Many architects, designers and builders have gotten in the habit of dealing with heat loss once the design is complete. Better windows here, more insulation there, and presto, you have cut the house's heat loss (or heat gain). That approach works, but there's another side to heat transfer. It's surface area.
By trimming the surface area of the building's thermal envelope, you can dramatically reduce heat transfer. Reducing surface area often cuts construction cost, too.
Surface area is closely related to the geometry or form of the building. The shape and complexity of a house design affects the surface area and the overall energy used for heating and cooling.
These three designs illustrate this idea. Each design contains 2,000 sq. ft. of conditioned floor area. The insulating value of all building components is the same. Windows are evenly distributed among the four elevations.
The key difference between the designs is the surface area of the building's thermal envelope. The compact two-story design has 4,210 sq. ft. of surface area--30 percent less than the complex, one-story design and 26 percent less than the ranch. More surface area means more heat loss. The complex design requires up to 24 percent more energy for space heating.
The data for these comparisons was generated with the Wattsun 5.4 computer program published by the Washington State Energy Office. The analysis uses weather data for Eugene, Oregon (4500 heating degree days).
Component Tradeoffs
Ceilings generally have more insulation than walls, and the same is often true of floors. In these examples, ceilings are R-38, floors are R-30 and walls are R-19. A square foot of wall allows twice as much heat loss as a square foot of ceiling. Trading one square foot of highly insulated ceiling for one square foot of wall with less insulation would be a mistake. However, the two-story design adds about 500 sq. ft. more wall area, while it subtracts 2,000 sq. ft. in the floor and ceiling. That's a net reduction of 1,500 sq. ft. The large difference in surface area more than makes up for the difference in R-value.
As these examples show, building form has a major impact on heat loss. That doesn't mean every house should be a two-story rectangle. But it does suggest that simplicity can save energy (as well as construction costs). One good tool for making design decisions is a computer program that estimates energy use. Several good energy analysis programs are available.
These examples have been kept simple to illustrate a point. Other factors, such as solar heat gain, also influence energy use. Extending the east-west axis could capture additional solar heat and more than make up for a the extra surface area.
Design Interest without Complexity
Designs do not need to be highly complex to be interesting or marketable. You could consult an architect or designer for ideas on getting a good design in a simple shape. In some cases, architectural interest can be applied to the exterior by adding covered porches, bay windows, roof lines and decks.
There's almost always a way to pack more insulating value into a house once it's been designed. At that point, cost can become a major obstacle. On the other hand, decisions that affect building form must be made early in the design process. Fortunately, simplifying building form doesn't have to cost anything, it often saves money.
