more than 50% compared to single glazing. Adding heavy drapes to double glazing further reduces the heat loss by ANOTHER 30%, down to 35% of the loss from single glazing.
The Solar Heat Gain Coefficient (SHGC) figures from the WERS (Window Energy Rating Scheme) web site show that double glazing reduces the solar heat gain of 0.72 for single glazing to 0.61 for ‘4/10/4’ double glazing and down to 0.23 for ‘5LowE/10Argon/5’ double glazing. This is a huge reduction of 15% to 68% of the free solar heat (equal to a peak of 6kW to 27kW for a typical Strine home where the drapes or blinds can be reefed to not obscure any of the windows), so if double glazing is used it should have a high SHGC to minimise this reduction. The insulation U value of the best double glazing with conventional aluminium frames is still only 2.7 or 58% better than the 6.4 U value for single glazing . The saving in the quantity of heat lost by using typical aluminium-framed double glazing in the large amount of Strines north glass is only 2.4kW (when there is a 15°C temperature difference across the glass – the average over a typical Canberra winter day). This is a significant benefit over the cold night hours but small when compared to the 6kW to 27kW reduction in free heat from double glazing, over an average of around 4 hours per day.
Wherever possible glazing to all other orientations (east, south and west) should be double glazed with insulating frames and shaded. Double glazed windows should have the lowest U value. Shading is an important component of glazing to these elevations: shading to east and west helps reduce summer sun penetration and overheating in the summer and shading to the south helps reduce radiation heat losses to the black night sky that the Canberra region experiences, more so than the coastal capitals. This black night sky is a perfect absorber of energy and because on the south it is not balanced by daytime winter sun there is most net heat loss from the south facade of a building. This is particularly so through the glazing on that facade, as it has a much lower insulation value than the wall. A verandah to the south facade is ideal to thermally shield the whole wall and any glazing.
Double glazing also helps to eliminate condensation inside a home, as it insulates the warm air inside from the cold outside. Drapes on single glazing need to be well sealed on all edges to prevent the warm air inside getting to the colder glass and condensing in troublesome quantities .
So, single glazing or double glazing – it’s a great question and it really depends on the amount of thermal mass, the quality of the double glazing and of course, budget. If you follow the four golden principles of passive solar design (see our fact sheet), experience has shown us that you do not need double glazing in Canberra.