ASHRAE Journal - January 2013 - 72

engineer’s notebook

Using Passive Design
B
by Caroline Hachem, Student Member ASHRAE; and Andreas Athienitis, P.e., Member ASHRAE

uilding design plays a key role in influencing energy consumption of neighborhoods. It has a significant effect on thermal performance

and can provide advantages in capturing solar energy.1,2 Solar radia-

tion can be exploited in passive design to reduce energy consumption for space heating and lighting and as a source for electricity and thermal energy generation using solar collectors.
The main components in the design of a solar house are improved envelope, thermal mass and adequate shading devices, in addition to favorable building orientation and shape. This column presents the main effects of building envelope design on the energy performance of a building, and it focuses on the effect of deviation from a rectangular building shape on the potential of a building to capture and use solar energy. and overheating in summer.3 Figure 1 shows the annual heating load for different levels of insulation and different window areas for a two-story rectangular house in Montreal (45° N). Glazing that combines high solar heat gain coefficient (SHGC≥0.5) and insulation should be adopted to exploit passive solar gains in the heating season. Using an appropriate shading device, such as controlled reflective blinds, on all windows is important to avoid overheating during the cooling season.

around curved roads, see Figure 2). Nonrectangular and particularly self-shading shapes (such as L-shapes) offer architectural and solar design flexibility, but their efficient design is influenced by several parameters. To design an energy-efficient shape, with an optimized solar potential, the following parameters should be considered at an early stages of the design.

Rectangular Shapes
Aspect ratio (AR): Ratio of southfacing façade to the perpendicular façade is a key parameter in the design of rectangular shape for increased solar potential. AR of 1.2 to 1.3 offers a balance between heating and cooling loads. For an AR that is less than 1.2, heating demand is significantly increased. For a cold climate, a ratio between 1.3 and 1.7 can offer a reasonable choice for reducing heating load in northern cold climates, increasing south-facing areas (especially the roof, which is discussed later), while maintaining the functionality of the plan. There are some situations where it is not possible to design simple rectangular shapes, due to the site shape, road layouts, or simply for aesthetic and functional considerations. In this case, the following parameters should be considered in the design: Depth ratio (DR): Solar radiation incident on self-shading shapes is significantly affected by the depth ratio—the ratio of shading to shaded façade lengths, assuming that the shaded façade is south facing (Figures 2a and 2b). A ratio not exceeding one-half is suggested in cold climate design to reduce the effect of shade on heat gain by windows. An L-shape with a DR of
January 2013

building envelope Characteristics
Heat loss from conduction through the envelope can be significantly reduced by increasing wall insulation levels, use of windows with lower U-values, and increased airtightness. However, costbenefit of insulation is characterized by a diminishing returns curve, whereby, beyond a certain level of insulation, the cost of energy saving is not matched by the cost of increased insulation. Additional measures should be considered, such as appropriate window design. Size and location of windows influence the amount and timing of solar heat gain. For northern cold climates, south-facing windows that cover 35% to 40% of the south wall surface can maximize solar gains in the heating season and minimize it during the cooling season. Glazing on east or west façades should be minimized to reduce heat losses in winter
72 ASHRAE Journal

setting of Dwellings
Location of a dwelling with respect to neighboring houses affects significantly their energy performance. Orientation of a dwelling relative to south (in the northern hemisphere) affects energy consumption for heating and cooling and electrical energy generation from building integrated photovoltaic system (BIPV). Heating load is increased by up to 30% with a rotation angle of 60° east or west from south, as compared to the south facing rectangular shape building.2

Self-Shading Geometries

optimization of building Form
A rectangular layout is generally considered the optimal shape for energy efficiency. However, in an urban context, this shape may not be optimal (e.g.,
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ASHRAE Journal - January 2013

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