How the Shape of a House Affects Energy Efficiency in Warm Climates

The shape and exterior structure of a house play major roles in determining its energy efficiency and the comfort of residents. The shape is comprised of the building’s height, width, and depth—also known as the footprint. The exterior structure—also known as the building envelope—includes the walls, roof, windows, doors, and cladding. The footprint and envelope of the home can either enhance efficiency or contribute to more energy consumption. Homes that have simple or uncomplicated shapes are typically more efficient to heat and cool than homes with complex or irregular shapes.

Heat is transferred through the envelope at different rates depending upon the efficiency of the materials. Heat moves only when there is a difference in temperature between two objects that are in contact, and it always moves from the warmer to the cooler object. Heat will continue to “flow” until all touching materials reach the same temperature.

However, we usually want the inside of a home to have a different temperature than the outside. Depending upon the climate, the shape of the home and the materials used in the building envelope are important in reducing the energy required to heat and cool the house. In hot, humid climates, the house shape should be designed to minimize solar heat gain, so as to reduce the energy required to cool the house.

Why are compact homes more energy efficient?

Compact homes, when properly designed, are more energy efficient than irregularly-shaped homes. They gain less heat in the summer and lose less heat in the winter since a smaller surface area has less exposure to the outside elements of sun, rain, and wind. Also note that compact homes use fewer building materials, and simplify the length and complexity of mechanical duct runs and plumbing pipes.

For example, the figures below show 2 designs that have approximately equal floor areas (2062 square feet and 2095 square feet respectively), yet significantly different wall areas (1817 square feet vs. 2639 square feet, assuming exterior walls are 10 feet). There is over 45% more wall area in the irregular shaped home. The compact design has less surface area exposed to the outside, allowing less heat gain in the summer and less heat loss in the winter.

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Two story homes are generally more efficient than one story homes, because of the reduced footprint and roof area, and because the summer sun is higher in the sky and a smaller area of the wall is exposed to the direct sun.

Siting and orientation

Along with building shape and envelope, it is also important to consider the siting and orientation of the house. The relationship of the home to the land, the sun, and the wind will greatly affect the overall efficiency of the home. Careful consideration should be given to the east and west orientations, as these exposures will have more significant solar heat gain in hot climates. Fewer windows should be placed in east- and west-facing walls to reduce solar heat gain. Furthermore, the narrowest elevations of the home should face these orientations to reduce the wall exposure to the summer sun, yet when needed in cooler regions, allows the low angle rays of the winter sun to warm the southern exposure. In cold climates, consider the cold prevailing winds and the orientation as well as landscaping and windbreaks.

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The nature of the building envelope includes proper placement of window and door openings, insulation, wall shading and roof overhangs, thermal mass, and color and reflectivity of wall cladding and roof coverings. Despite the fact that a home has a compact shape, proper attention must also be given to the systems that make up the structure to ensure a well balanced energy efficient home.

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