When Does A Sustainable Architect Use Single Glazing? Why?

Double Glazing or Single Glazing? It depends on the Design.

Ric Butt LFRAIA, HonAIA, Past President Australian Institue of Architects.

Strine Environments specialises in sustainable Passive Solar design and our homes require no cooling in summer, (even in 40 degrees plus temperatures) and minimal heating in winter. Our designs have a full glass façade to the northern side of the building which is not usually seen in standard brick veneer homes. The type of glazing and window frames used in sustainable design is part of a critical equation including; building orientation, how much thermal mass, how much insulation, quality of glazing and window frames and of course, budget.

Double glazing by far provides better insulation, which in the grand scheme of things is of great benefit, and we highly recommend using it. Wherever possible, glazing to east, south and west should be double glazed, ideally with insulating frames, and shaded. Double glazed windows should have the lowest U value which is a measure of heat loss in a building element such as a window, wall, floor or roof.

The northern side in a passive solar design is a little different. There is a trade-off in a Passive Solar house as double glazing also reduces the amount of warmth (solar radiation) coming in to the building on the northern side in winter which is critical to solar performance.

The Strine Approach

Maximising the glazing to the north takes full advantage of the solar heat gain in winter. We always get comments that our homes are lovely and warm in the middle of a sunny winter’s afternoon. A Strine home capitalises on this free heat gain by using only single glazing to the north. We then use thermal mass to store this winter warmth and release it slowly over the course of the day and night. A Strine home also incorporates an optimised eaves overhang and other shading devices (particularly pergolas) to keep the high summer sun from shining on these generous northern windows.

The result is a home that stays at a constant temperature all year-round of about 23 degrees without the need for heating and cooling, and without double glazing to the north which can be expensive. When using single glazing, drapes need to be well sealed on all edges to prevent the warm air inside getting in contact with the colder glass and condensing in troublesome quantities which can be a drawback. If you don’t want to use insulated window coverings to the north, you might be better off with double glazing to the north. Double glazing helps to eliminate condensation inside a home, as it insulates the warm air inside from the cold outside (Warm air holds more moisture vapour).

Shading is important in sustainable home design

Shading is an important component of glazing to all sides of the house: shading to east and west helps reduce summer sun penetration and overheating in the summer, while 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. A verandah to the south facade is ideal to thermally shield the whole wall and any glazing. A northern pergola with deciduous vines allowing winter sun penetration is ideal for this type of design.

The Science of single vs. double glazing

The figures below indicate that single glazing is better than double glazing on the north face of a well-designed passive solar house. They support why, in Strine’s three‑decades‑long experience, single glazing to the north works well in the Canberra region climate, negating the need to spend money on expensive double glazing on the north facade.

The Solar Heat Gain Coefficient (SHGC) is a measure of how much solar radiation passes through a window. The SHGC for Canberra winter days is very significant because of the clear skies and sunny days. Around noon, Canberra gets approximately 800 watts per square metre of northern facade; that’s 0.8 of a kilowatt. 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’s 40 kW of free heat energy coming in to the home on a clear winter’s day. However, the 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. This heat loss can be offset by more than 50% by installing 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). 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 6kW to 27kW for a typical Strine home where the drapes or blinds can be reefed to not obscure any of the windows). This means that if you are using double glazing, it is important that it has a very high SHGC to minimise the reduction of free solar heat coming into the building. 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 Strine’s 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 in the middle of a winter’s day.

As you can see, the glazing question is a complex one. Strine has perfected the use of different types of glazing to achieve an energy efficient solution which delivers a home that is full of natural light and warmth, all year round.

* A U value is a measure of heat loss in a building element such as a window, wall, floor or roof.

Ric Butt is a Life Fellow, and Past President of the Royal Australian Institute of Architects. He is also an Honorary Member of the American Institute of Architects. His Sustainable practice, Strine Environments, is based in Queanbeyan, and caters for Canberra and the surrounding Region. Learn more about Ric…

Note that Ric is only advocating the use of single glazing on one orientation, in homes that he has designed, and only under nominated conditions. In other circumstances he is a career-long champion for the benefits of double glazing.- Ed.