Windows can have a major impact on the heating and cooling loads of a building. Up to 40 per cent of a home’s heating energy can be lost and up to 87 per cent of its heat gained through windows.
Improving windows’ thermal performance reduces energy costs and greenhouse gas emissions. The increasing propensity for highly glazed skyscrapers, especially, is seeing the increasing consideration of low-e glazing to combat this.
The Solar Heat Gain Co-efficient (SHGC) for windows measures how readily heat from direct sunlight flows through a window system. The lower a window’s SHGC, the less solar heat it transmits.
In temperate and cool temperate climates such as Sydney, Perth, Melbourne, Adelaide and Hobart, northerly glazing should have a high SHGC as they are the building’s primary solar collectors. In hot climates such as Darwin and Brisbane, there is little compromise: a low SHGC is best.
To control space heating costs and rein in the rapid growth of space cooling costs, there is an obvious solution: improve the thermal performance of window glass. This is where low-e glazing comes into play.
Low-emissivity glass is designed to prevent heat escaping through your windows to the cold outdoors. Low-e glass has an invisible coating which dramatically reduces heat transfer and reflects interior heat back into your room.
For cooling-dominated climates, low-solar-gain (LSG) low-E coatings are best. Typically made with sputtered low-E coatings consisting of either two or three layers of silver, they have the ability to reduce solar heat gain while retaining high visible transmittance.
High-solar-gain [HSG] low-E glass products are best suited for buildings located in heating-dominated climates and are the product of choice for passive solar design projects. These windows are designed to reduce heat loss but admit solar gain and are often made with pyrolytic low-E coatings, although sputtered high-solar-gain low-E is also available.
New research in Canada has compared both LSG and HSG products to understand more clearly the effects they have on occupant comfort and energy use.
Three almost identical southeast-facing, upper-level apartments were selected; one was refitted with HSG low-e coated glass, one with LSG and other left uncoated. Each apartment was occupied by one resident.
As solar gain increased and outdoor temperature decreased, perceived comfort in the LSG apartment improved. Meanwhile, those in the control and HSG apartment experienced uncomfortably warm periods during warm, sunny days which had to be relieved by opening windows and doors.
During the winter months as solar gain hit its highest point and outdoor temperatures their low, those in the LSG apartment reported improved thermal comfort and they set their thermostats higher accordingly.
In spring, those in the LSG apartment again reported improved thermal comfort and, although all three apartments were able to reduce thermostat settings, those in the ‘control’ and HSG apartments reported periods of discomfort which had to be relieved by opening doors and windows.
In summer, with solar gain at its minimum, there was little difference across all three apartments.
The study concluded that LSG low-e coated glass for residential apartments with sunny exposure is best for improving thermal comfort in spring but also had benefits in autumn and winter. The downside is greater space heating usage.
The researchers suggest that double- or triple-pane IG units, warm-edge spacers and argon or krypton gas fill, and non-metal frames could solve this issue.
In the summer, there appears to be little benefit to LSG and HSG low-e. Here the researchers suggest other means of reducing solar gain should also be considered, such as exterior sun shades or dynamic glass.
“The findings demonstrate that those specifying glass need to be careful when choosing the type of low-e to suit façade orientation and solar exposure,” said George Torok, a building science specialist and member of the Façade Engineering Group at Morrison Hershfield in Ottawa, who led the research.
“For example, east-facing windows could benefit from LSG low-e in the early morning, but HSG low-e may be more beneficial since morning temperatures are usually cooler and for most of the day there would be little direct solar energy.
“Champions of passive solar heating design have long harped on about concept of ‘tuning’ each building façade to suit solar exposure. The ability to do this now exists using modern HSG and low-LSG technology."
Torok is in no doubt as to the potential value of LSG and HSG low-e if used appropriately.
“As building codes become more stringent, the clever use of LSG and HSG low-e on different elevations, zoning of space heating systems and provision of space cooling systems to allow the indoor environment to be heated or cooled appropriately in response to solar gain will become more important,” he said.