What's Your Ratio? Your home's surface to volume ratio counts for more than just math!

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Last week we took a look at the how the number of surfaces (different planes, an abundance of corners, etc.) can impact a home's performance. In addition to the number of surfaces, the surface area-to-volume ratio is also an important consideration when building a green home. The more surface area a home has (the total area of the exterior walls, roof, and floors), the more opportunity there is for heat to escape or enter. Likewise, the higher the ratio, the greater the risk of loss.

The geometric shape that has the minimum surface area to volume is a sphere, but that’s hardly practical for a house. A cube is the most reasonable, compact shape for a home to minimize heat transfer. Of course, other factors come into play, such as optimizing glass area on the south-facing wall and ensuring that sufficient light can penetrate to the interior spaces, often making a rectangular shape more desirable than a cube. Plus, sloped roofs make more sense in regions that receive a lot of snow, such as New England.

The key is to strike the right balance between all of these factors to produce a green home that’s right for you and your building site. An architect with a background in green building can use sophisticated modeling tools to calculate how adjusting various factors, including surface area and volume, will impact the performance of the building.

To illustrate why this metric is so important, consider two shapes that have the same volume but a very dfferent surface area:

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Both a 10’x10’x10’ cube and a 10’x50’x2’ rectangle have a volume of 1,000 cubic feet, but the surface area is quite different. The cube’s surface area is 600 square feet and the rectangle’s is 1,240 square feet. That’s more than twice the opportunity for heat loss on the rectangular building. The rectangle in this example also requires more building materials for the walls, roof, slab, and flooring, which means a higher cost for the building.