Masonry Magazine June 1987 Page. 29
NCMA-TEK
An Information series from National Concrete Masonry Association
161
Energy Efficient Concrete
Masonry Basement Design
Introduction
This TEK provides a simple, yet reasonably accurate tool for use by builders, designers, engineers and architects to predict heat losses and the effectiveness of insulation in the design of energy efficient basements.
While much is already known regarding how basement heat losses may be estimated and methods presently available to reduce them at a reasonable cost, research to develop more reliable systems for design and analysis of foundation heat transfer is continuing. The need for this type of information is being stimulated by proliferating energy standards and today's codes, as well as those presently in the stage of development.
Background
Concrete masonry basements are being installed under today's energy efficient homes in increasing numbers. According to recent studies) basements are provided under 83% of one-story homes and 78% of two-story homes in the northern region of the United States. The percentage of basements drops from between 35 to 26% in the central region and considerably less in the South, where slab-on-grade construction predominates. These studies also indicate that approximately 35% of all new homes have basements. According to the Underground Space Center at the University of Minnesota, roughly 30% of all residential basements are heated. This represents a tremendous potential for energy savings since the same researchers have also concluded that only about 5% of all basements are insulated.
Commercial building owners are also becoming aware that a dry, insulated basement or lower level can be used for purposes other than mechanical rooms, parking garages or storage. When properly insulated, waterproofed and provided with lighting, ventilation and egress, below grade spaces offer potential for retail, recreation areas, office space, class rooms and other non-residential uses at construction costs significantly below those required for enclosing above grade spaces.
The issue of below grade spaces being lower in cost is by no means insignificant. Calculations using recognized construction cost data show that the price of above grade residential space is in the range of $47 to $62 per square foot (1986 prices). Conversely, the cost of below grade spaces constructed of concrete masonry drops to just between $18 to $32 per square foot depending upon the level of insulation and whether the use of the space requires special added features.
In today's market a basement can be transformed into an affordable living area through proper design at a more cost effective basis than adding on above grade space. Proper design includes such considerations as effective waterproofing, insulation, adequate lighting levels, and proper heat and ventilation.
Foundation Energy Research
In a study for the U.S. Department of Energy, researchers at the University of Minnesota conducted detailed computer simulations of basement and slab-on-grade thermal performance in several U.S. climate zones,
Results for Minneapolis showed energy savings realized by adding full R-10 exterior insulation extending down to the footer were 18 to 27 million BTU per heating season. Little cooling energy was saved, but little is used in Minnesota. Energy savings for slab-on-grade foundations were 5 to 11 times less for the same size house.
Kansas City climate studies indicated basement insulation resulted in a savings of from 10 to 16 million BTU per heating season and 0.4 to 2 million BTU per cooling season, with R-10 insulation extending down to the footer. In this milder climate, returns of energy savings are slightly over half those for Minneapolis, suggesting a lower insulation level might be more cost effective.
For the climate surrounding Tucson, Arizona, the R-10 options for slabs and basements were not shown to save very much energy. Only 3.3 to 4.5 million BTU were saved in the heating season and up to 4.5 million BTU were saved on cooling using basement insulation. Few basements are built in Tucson, where slab-on-grade construction prevails.
The analysis showed that 10 to 35% energy savings for insulated basements is achievable. As the energy efficiency of above grade construction has improved, the overall percentage effect of uninsulated, heated basements on total residential heating and cooling costs has increased.
Researchers surveyed seven major basement heat loss calculation models in an ASHRAE technical paper). The methods produced a wide range of results when predicting basement energy use of a prototype house in five U.S. cities. Heat losses were calculated for masonry walls at three insulation levels: R-1.5 (bare), R-6.5 and R-11.5, and at R-5 and R-10 added. The researchers also performed sensitivity checks on parameters such as soil conductance and basement depth below grade to check the comparability of the models.
Concrete Units
TEK 161 © 1987 National Concrete Masonry Association