Advanced HVAC systems can provide marvellous energy efficiency benefits to virtually any building. The problem is that many such systems work harder and claim a larger footprint simply because buildings are often designed with little regard for the thermal performance of their enclosures or the way in which their design influences heating and cooling loads.
"The window-to-wall ratio of most Canadian buildings is extremely high for the cold climate, and that results in very low insulation value," says Scott Armstrong, associate with MMM, a WSP Company.
"If you build an R-30 wall in which the surface is 60 per cent double-glazed windows, the total wall insulation value probably winds up being R-5 or less. If you switch to triple-glazed windows, the total wall insulation value jumps to nearly R-10."
Armstrong notes that curtain walls composed of glass panels are popular from a construction perspective for several reasons: they represent a modular product precision-manufactured off site; they’re less expensive than brick and other construction materials; and they’re easy to install quickly from within the building floor plate.
"Clearly we could be building walls with better insulation value, but it’s difficult to determine whether people prefer to lease buildings with glass exteriors, or whether they lease them because that’s all that the market is offering them," he says. "On one project I know, the walls had a 55 per cent glazing area, with the remainder made up of spandrels insulated to R-12. That bought the entire exterior wall to at least R-7. While that’s not a heroic effort, it’s a really good R value for a commercial building."
The intended use of the building — whether residential or commercial — will also generally determine the type of heating and cooling loads the building’s mechanical system must overcome.
"A lot of commercial buildings are cooling dominated," says Armstrong.
"They’re more densely lit compared to residential buildings and you might have 100 square feet available per person and those people are generating heat, along with photocopiers and other equipment. These buildings have large core zones and these zones are almost entirely cooled year round."
On the other hand, residential buildings are heating dominated because there’s a low occupant density with one or two people typically occupying 800 square feet. "If the condo suite is only 40 feet deep, there’s a lot of influence by the building skin on energy loads because less of the building could be classified as a core," says Armstrong.
Under the current energy market, electricity required for cooling is much more expensive than the gas typically used for heating. "Cooling loads are significant with a lot of glass," says Armstrong. "This should create much more incentive for commercial buildings to more carefully manage their cooling loads."
Building orientation can also affect the energy consumption of mechanical systems. Armstrong points out that many of the condominiums and apartment buildings built along the waterfront in Toronto in the 1970s are oriented north to south — presumably to provide occupants a fleeting but equal glimpse of Lake Ontario.
"You might have been able to affect the cooling load with shading, however, these buildings are difficult to shade on the east and west elevations," he says. "You can see occupants voting on their comfort level with their shades to see how well this has worked out."
Armstrong notes that, if the building enclosure is well conceived, building massing — the shape and size of a building — and form ultimately have little impact on energy loads. However, in general, building height tends to increase cooling load intensity, while decreasing heating load intensity.
"Really amazing mechanical systems can cover the sins of an inadequate building enclosure, and they’re a way to meet Code and LEED energy performance targets," says Armstrong. "However, the truth is that we haven’t worked very hard to reduce heating and cooling loads in the first place. The better we design our enclosures, the more opportunities we have for taking advantage of higher-performance HVAC systems."
Armstrong spoke at Construct Canada 2015 in Toronto.
Curtain walls composed primarily of glass provide challenges in creating significant insulation value.
Photo: Skeezix1000/Wikimedia Commons"
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