By: Concrete Homes
Back in 1995, Mark D. Olson simply wanted a nice house for his family when he built his first concrete masonry home. Six years and eight homes later, Olson has the beginnings of a solid residential construction business that is making an impact in Minnesota's Twin Cities market. His most current offering was a featured entry in the local 2001 Parade of Homes Fall Showcase where it was a Finalist in the Best of Show Awards Competition.
Olson is a mason contractor with more than 20 years of commercial and residential experience in the Minneapolis and St. Paul areas. But, four years ago, he decided to embark along a new path in the residential marketplace. He knew the value of concrete masonry construction and knew it functioned well in many commercial projects, but was unclear as to whether or not a homeowner would find that same value and excellent performance. And while concrete masonry residential construction was accepted and popular in the southern United States, wood-frame construction dominated the Twin Cities market.
Just like any other builder, Olson promotes these homes by pointing out the hardwood floors, the fireplace or the great room, perfect for entertaining family and friends. But when other builders run out of features to promote, Olson has something more to say. These homes aren't simply a pretty face, but rather represent "a new concrete residential building system, in which long-lasting concrete masonry components carry the load in solid structural walls," Olson says. And just in case the home buyer starts thinking of something cold and institutional, they are reminded that these concrete masonry walls "are scaled for residential use and come in warm aggregate colors and textures," Olson adds. And finally, if any resistance remains to buying one of these homes, Olson simply points out that a concrete masonry home is "less expensive than a wood-frame home with a brick veneer and about the same as one with a stucco exterior."
Olson began with a 2,400-square-foot (223-square-meter) rambler home that included a full, finished basement with a walkout feature. This is a very popular model in the Midwest, and in Minnesota it is well established in the local market. Starting with plans based upon a wood-frame structure, Olson began to modify these for an all concrete masonry structure and could have continued along these lines using the empirical design provisions of the building code. Instead he chose to contract with a local engineering firm, LHB Inc., to design the structure and provide stamped drawings for local building officials to review. This proved to be a good investment as the plans were used repeatedly and the engineer's stamp helped to reassure skeptical building officials unfamiliar with residential concrete block construction. This practice continues, and Olson has used these designs, or slight modifications thereof, in all of his homes.
Where the residential building code allows for empirical design, the code includes prescriptive requirements to incorporate 8-inch concrete masonry in all bearing walls. The engineered review of the home's structure developed a design based upon the use of 12-inch concrete masonry units for the foundation of the home and 6-inch hollow core concrete masonry in all of the walls. The 12-inch foundation units are reinforced with No. 5 vertical bars at 48 to 72 inches, depending on soil conditions, while the 6-inch walls use No. 5 bars placed vertically at 36 to 48 inches as required by the design.
Olson has taken advantage of this decision and makes a point of it in promotions aimed at the home buyer. His homes are a concrete masonry residential building system in which long-lasting concrete masonry components carry the load in solid structural walls. And unlike brick, these walls are loadbearing, offering a structural capacity with significant benefits.
In the design engineered for Olson's initial project, the required lateral support was engineered into the wall system and, therefore, not subject to the prescriptive requirements of the building code. The 6 inch thick walls were designed with lateral support at the footings, the second level floor assembly and the roof system. This strategy provided an unsupported height of 8 feet, 2 inches at the lower level and 9 feet for the upper level. While Olson had covered the structural issues, there remained a number of design decisions that dealt with the functionality of the home and which the home buyer would consider in the decision-making process. Olson gave considerable thought to each of these as the process of building concrete masonry homes developed.
Promoting a home for its solid, long-lasting structural walls meant that the design had to ensure that the walls would not crack due to tensile stresses associated with temperature and moisture changes or differential settlement of the foundation soils. Such cracking is not as likely in reinforced concrete masonry because the steel resists the tensile stress and the design incorporates a bond beam every 8 feet vertically that contributes to the resistance of these stresses. Six years after the first homes were built, they remain as durable as ever. To mitigate the transmission of wind-driven rain and snow through the wall, Olson chose a siloxane treatment, which is sprayed onto the wall after construction and is a very effective treatment that has served Olson's customers well.
With loadbearing architectural concrete masonry walls on the exterior, Olson's homes required a system that provided an effective interior insulation. The thermal mass of the concrete masonry wall assists in buffering the impact of temperature changes, but for the 8,200 heating degree days in Minnesota, an appropriate insulation system is required. The insulation in the home is a 2.5-inch polyisocyanurate panel that is glued across the entire concrete masonry wall. Olson promotes this wall system to the home buyer as having an R-value of 22.3, but points out that "unlike conventional wood-frame homes, which are insulated between the wall studs, this system creates a complete thermal break between the exterior and interior of the home. Along with the thermal mass of the concrete products, it keeps your home cooler in the summer and warmer in the winter."
As an added side benefit, Olson notes that the insulation system also provides a complete vapor barrier. Against the insulation panel, light gauge metal studs are affixed to the floor and ceiling assemblies, providing a cavity for the electrical wiring. The interior finish is wallboard fastened to the interior side of the metal stud wall with screws.
One obvious benefit that comes from Olson's use of a concrete masonry wall system is in the reduction of noise. To the home buyer Olson makes it clear that with his homes, "you keep the outside noises where you want them, outside. Distractions such as auto traffic, overhead planes and strong winds are blocked out by the solid exterior envelope." To ensure success in this area, Olson designed an effective system to prevent wind noise and water from entering through wall openings such as windows and doors. In his home designs, these openings range from 3 feet to 12 feet wide. To support the wall across these areas, Olson uses a concrete masonry lintel from a soldier course header with half-high concrete masonry units. These units are set on a temporary steel lintel and then grouted in place. To install the windows, Olson used a modified buck approach where a pressure-treated lumber buck is attached to the inside face of the concrete masonry wall with both adhesives and screw fasteners. The window is then nailed to the 1.5-inch dimension of the buck and trimmed out with two-by-fours ripped to the desired width.
Quiet is the watchword here as Olson is fond of telling the story of cleaning snow from the driveway one morning with a roaring Bobcat while the family slept undisturbed.
Connections for the floor and roofing systems are also important parts of Olson's concrete masonry masterpieces. These elements are critical to the effective function of the home and could be sources of noise that reduce the solid feel that these homes otherwise provide. In the first few homes that Olson built he chose to support the 20 inch deep floor truss system on two-by-four stud walls on the lower level and not on the concrete masonry wall. While the floor deck assembly is connected to the concrete masonry wall with a 5/8-inch expansion bolt, this is done to provide lateral support to the wall. The stud walls are placed along the side walls of the home and, although they do provide a true thermal envelope without breaks, Olson found that by living in the home this did not appear to provide much value. Olson's local block producer recommended he build with joist hangers that are tied directly into the bond beam at the floor level. This system works well where the thickness of the concrete masonry wall does not change at the floor level which Olson has chosen with the 6-inch reinforced walls.
Despite all the innovations and benefits of this concrete masonry wall system, Olson is quick to point out that it comes without penalty. Concrete masonry "on the outside does not impact the design of the home on the inside. In fact, construction details in the interior give no clue that advanced masonry technologies have been utilized in construction. From traditional to unconventional, the concrete masonry system allows free reign with interior plans." The Olson home buyer becomes immediately focused on selecting cabinets, flooring and bath fixtures just as if this were simply any other home.
This article originally appeared in Concrete Masonry Designs and was contributed by the National Concrete Masonry Association.