House Design Geometry

How to improve the energy efficiency of your home's overall shape, size, and design

R-2000 is a performance-based program where each registered house is challenged to meet an energy budget. Each home is allowed to use a maximum amount of energy based on its interior volume of heated air and the home's geographic location.

Homes with a lower surface area to volume ratio are inherently more energy efficient and have an easier time reaching their energy budget with only modest upgrades. Home designs which incorporate a higher surface area to volume ratio use more energy and require a greater degree of envelope and mechanical upgrades in order to meet their R-2000 energy budget.

Let's consider a typical 1,600 sq. ft. three bedroom house in four different design configurations:

Modest bungalow design:

Most efficient
Single story
Minimum number of corners
No cantilevers
The volume of heated air in this design is 27,200 cu. ft.
Total surface area of all the walls, ceiling and basement floor is 5,990
Volume to surface area is 27,200 - 5,990
For every one cubic foot of heated air, 0.22 square feet of exterior surface area exist through which to lose heat - 1:0.22.

With modest improvements to the building envelope and mechanical systems, this design easily meets the R-2000 energy budget.

Complicated bungalow design:

Less efficient than modest bungalow design. Still single story but increased number of corners, cantilever floors, vaulted ceiling, and raised basement wall all increase the exposed surface area.
Volume of heated air goes up by 1200 to 28,400 cu. ft.
Surface area increases by 470 to 6460 sq. ft.
Volume to surface area is 28,400 - 6,460
For every one cubic foot of heated air, 0.23 square feet of exterior surface area exist -1:0.23.

This design would be more difficult to meet the R-2000 energy budget. It will require major improvements to the building envelope and mechanical systems.

Modest two-story design:

Even though at first glance this modest design appears to have a fairly energy efficient basic shape, two story designs inherently have larger surface area to volume ratios. When the 1,600 sq. ft. of living area is split between two stories at 800 sq. ft. per floor, the volume of heated air in the basement is reduced by half. Two story designs typically retain a significant surface area but reduce the heated volume of air by a large amount.

Volume of heated air in this design is 20,800 cu. ft.
Surface area is 4,720 sq. ft.
Volume to surface area is 20,800 to 4,720
For every one cubic foot of heated air, 0.23 square feet
of exterior surface area exist - 1:0.23.

This design is similar in efficiency to the 1,600 sq. ft. of living space laid out as the complicated bungalow design.

Again, this home requires major improvement to the building envelope and mechanical systems to balance off the less efficient design.

Complicated two story design:

Less efficient than modest two story design increased numbers of corners, cantilever/exposed floors, and raised basement wall all increase the exposed surface area.

When compared to the basic two story, the volume of heated air in this layout actually goes down slightly to 19,825 cu. ft. but the surface area increases to 5,140 sq. ft.
Volume to surface area is 19,825 to 5,140
For every one cubic foot of heated air, 0.26 square feet of exterior surface area exist - 1:0.26.

This is the most difficult design to meet the R-2000 energy budget. It will require maximum improvements to all areas of the building envelope and mechanical systems to balance off the inefficient design.

While we don't advocate that all homes be designed as plain boxes, we do need to consider the overall shape and the details of the design in the context of the architectural advantage that design detail provides.

Is that extra 45-degree corner jut out really necessary? Not only does it add to the cost of construction, the additional framing inherent in making three corners and increased wall area will increase heating costs.
Is the energy penalty imposed by the china cabinet cantilever worth it? Or is there a better room layout which can accommodate the china cabinet in another location?
Do you need to cantilever the floor over the side of the basement wall? Can the basement be built a little larger or can those extra square feet eliminated?
Can the master bedroom be relocated from over the garage where it has its floor exposed to another location within the house to decrease exposed surface area?
Would a straight 9' ceiling height throughout the house provide the same sense of interior spaciousness as several sloping and vaulted ceiling areas at less construction and operating cost?

Being aware of these design implications and considering them at the preliminary design stage is the first step to achieving maximum R-2000 house performance.