CITIES IN FLUX

Passive Solar Design in A Cold Climate
— Caroline Hachem-Vermette

Why lost skills in passive design are resurfacing in today’s energy efficiency designs and what that means as cities monitor carbon more closely

Passive solar design seems like common sense: if we design buildings thoughtfully and holistically, in consideration of the annual path of the sun, we will maximize solar radiation when needed and reduce energy consumption. Why, then, is not incorporated more often in new design? How easy is it to implement, and what are the principles and techniques behind it?

The practice of capture and utilising natural solar radiation in buildings, thus reducing energy consumption must be considered a priority in contemporary architectural design. Particularly with the increasing practice of green real estate and increasing energy demands, architects can play an important role in reducing the negative environmental impact of buildings if they incorporate passive solar design principles and related technologies in every new structure.

Passive solar design, without the use of mechanical or electronic devices, is not a new approach. Historically, ancient Greeks designed their houses and cities to take advantage of passive solar radiation. Romans became the first people to use transparent glass to trap heat gain inside their buildings. A prominent application of this principle is the roman baths, where water was kept at warm temperatures using passive heat gain. The cliff dwellings at Mesa Verde in southwest Colorado are a striking North American example of passive solar design. These dwellings face south, have a large overhang that protect the interior space from the summer sun, and store winter solar thermal radiation in their solid earth floor and walls, to be released passively later on in the day.

It is only in the last hundred years that we started to steer away from applying natural solar design in our buildings. Take, for example, our excessive use of glazed surfaces in our buildings without the consideration of the orientation of these surfaces, and a disregard of whether or not they are properly shaded (see below for the main principles of passive solar design). Our buildings increasingly rely on mechanical systems to provide a comfortable indoor condition, rather than on proper design that can reduce the need for and reliance on these systems.

The main components of passive solar design are an improved building envelope, thermal mass and adequate shading devices, and favourable building-to-sun orientation and building shape. A holistic approach that aims to optimize all these design components should be applied to achieve an efficient and comfortable indoor environment.

Contrary to common belief, most Canadian cities receive abundant solar radiation. In fact, some of our cities receive the same amount of solar radiation as hot-climate Mediterranean cities such as Athens and Rome. Solar radiation can be exploited in passive design to reduce energy consumption for space heating and daylighting, thus providing building occupants with comfort while reducing energy consumption. To that end, many architectural or engineering decisions must be made in the early stages of design and should be coordinated for a holistic approach.

The main rationale behind passive solar design is to design with the climate, considering thus the climatic variables. These are summarized in five main principles aiming to increase the passive solar design potential of a building (figure 1). An example of the application of passive design in cold climate is presented in Figure 2, 3, 4, depicting a chalet designed in Lanaudiere Northeast of Montreal (Architect: Dominique Laroche, Passive solar consultant: Dr. Caroline Hachem-Vermette).

• Gain: getting enough solar radiation into the building.

• Storage: store solar energy to keep the home warm at night, and from overheating during the day.

• Conservation: keep the heat in during the winter, and heat out during the summer.

• Distribution: distributing solar gain collected in a specific location in the building to other building locations.

• Control of heat gain: use of shade (natural or architectural) to control solar heat gain.

Solar design must take place early in the building design process to have meaningful impact on the design

Heat Gain 
There are three main techniques to capture and use passive solar energy in a house. These techniques, characterized by the way they collect, store, and distribute heat are summarized below.

• Direct gain relies on south-facing windows.

• Indirect gain collects and stores energy in one part of the house and uses natural heat movement to warm the rest of the house. One of the most inventive indirect gain designs is the Trombe wall – a thermal mass placed at a small distance inside south facing glass.

• Isolated gain uses solar collection and thermal storage that are separate from the actual living space. An example of this technique is the use of a solarium or greenhouse attached to the house. Ventilation is essential in this method of solar heat gain to distribute heat.

• Orientation is an important parameter to optimize heat gain. The best orientation is south-facing (for northern climates). Deviation from this orientation towards the east or west leads to increased heating and cooling loads. A successful solar house should have the long facade with good percentage of high performance glazing oriented within 30° east or west of south.

Storage: Thermal Mass
Thermal mass is necessary in the design of passive solar house. It allows for the storage of solar heat gain during the day when solar radiation hits the building mass, and releases passively the heat during the night. Thermal mass can be of advantage in both the summer and winter seasons. The mass acts in the summer in an opposite way to the winter: it is cooled during the night and absorbs heat from the indoor air during the day. The most effective thermal mass in a house is a solar exposed concrete slab. Reflective materials or carpets should not cover this slab to allow an effective absorption and release of passive heat.

Conservation
The building envelope should be designed to reduce heat loss through good insulation, airtightness, elimination of thermal bridging, and high window performance. Increased insulation levels can reduce heating load of a house significantly. Cost-benefit of insulation is, however, characterized by a diminishing returns curve, whereby beyond a certain level of insulation the benefit of energy saving is exceeded 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. Glazing properties is an important factor affecting the performance of the building envelope. Low-e coated glass plays the most significant role in reducing heating load.

Distribution
The house layout should be designed to allow distribution of heat gain from the south side through the rest of the space. Usually "open floor plans" are a good option to facilitate the movement of passive solar heat. Otherwise, kitchens, dining rooms, and sitting areas should be located along the south perimeter to take advantage from direct solar heat gain during the day.

Heat Gain Control
Control of heat gain goes hand in hand with maximizing solar heat gain to avoid overheating. Appropriate shading devices should be designed to assist in reducing solar heat gain when it is not needed. In our climate and geographic location, overhangs are optimal shading devices for south windows. They allow for the low winter solar radiation to penetrate the space while blocking summer radiation.

Building a passive solar house is not a complex procedure. However, every energy conservation system has pros and cons, and therefore a holistic approach needs to be considered. Many of the decisions regarding passive solar design are made in the early stages of building construction and should be coordinated to allow for a significant improvement in the performance of the building while reducing its negative environmental impact.

Dr. Caroline Hachem-Vermette is an assistant professor at the University of Calgary, Faculty of Environmental Design. Her research area includes the investigations of solar potential and energy implications of building shapes and multifunctional neighbourhood patterns, to reach net zero energy status. She is currently an expert member in the International Energy Agency Task 51- Solar energy in urban planning.