Article No: 117

2006-05-02 09:33:16
Sweet Home Alabama
By: Concrete Homes


Photography courtesy of Concrete Masonry Designs

After building other people's dream homes for more than 20 years, Robert Pollock is finally taking his turn. The building contractor and his wife are finishing construction on their new house outside of Birmingham, Ala. Pollock's design is driven by his holistic approach, born out of his practical knowledge that the various elements of a building, though they may seem unrelated, are in reality interdependent.

What is his material of choice? Concrete masonry. A masonry cavity wall with rigid insulation, high-performance windows and careful attention to controlling both moisture and air infiltration combine to provide a comfortable, economic and beautiful home. Pollock admires concrete masonry's solidity.

"In the past, after framing a two-story wood-framed house, while installing plywood on the roof trusses, I've been able to shift my weight from side to side and cause the whole house to move. You can't do that with this rigid house. It's solid as a rock." He finds building with concrete masonry to be flexible and economic, with unbeatable durability. In his words, "This house will last about eight times longer than a 30-year mortgage, and the wall system is basically inert, being of no interest to termites and being less hospitable to fungus and mold than the materials of a wood framed house."

The envelope house
Pollock has relied heavily on his experience and knowledge to design a home with minimum reliance on purchased energy. Pollock's approach results in what he calls an "envelope house," meaning that the building envelope, or exterior skin, is designed and constructed to control heat, air and moisture. The insulated masonry cavity wall combines concrete masonry's thermal mass for absorbing energy and moderating indoor temperature swings with the insulation's high R-value. Double-paned, low emissive windows help control heat flow and limit solar heat gains.

"We have a concrete masonry wall with insulation on the exterior, so the masonry assumes the temperature of the inside," Pollock said. "We've got all that thermal mass maintaining a comfortable temperature, and it couldn't be simpler — there are no moving parts." Because the energy load in Birmingham is primarily cooling, the slab-on-grade floor is not insulated, allowing the earth below to act as a heat sink, helping to keep the interior cool. Air infiltration is carefully controlled to moderate two phenomena — heat loss and moisture movement through the walls.

"My approach is to make the house as tight as possible, then use equipment and natural ventilation to control the temperature, humidity and air quality. The whole house works better if you can control the air flows."

Because a large portion of moisture in a building typically comes in with unwanted air flows, such as around windows and other wall penetrations, controlling air infiltration is paramount to keeping a dry building envelope. Pollock noted "almost any damage you see in buildings — poor indoor air quality, structural problems, energy loss, stains, mold — can be traced back to moisture in some form. Even termites love water in a wall. With wood construction, you're providing a buffet for those guys."

Builders who work in different geographic areas are accustomed to using the same basic strategies to protect walls from liquid water sources such as rain and groundwater. Wetting because of air and water vapor movement, however, varies with climate, interior conditions and often the time of year as well. For this reason, recommendations for using elements such as air and vapor retarders, ventilation and dehumidification vary with climate.

Pollock, who moved to Birmingham from Canada, understands these subtleties, pointed out that the vapor barrier always goes on the warm side, which is the interior in Canada, but the exterior in Birmingham.

Because the house is designed to have low air leakage, hot air can build up inside. To improve comfort, Pollock installed skylights that automatically open to exhaust hot air when the indoor air reaches 80 degrees. To replace this discharged air, the house has fresh air vents that passively draw in outdoor air as needed. In this way, passive ventilation is used to improve comfort and further reduce reliance on air conditioning.

In the winter months, heating will be provided by an airtight masonry fireplace, which is supplied with outdoor combustion air. Conventional fireplaces have extremely high air flows through the fireplace and up the chimney, while the fire is burning and especially when the fire is dying and the embers are cooling. Even with the typical glass fireplace doors closed, traditional fireplaces typically waste more energy than they supply in the form of heat.

Pollock's airtight masonry fireplace, however, is designed to reverse this trend. The fireplace operation is similar to a wood stove, providing excellent heat with little to no loss of conditioned air from the home. An added benefit is the masonry construction.

"You can't stand near a steel wood stove with a hot fire — it's too hot — so you tend to damp the fire down, meaning that it burns slowly and only at about one-quarter of its efficiency. With a masonry fireplace, you can burn the hottest fire you possibly can, and it doesn't overheat."

A hotter fire also means more complete combustion of the wood, less creosote and lower emissions. To help verify some of Pollock's claims on low energy use, the Energy-10 building design software, distributed by the Sustainable Buildings Industry Council, was used to evaluate Pollock's design. The program accounts for the energy efficiency of concrete masonry's thermal mass and the home's low air infiltration rate, and compares the resulting energy performance to the same home built in Birmingham using typical wood-stud wall construction.

The results for a year showed Pollock's house using almost 40 percent less energy for heating and cooling than the comparable wood-frame house. The accompanying lifecycle cost analysis during a typical 30-year mortgage life projects a savings for the concrete masonry homeowner of more than $600 per year (note that this does not include the additional economy of burning wood from the site in the airtight masonry fireplace). In fact, Pollock said he expects even better performance than indicated by the program. He's gotten some practical evidence of this during the summer, when the exterior walls of the home were still under construction, but an interior concrete masonry tornado shelter was in place.

"Because the house was not yet enclosed, we used the shelter to lock up the tools. During the extreme heat of August, that room stayed consistently between 68 and 70 degrees. I expect little to no air conditioning load for the finished house because of all the thermal mass."

Safe shelter from tornados
Although Alabama's mild climate is a boon for gardening and golf, it is also in prime tornado country. Birmingham falls under the Federal Emergency Management Agency's (FEMA) highest wind zone for tornadoes, Zone 4, characterized by design wind speeds (3-second gusts) up to 250 mph and one of the second highest areas of tornado activity, with 11 to 15 tornados recorded for each 1,000 square miles.

To encourage construction of safe havens, the State of Alabama provides a $2,500 credit to homeowners who incorporate a tornado shelter in their home. Pollock's shelter, constructed of 8-inch solid grouted and fully reinforced concrete masonry, meets the stringent structural requirements of FEMA publication 320, "Taking Shelter From the Storm, Building a Safe Room Inside Your House."

The publication was developed to address the lack of safety from tornados and other high wind events prevalent in most new construction. According to the report, "extensive testing by Texas Tech University and other wind engineering research facilities has shown that walls, ceilings and doors commonly used in house construction cannot withstand the impact of missiles carried by extreme winds."

Not only does Pollock's shelter meet the FEMA requirements, recent research shows that it may in fact be over-designed. Six- and 8-inch fully grouted but lightly reinforced panels (as opposed to reinforcement in every masonry cell as dictated by the current FEMA recommendations) recently tested by Texas Tech University were found to exceed the tornado shelter wall requirements.

Despite all the "extras," Pollock found that his high performance house was also economical to build, with an overall cost of about $68 a square foot, which is comparable to wood-frame construction in his area. "The whole secret to this house is concrete," he said. "I'm a very practical person — it's not just about making it energy efficient, it's about making the house comfortable and affordable."

Taking advantage of the energy efficiency, moisture resistance, insect resistance, economy and structural integrity of concrete masonry construction, the Pollocks' new home is a prime example of design excellence and high quality.

"My feeling is that this house offers much more value, above and beyond the energy savings and low maintenance factors," Pollock said.

However you look at it: comfort, low energy use, durability, low maintenance, structural integrity and aesthetics, concrete masonry is contributing to outstanding home design.

This article is reprinted with permission from Concrete Masonry Designs, courtesy of the National Concrete Masonry Association. Photography courtesy of Concrete Masonry Designs.