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Window shading devices have been used for many years to prevent overheating of houses. Few studies have been done, however, to quantify the benefits in a meaningful way.
The twin houses of the Canadian Centre for Housing Technology (CCHT) afforded the National Research Council’s Institute for Research in Construction (NRC-IRC) an excellent opportunity to conduct tightly controlled studies to assess the benefits of using two types of window shading devices: exterior insulating rollshutters and highly reflective interior screens. These devices were installed in the CCHT test house in two different experiments and their performance was compared to that of typical interior (Venetian) blinds installed in the otherwise identical reference house
Exterior Rollshutters
The exterior rollshutters were studied in the winter and summer of 2008. They were made of fixed and articulated aluminum slats (beige color) with sandwiched polyurethane insulation. The slats could be arranged so that they were tightly abutting (winter), or with a small gap between them to admit some light and provide a view from the inside (summer). They were not designed to allow the slats to be angled, and could only be adjusted up and down in the vertical plane of the window using side railings installed on the brick walls. A rubber gasket installed between the side railings and the walls sealed the air space between the shutters and windows. The bottom of the shutters was unsealed to allow for water drainage.
In the winter, the shading devices in both houses were kept open (shutters fully retracted and slats of interior blinds horizontal) from 9:00 a.m. to 5:00 p.m. to admit daylight and solar heat gains indoors, and kept closed (shutters drawn down and slats of interior blinds tightly squeezed) at other times. At night, the rollshutters reduced the heat loss through the windows of the test house by about 20 per cent compared to the reference house. They reduced the heating energy use by four per cent ± two per cent. These energy savings were proportional to the ratio of the total window surface area to the total envelope surface area of the house (windows make up 11 per cent of the liveable surface area).
In the summer, the shading devices in both houses were kept closed throughout the testing period to explore the maximum effects; it is important to note that most residents would open shading devices for some of this period, and thus the savings reported below would be reduced. Indeed, a follow-up NRC survey on household indoor climate controls found that about 50 per cent of households would open or close their shading devices during daytime. Effects of such realistic shading controls on house energy use are included in a whole-house energy simulation model to predict the energy performance of other shading types in other Canadian building sites.
The rollshutters reduced the daily energy use of the air conditioning unit of the test house by about 45 per cent compared to the reference house. When the energy use of the furnace fan was factored out, the net reduction in daily cooling energy use was roughly 26 per cent ±10 per cent. The fan thus accounted for more than 30 per cent of the energy use of the air conditioning unit. The fan ran continuously on a full nominal speed with the air conditioning unit, and on half the nominal speed with the heat recovery ventilator when space cooling was not required.
Interior Reflective Screens
The experiments with the interior reflective screens were conducted in the summer only. The shades were mounted outside (against) the window frames leaving an open air space between the shades and the wall to reduce the risk of excessive window glass temperatures and glass breakage caused by heat build-up. The shades were made of a PVC-coated glass fibre material with an openness factor of four per cent, which allowed a view-through to the outside.
The results with the interior reflective screens showed that the daily air conditioner energy use of the test house was about 13 per cent lower than that of the reference house. The maximum difference in the daily energy use was about 18 per cent, and the lowest about 10 per cent. When the energy use of the furnace fan was factored out, the daily cooling energy use of the test house was on average roughly eight per cent ± two per cent lower than that of the reference house.
Window Moisture also Reduced
In addition to saving energy and reducing peak electricity demand, the exterior rollshutters reduced the risk of moisture condensation on the interior window surfaces during the night in winter by keeping interior glass surfaces 4oC warmer than those of the reference house. The colder window surfaces of the reference house were, however, shielded by the closed interior blinds. Consequently, the rollshutters and interior blinds resulted in similar thermal conditions near the windows.
Additional information about the energy performance of the tested shading devices is available at: http://irc.nrc-cnrc.gc.ca/pubs/ci/v12no4/v12no4_13_e.html and http://www.ccht-cctr.gc.ca/projects/solar_e.html
Dr. Aziz Laouadi is a research officer in the Indoor Environment program of NRC-IRC. Anca Galasiu is a technical officer in the same program.
