FOUNDATIONS—THE BASE OF SUSTAINABLE RESIDENTIAL BUILDING DESIGN

INTRODUCTION

The foundation is perhaps a building’s most challenging envelope component from energy transport and hygrothermal perspectives. As such, the foundation literally provides the energy and materials basis on which the sustainability of the entire structure depends from a number of perspectives. The foundation can account for as much as 40% of the building envelope conduction loss and, more importantly, in unoccupied conditions, almost all of the latent load (or energy required for dehumidification). From a materials perspective, the foundation has to provide a durable interface between the surrounding ground and the building interior in the presence of bulk water, soil gasses (such as radon, water vapor, and even, on occasion, hydrogen sulphide), frost, biotic activity, and pest infestation while simultaneously being the building’s structural basis. From an energy perspective, the foundation must insulate the interior from the surrounding soil and ambient environment, and provide an interior surface temperature producing comfortable conditions for the occupants while minimizing both sensible and latent thermal loads to the greatest possible extent.

Achieving all these requirements in a cost-competitive residential housing market is difficult. Certainly, national residential building codes such as the International Residential Code (IRC) and theInternational Energy Conservation Code (IECC) do provide a least common denominator basis for building adequate foundations, but they do not go nearly far enough from a sustainability perspective, particularly in light of twenty-first century energy prerogatives. It is reasonable to state that these codes are a generation or two behind what would be an appropriate building regulatory policy for the current times, namely, that residential buildings in particular should be built to a zero net energy standard (that is, no net energy importation across the property line for a given calendar year). The technology to accomplish this standard does exist and is being demonstrated by agencies such as the United States Department of Energy (DOE).

It is in this context that the State of Minnesota recently undertook a major revision of the foundation rules in its state energy code. The revision was prompted by widespread failures of foundation systems across the state, largely as a consequence of biotic activity caused by condensation within foundation wall assemblies. An example of the condensation that can be produced by wall assemblies compliant with Minnesota Rules chapter 7672.0600 is shown in Figure 1.

The energy code foundation rules revision was guided by the principle that the code rules must have an experimentally validated basis in the physics of building foundation heat and mass transport. The resultant rule as currently expressed in the public review draft (at the time of writing) has made significant progress in establishing a well-founded basis for constructing durable foundation systems that can fulfill the requirements of material and energy sustainability. Because the rule is centered on a set of generalized building envelope hygrothermal performance criteria, the rule is not limited to foundations in a cold climate, but is applicable to the entire envelope in all terrestrial climates.