by Katie Daniel | July 8, 2015 10:49 am
By George Torok, C.E.T., BSSO
In many large, urban areas of Canada, most of the population lives in apartment buildings. In the downtown core of cities like Toronto, the proportion is up to 70 per cent. With the current trend to intensify urban areas to limit sprawl into surrounding valuable farmland, the proportion of high-rise multi-family dwellers is expected to increase.
Building owners, operators, and residents in apartment buildings often report thermal discomfort in the fall and, especially, spring. This is due to the degree of solar radiation exposure, which is highest in these seasons due to a combination of low altitude and narrow azimuth range that causes the sun’s rays to be closer to normal (perpendicular) to the face of window glass (low angle of incidence). This results in maximum transmission to the building interior.
The angle of incidence is increasing in the spring and solar energy transmission is therefore falling, but the discomfort is often worse than in the fall because daytime hours are also on the rise. Additionally, outdoor temperatures are typically colder so windows and exterior doors are closed to keep out cold winter air.
Understanding the problem
Apartments with sunny exposures need space heating turned off early in the spring and turned on late in the fall; apartments with less exposure need space heating turned off later in the spring and turned on earlier in the fall. Typically, space heating systems are not zoned, with residents often having only limited control over space heating to compensate for solar radiation gain in condos and apartments with high solar gain.
When provided, cooling systems cannot always operate at the same time as space heating to provide relief. The thermal comfort of apartment residents suffers, as does the sanity of building owners and operators. Residents may try to cope by venting excess heat through open doors and windows or reducing solar gain by covering windows with curtains, blinds, shades, or aluminum foil. An apartment building with many covered windows during a sunny day is often indicative of a solar gain problem.
Energy use in residential apartment buildings accounts for 16 per cent of total energy used by Canadians and about 14 per cent of overall greenhouse gas (GHG) emissions. Space heating accounts for 51 per cent of residential energy use. Despite a 30 per cent increase in Canadian high-rise apartment households and condos in the current construction boom, the rate of growth of heating energy consumption has been modest—only about two per cent. This reflects the increased energy efficiency requirements in building codes (Figure 1).
Global warming, giving rise to milder winter temperatures in many locations, has also helped reduce space heating demand. However, it likely adds to space cooling demand; while space cooling energy usage is low, it is increasing rapidly—about 43 per cent in the same period. This also mirrors the increasing use of highly glazed building enclosures—window walls—that allow much more solar gain than traditional designs.
What can be done to further control space heating costs and rein in the rapid growth of space cooling costs? Anyone who has sat next to a south-facing window on a bright, sunny day and again at night will realize there is an obvious solution—improve the thermal performance of window glass. This can be done by including glass that has a low-emissivity (low-e) coating—microscopically thin metal layers that reduces the glass’ ability to emit radiant heat energy from the inside to the outside and/or reduce the amount of solar energy entering from outside to inside.
These coatings are applied when the glass is manufactured in large sheets, prior to being shipped to insulating glass (IG) unit manufacturers or later, after uncoated glass has been cut to size by the unit manufacturer, before the units are assembled. For retrofit projects, some low-e coatings are available on a plastic sheet that can be applied to the room-side surface of a unit later, after the completed glazing is installed and the building is occupied.
The original type of low-e coating is the high-solar-gain (HSG) type developed following the Arab oil embargoes of the 1970s to improve the energy efficiency of houses. This type allows high amounts of solar gain, but restricts radiant energy heat loss, so for windows with sunny exposure, there is an overall net heat gain, reducing the amount of space heating required.
These early low-e coatings also have high visual transparency—architects took advantage of this to improve daylighting in commercial buildings. Unfortunately, the high solar gain sometimes created uncomfortably warm indoor conditions and high space cooling loads. Low-solar-gain (LSG) low-e coatings were then developed to better control solar-gain-related cooling load, in commercial buildings.
In a double-pane IG unit, HSG low-e coatings are most effective when placed on the outward facing surface of the inside pane of glass to reduce heat loss through the unit cavity and outboard pane of glass. LSG low-e coatings would be most effective when placed on outward-facing surface of the outside pane of glass, but since almost all such coatings are sensitive to moisture, they are located on the inward-facing surface, protected inside the unit cavity.
Nature of the experiment
In order to determine the effects of low-e glazing on resident comfort and energy use, an experiment was conducted on an apartment building owned by Centretown Citizens Ottawa Corporation (CCOC) in Ottawa. The study was initiated by this author while at GRG Building Consultants, and completed at Morrison Hershfield. Funding was provided through the Canada Mortgage and Housing Corporation (CMHC) External Research Program and from Natural Resources Canada (NRCan).
