Article No: 67

2006-05-01 12:55:16
When things get hot in Texas, a masonry plantation home is a great way to be cool, stay cool
By: Concrete Homes

While the image of a plantation home may be the archetype of Southern living, it is more likely to evoke thoughts of times past than of modern residential technology. But for Bob Whisnant of Columbus, Texas, and his wife Julie, their longtime wish to own a plantation-style home included a requirement that it be a modern home in all respects.

While that means including all the amenities and design features of a modern-day home, the Whisnants went further to build energy savings and durability into their plans with the transfer of a traditional, wood-frame structure into a home built with concrete masonry. According to Whisnant, "A concrete masonry wall system will meet our energy efficiency objectives and also provide a home safe from tornadoes and other problems homeowners face in this area."

Starting with a floor plan that captured the style of the South and comprising just over 3600 square feet, Dick Weiler of R. D. Weiler and Associates began writing the specifications for constructing the Whisnant home from concrete masonry units. General Contractor for the project was Saddlewood Homes, and the mason contractor was Easthaven Masonry.

Whisnant knew that buildings constructed with insulated wood or steel frame walls will consume larger quantities of energy for both heating and cooling than a concrete masonry structure with less insulation.
This is due to the thermal mass effect created by the heat capacity of concrete masonry, which reduces the (inward) heat flow during periods of hot weather and thereby results in a smaller cooling load per unit time distributed over a longer period.

The effect of concrete masonry walls on residential load reduction in cooling dominated climates is especially significant. Whisnant and Weiler reviewed analyses illustrating considerable residential cooling load reductions with the use of concrete.

These benefits were so significant that it was determined that greatly reduced insulation levels are capable of providing equal or considerably lesser energy expenses. The State of California took this into consideration when implementing its new State Energy Code and Whisnant knew these data favorably transferred to application in Texas.

Another implication is that a well-insulated concrete masonry home may not require an expensive air conditioning system in a climate where average light-frame houses would be uncomfortable without one. The cost savings realized by the elimination (or size reduction) of the air conditioner may best be used elsewhere in the structure, or may result in more affordable housing.

According to Archie Ameigh of Easthaven Masonry, "The walls in Whisnant's home were solid grouted, which is used frequently in this area of Texas because concrete masonry thermal mass reduces the transfer of unwanted heat through building walls."

Concrete masonry also acts as a heat sink for temporary storage of unwanted gains indoors. Solid or solidly grouted walls have high heat capacities per square foot of surface area and
are preferred for exterior walls.

The choice of interior insulation consists of foil-backed material installed between studs, as shown in Figure 1. The insulation selected in the Whisnant project was a fibrous batt product, although rigid board (polystyrene or polyisocyanurate), cellular glass, or fibrous blown-in insulation would also have been suitable. As an alternative to studs, systems have been developed to attach insulation using specially designed clips and channels. The interior wall surface was finished with gypsum wallboard.

As an alternative, studs may be held away from the face of the concrete masonry with spacers. In this case, the insulation should be stapled, or otherwise attached to the studs to prevent the insulation from becoming dislodged. The space created by the spacers provides moisture protection, as well as a convenient and economical location for additional insulation, wiring or pipes.

Because the studs penetrate the insulation, the properties of the stud should be considered in analyzing the wall's thermal performance. Metal stud penetrations through insulation significantly affect the thermal resistance by conducting heat from one side of the insulation to the other. Wood stud penetrations also affect the thermal performance.

Typically, non-structural wood or metal studs are installed on the interior of the concrete masonry and attached at the floor and ceiling. The size of the stud is determined by the type of insulation chosen and the R-value required. In the Whisnant home an R-6 value was installed.

Though not as conductive as metal, the thermal resistance of the wood and the cross-sectional area of the stud penetration should be taken into account. In areas such as Texas which require building cooling, exterior walls should be shaded from unwanted sun-gain whenever possible. Weather data shows that in cooling dominated regions, any wall can be subjected to significant amounts of solar energy during various times of the day. In the northern hemisphere, the west-facing wall receives the greatest amount of sunlight coupled with higher afternoon temperatures. This can make the west wall a large contributor to cooling loads if left unmanaged.

According to Weiler, window openings should be minimized in the west or east facing sides except for ventilation; and south-facing walls should be provided with adequate overhangs.

The plans for the home selected were adapted by the Weiler Architects to accommodate these requirements, particularly the large front porch typical of Plantation home. North walls can become good sources of ventilation air if they too are properly shaded. Exterior walls should be light in color to reflect indirect sunlight; and the use of light colored ground cover materials, such as concrete sidewalks, in close proximity to the walls should be avoided. Dark colored ground cover, vegetation, or lawns that extend up to or near the walls are recommended to minimize the reflectance of solar energy to the walls.

The Whisnants have been in the home for just one year, but are very happy, and hope their experience can be a valuable lesson for other home buyers in the Eagle Lake community.

"We are fully satisfied with choosing concrete masonry construction in our dream home," Whisnant said. "Electrical costs have averaged $280 per month while comparable non-masonry homes in the area are approximately $100-150 per month higher."

This article is courtesy of the National Masonry Association and is reprinted with permission from the Concrete Masonry Design magazine.