Q&A: Optimal R-Values for Building Shell

A.Michael Holtz responds: Designing the most cost-effective building envelope requires analyzing the entire house as an energy system and making tradeoffs between insulation levels, glazing type and area, and mechanical equipment efficiency. The various energy components in a house should be balanced to get the maximum savings for the money invested in energy improvements.

To determine how far to go with energy conservation, you need to evaluate how much energy each measure will save and what it will cost. Only then can you decide whether the additional expense is justified. For example, let’s look at a 1,500-square-foot, single-story, wood-frame ranch house built on a slab foundation. The base design has R-11 walls, R-19 ceilings, no slab insulation, a 0.6 air changes per hour (ach) infiltration rate, a window area equal to 10% of the floor area equally distributed on all walls, double glazing, and a furnace seasonal efficiency (AFUE) of 65%. The "Annual Heating Energy Use" table at right shows the heating consumption for the base design in three climates: cold/cloudy ( Concord, N.H.); cold/ sunny (Denver, Colo.); and warm/ sunny (Atlanta, Ga.).

The first package of envelope improvements (Design 1 in the "Energy Conservation" chart) reduces heating costs by about 30% in all locations (based on our company’s REM/Design energy software). The second package (Design 2) reduces costs further, but by a smaller increment. Simply increasing the furnace AFUE from 65% to 95% (Design 3), on the other hand, reduces heating costs by roughly 35% compared to the base case —yielding greater savings than Design 1 in all three climates. Similar relationships will exist if we analyze two-story houses or houses with basements or crawlspaces.

In general, you should offer reasonably high insulation levels, such as those in Design 2, as a standard package. Even if such levels cannot yield a fast payback based on today’s energy costs, they offer your client insurance against future hikes in energy costs. To help you decide which specific combination of energy improvements to include in a building, a user-friendly computerized design tool is indispensable.

Michael J. Holtz is president of Architectural Energy Corporation (AEC), in Boulder, Colo., and the former head of building systems research at the Solar Energy Research Institute. AEC has developed software called REM/Design to help builders and designers analyze residential energy use.