Three southeast-facing, upper-level apartments were selected as the subjects for this study. The apartments were almost identical in layout, size, and solar exposure (i.e. facing southeast), as shown in Figure 2). Each unit was occupied by one resident with occasional guests. In two of the apartments, windows and sliding doors were refitted with low-e coated glass—one with HSG and the other with LSG. In the third apartment, the existing, uncoated glazing was left as is to act as a ‘control’ against which performance in the refitted apartments was compared.
Each of the apartments was fitted with equipment to track the interior air temperature and relative humidity (RH) to gauge thermal conditions. Incident solar radiation was monitored at the exterior of the building and transmitted solar energy was measured inside each unit directly behind window glass with and without low-e coatings.
Residents were interviewed for their perceptions of thermal comfort and operation of the apartments (e.g. position of window coverings, windows and doors being open or closed, thermostat settings) were observed on a monthly basis to estimate effects on energy use. (The apartments were not metered for energy use so energy consumption was not measured.) The experiment was conducted over a 12-month period from September 2010 to 2011.
During the fall, when solar gain was increasing and outdoor temperatures decreasing, perceived comfort in the apartment refitted with the LSG low-e coated glass improved. Meanwhile, during warm and sunny periods, the residents of the ‘control apartment’ and the HSG low-e refitted apartment experienced uncomfortably warm conditions from time to time, which they relieved by opening windows and doors.
In the winter, as solar gain reached yearly maximum values but outdoor temperatures dropped further, the LSG low-e apartment resident continued to report improved thermal comfort. Residents of all apartments began setting their thermostats higher. Those in the HSG and LSG low-e apartments had higher thermostats settings than the control unit (Figure 3).
As solar gain and outdoor temperature increased in the spring, thermostat settings were reduced in all apartments. The resident of the LSG apartment continued to report improved thermal comfort whereas from time to time, on warm sunny days, those in the ‘control’ and HSG apartments reported discomfort which they attempted to relieve by opening windows and doors (Figure 4). In the summer, solar gain reached yearly minimum values with little variation between the apartments. All residents reported uncomfortably warm indoor conditions with no improvement compared to pre-experiment conditions (Figure 5).
The study revealed using LSG low-e coated glass in residential apartments with sunny exposure can improve thermal comfort in the fall and winter, but especially in the spring. However, this comes at the expense of greater space heating usage. If LSG low-e is used, compensating actions should be considered such as using:
In the summer, there appears to be little benefit to LSG and HSG low-e. Other means of reducing solar gain should also be considered, such as exterior sun shades or perhaps dynamic glass (e.g. thermochromic or electrochromic glazing). Reducing the amount of vision glazing should also be considered—after all, what benefit is a window behind a couch? Smaller glazing areas would also help in controlling fall, winter, and especially spring thermal discomfort since less solar gain would occur and also because opaque wall systems generally have lower rates of heat loss.
Since the effectiveness of LSG low-e is linked to combinations of sun altitude and azimuth angle for which the resultant angle of incidence to the glass is low, the study’s findings could be applied to other building elevations. For example, in this experiment solar gain was reduced in the southeast-facing LSG apartment from mid-morning to early afternoon; southwest-facing apartments would benefit from LSG low-e throughout the afternoon. There would be little benefit to LSG low-e in a north-facing window—there, an HSG low-e would be a better choice to control heat loss. 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 (Figure 6).
The path forward
The findings demonstrate there is a benefit to using HSG and LSG low-e glazing, but one must be careful when choosing the type of low-e to suit façade orientation and solar exposure. The concept of ‘tuning’ each building façade to suit solar exposure has long been advocated by proponents of passive solar heating design. The ability to do this now exists using modern HSG and low-LSG technology.
Many low-e coatings have neutral appearances with little colour and low visible light reflectance, enabling LSG and HSG coatings to be used on different elevations without difference in appearance. Computer energy modelling software such as EnergyPlus allows the design team to quickly and efficiently determine the effect of LSG and HSG low-e on building energy consumption and indoor thermal conditions.
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 (and other weather effects), and smarter use of the extent of vision glazing and opaque wall areas will become more important.
George Torok, CET, BSSO, is a building science specialist and member of the Façade Engineering Group at Morrison Hershfield in Ottawa. He has more than 25 years of experience in building envelope design and construction, performance failure investigation and remediation, and specializes in design and construction of new glazing systems and in repair, upgrade, and replacement of existing glazing systems. Torok is active in the development of Canadian Standards Association (CSA) standards governing design and installation of fenestration systems. He can be reached at firstname.lastname@example.org.
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