High or Low-e? Low-emissivity coated glass for apartment buildings

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Condo dwellers often report thermal discomfort in the fall and spring due to the degree of solar radiation exposure hitting their windows.

Product solutions
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.

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Figure 2: These photos show the study apartment building in Ottawa. Superimposed are the positions of the sun at sunrise, noon, and sunset during summer and winter solar solstices and the fall and spring solar equinoxes. Each unit was occupied by one resident.
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