Home Depot Corporation is carrying out rapid expansion plans - opening 135 new stores within the next three years in the northeast United States alone. In Massachusetts, for example, two stores in Plymouth and Salem are underway, with six more slated. The prototype Home Depot has 116,000 ft2 (10,777 m2) of floor area under roof, with an adjacent garden center measuring 25,000 ft2 (2,323 m2). Used as the basis for each new store design, this prototype is then remodeled as needed to adapt to local site conditions and prevailing architectural styles.
Energy efficiency codes also vary from one state to the next. Meeting these requirements, however, is straightforward using tall slender concrete masonry walls with insulated cores. Exterior walls use concrete block exposed on both the interior and exterior for maximum durability. The 30-foot (9-meter) high walls are constructed of a single wythe of 12-inch (30-cm) concrete masonry unit, with vertical reinforcement incorporated to meet structural requirements.
INSULATING SINGLE WYTHE WALLS
Concrete masonry walls can be insulated on the interior, exterior, or integrally - i.e., with insulation placed in the hollow concrete masonry cores. Integral insulation is ideal when exposed block is desired, as it is for Home Depot. Insulation types for concrete masonry walls can be granular fills, foams or molded polystyrene inserts.
Granular fills, such as perlite, vermiculate or expanded polystyrene (EPS) beads, are poured into masonry cores as the wall is laid up. The free-flowing material easily fills all the empty spaces. Although a small amount of settlement may occur after installation, it typically has a relatively insignificant effect on overall thermal performance. In addition to increasing the wall's R-value, perlite and vermiculite also increase the fire resistance rating of concrete masonry walls and decrease sound transmission.
Because of their free-flowing nature, however, granular fills tend to flow out of any holes in the wall system. Therefore, mortar joints should be examined for "bee-holes" or other gaps, which should subsequently be filled. In addition, any wall penetrations or anchors placed after the insulation has been installed must be placed with care to prevent insulation loss. Weep holes should be installed with wicks or non-corrosive screens on the interior to contain the loose-fill insulation while still allowing water drainage.
Foamed-in-place insulation is installed after the wall is built to full height. The installer pumps the foam into masonry cores from the top of the wall (on new construction) or through small holes drilled in the face of the wall (for existing masonry). These foams then expand to completely fill the core spaces. Some foamed-in-place insulations have been tested to certify acoustic and fire ratings as well, hence improving these aspects of the masonry performance. Because foams may be sensitive to temperature, mixing conditions or other factors, manufacturers' instructions should be carefully followed to avoid excessive shrinkage due to improper mixing or placing of the foam.
Rigid expanded polystyrene inserts may also be used, either in the cores of conventional block or in specially designed units. Various inserts are available to provide a range of R-values and accommodate various construction conditions. Most rigid inserts are designed to accommodate reinforcing steel and grout, to provide both thermal protection and structural performance in the same masonry core.
Specially designed concrete masonry units often have cross webs that are notched to accommodate rigid insulation insets while reducing heat loss through the webs. To further reduce this heat loss, some manufacturers have developed concrete masonry units with two cross webs rather than the conventional three.
BUILDING A HOME DEPOT WALL
Typical Home Depot walls are about 30 feet (9 meters) high, constructed with a single wythe of 12-inch (30-cm) concrete masonry. Walls are built to full height, insulated with foamed-in-place (or sometimes-loose fill EPS bead insulation), then topped with a bond beam course.
In northeast stores, Home Depot uses vertical reinforcement at 32 inches (81 cm) on center to meet structural requirements. Cores to be grouted are isolated from cores to be insulated by mortaring the cross webs to confine the grout. After the walls are built to height, foamed-in-place insulation is pumped into all ungrouted cores. The insulation consists of a resin and expansion agent, which are mixed together in a prescribed ratio, then propelled into the masonry cores using compressed air. The insulation expands and sets up within one to two minutes, effectively filling all void spaces in the wall.
After the walls are insulated, Home Depot specifications call for infrared (IR) termography testing to insure there are no gaps in the insulation. IR testing indicates thermal performance using special cameras and film that record temperatures. In new construction, entire buildings can be quickly surveyed to determine whether there are any major air leaks or any missing or misplaced insulation. Missing, improperly installed or damaged insulation shows up on a thermogram as a light (warm) patch with distinct edges, which generally outline the uninstalled area. If any such areas are detected, small holes can be drilled into the wall face, and additional foamed-in-place insulation added at the affected locations.
A Home Depot building is designed to meet a variety of goals, including aesthetics, economy, structural strength, low maintenance and energy efficiency. Concrete masonry walls help fulfill these needs with a versatility that allows the prototype store to meet building code and architectural requirements in locations nationwide.
ENERGY REQUIREMENTS FOR LARGE RETAIL STRUCTURES
Although Home Depot uses a basic prototype design for new stores in all locations, energy code requirements can vary even within the same state, meaning that the same building design could have different insulation requirements depending on its location. Each Home Depot store, then, must be evaluated to ensure it meets the local energy requirements.
Energy codes generally prescribe a minimum wall R-value. R-value indicates resistance to steady-state heat flow - walls with higher R-values are typically better insulators. Minimum R-value requirements for a typical Home Depot range between 1 and 4 hr/sq.ft./0F/BTU, based on Energy Efficient Design of New Buildings Except low-rise Residential Buildings (ref - 1). These requirements are easily achieved using integrally insulated single wythe concrete masonry.
REFERENCES
1. Energy Efficient Design of New Buildings New Low-Rise Residential Buildings, ASHRAE/IES 90.1 - 1989 American Society of Heating Refrigerating, and Air Conditioning Engineers, 1989.2. R-Values for Single Wythe Concrete Masonry Walls, TEK 6-2A. National Concrete Masonry Association, 1996
Maribeth Bradfield, P.E., is a consulting engineer practicing in Arlington, Virginia.
To learn more about concrete masonry walls go to the National Concrete Masonry Association Website.
