Concept

Minimize surface area for the volume you need. Elongate the East-West axis to allow for more South facing windows. We use volume, we lose heat through surface area – read on, I’ll explain.

Heat travels toward cold – a version of the second law of thermodynamics. The amount of heat (BTUs) traveling (lost) is directly proportional to the area of the surface containing the heat (square feet) and the degree to which the material conducts heat (U-value, which is the reciprocal of the R-value). OK, enough physics. The bottom line is that the smaller the thermal shell or envelop per unit volume of the house and the better insulated it is, the less heat is required.

The greatest volume for surface area is provided by a sphere, but while some have built such houses, utilizing their shape is problematic and construction is not standard or typical.

An elongated rectangle, the longer it is elongated, the less volume per surface area is available. The greater the surface area, the more energy it takes to heat the volume. So elongation to an extreme is not good.

A cube offers a good volume to surface ratio, but it conflicts with the concept that it is good to elongate East-West axis to allow more South facing windows and to allow greater penetration of sun light on winter days. If you had minimal or no access to the sun, a cube would be best.

Finally, bump-outs or “additions” are high in additional surface area (and cost) relative to volume. This is mostly a variation of elongation, but with extra corners that are thermal weak points.

Keep both concepts (high volume to surface area is good & the need to elongate for southern exposure) in mind when working back and forth between the overall shape and planning spaces (next section)..

Application

Originally, I wanted a cape with a 24’x36’ main section, a single floor 16’x24’ master bedroom suite attached to the west end, and children bedrooms in the cape second floor. This gave me the spaces I wanted and a total area of less than 2000 square feet (1853 SF). But…. it is a very elongated rectangle, with extra corners and a lot of “extra” surface area to insulate and lose energy through. In the end, we made compromises to stick with the Design Principles, as much as possible.

What we settled on (after much debate and deliberation) was a house 42’ long (East-West) x31’ wide (North-South) with fully insulated basement and roof, with a nearly full shed dormer to the South – you know, a cape, without the thermal complexity of two or three cute little dormers. See the example in the above popup. This is close to what we built. It has a very efficient surface to volume ratio – better than a cube due to the added volume and little added surface due to the roof line.

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