Summer shading performance at the Canadian Centre for Housing Technology

Authors:

J. Gusdorf (Consultant)
M. Armstrong (NRC)
F. Szadkowski (NRCan)
J. Sager (NRCan)

Publication date: March 24, 2011

Discussion

Maximizing free passive solar space heating should be a prime strategy in any building project. If a building is well exposed, and can be built with a southern orientation, then the main limiting factor on passive solar is overheating, which can occur on warmer, sunny days during the heating season, during shoulder seasons, and during the cooling season. This problem cannot be dealt with by fixed overhangs above windows, since they reduce useful solar gains by at least as much as those that increase cooling loads.

A solution to overheating is some form of adjustable shading device. If the shading devices were located inside, they could be easily adjusted by occupants in the way Venetian blinds are. However, experience to date suggests that with energy-efficient windows, internal shading is either ineffective or causes thermal stress on the glass. Therefore, moveable external shading devices are likely to become important parts of (near) net-zero energy houses and should be deployed in any house that seeks to maximise passive solar gains while minimizing potential for over-heating. These could be manually adjusted from the outside, in which case users could deploy them during the cooling season, and retract them for the rest of the year. They could be manually adjusted from the inside, either electrically or physically, as is the case with some rollshutters. Or they could be automatically controlled so that they would only be deployed when needed to prevent excessive solar gains, with manual overrides for privacy or security, or when occupants consider natural light or views more important than excess gains.

This study has shown that retractable canvas awnings that cover approximately the top half of windows can significantly reduce cooling energy. The awnings used were typical commercially available models, and no attempt was made to change their characteristics in order to optimize their performance. They were installed on three of the four south windows, and not on the west window or either of the east windows. Because they cover only about the top half of a window, they allow significant amounts of natural light, and leave much of the view to the outside unobstructed. The awnings reduced cooling energy by 18%, and also kept maximum temperatures much closer to the set-point, in the rooms with awnings and in other rooms, thus providing both energy savings and increased comfort.

Although awnings may reduce light levels less than other shading devices do, it is clear that they do significantly reduce light levels in the rooms to which they are attached, and significantly increase the amount of time that illuminace is below recommended levels. However, the effects this will have on occupants will depend on a number of non-quantifiable and subjective factors. For example, whether occupants are in those rooms during times of low light levels, what activities they are engaged in, and their subjective reactions to particular light levels. It is possible that some will find the reduced lighting unacceptable, and will react by turning on lights, thus negating some of the energy savings. Increased use of lights would increase energy use directly and also indirectly by increasing cooling loads. However, since savings due to awnings are in the order of three kWh per day, it seems unlikely that occasional increased use of lights would negate a significant amount of the energy savings, especially in energy-efficient houses with energy-efficient lighting. It should also be noted that in very energy-efficient houses, shading devices may be controlled automatically, so that they are only deployed when needed to avoid overheating or increased cooling loads. This is unlikely to occur during periods of low light levels.

For access to the full publication, please contact the CanmetENERGY-Ottawa Business Office.