June 2005, By Rex A. Bost
ENERGY EFFICIENCY AND WEATHER RESISTANCE MAKE CMUS DESIRABLE FOR HOMEBUILDING
Huddled on the den floor with my family all night while Hurricane Fran’s eye passed 10 miles to the south of our central North Carolina home, I had time to re-think the way we build custom homes. I had frequently visited residential construction sites in southern Florida, Arizona, and other parts of the world and wondered why we didn’t use more concrete block walls provide protection from the weather, fire resistance, and more efficient thermal mass, it was a “no brainer” that Bost Customs Homes needed to build a concrete block project. So, in 1999 we designed and built our first masonry framed home.
In order for the construction techniques to make sense, I needed a system that did not require major retraining for our subcontractors (including masonry contractors), vendors, and inspections. As it turned out, our local structural engineers required the longest training period. My goals included bringing the cost as close to a conventional stick-built home with brick veneer as possible—we are currently within 3%.
For our first projects, we constructed the concrete block walls first and furred out the interior walls to accept electrical, structured wiring, plumbing, and insulation. Although we add fiberglass insulation in the 2×4 furred walls for added thermal and sound insulation, our walls meet the insulation requirements without this layer.
We had difficulty getting window and door openings exactly right, especially in the more complex larger homes and spent as much time on this detail as we would have framing 2×4 walls. I didn’t want to surface-mount the windows and doors and leave the “shadow boxes” on the inside, so we mounted the windows on the furring, which in our case ended up being the 2×4 framed walls. This creates an 8-inch-deep shadow line on the exterior and allows the interior windows and doors to be trimmed in a traditional manner. Today, we work on the other direction, first erecting 2×4 interior (non load-bearing) walls, installing windows and doors, and then constructing the 8-inch block walls.
FROM THE GROUND UP—THE DETAILS
The topography in our market is not conducive to building on slabs. So whether over a crawl space or a basement, we start with a 12-inch wall. We frame the floor system on the inside 4 inches and the 2×4 wood walls with the appropriate openings for doors and windows. We do not apply exterior sheathing but use diagonal metal bracing to keep walls plumb. The construction of the 2nd floor/ceilings is on the 2×4 framing. All of our beams are cantilevered beyond the 2×4 wall to be “picked up” (load transferred) by the masonry walls when they come up. Minor load points coming down through the middle of the house can be carried with typical wood jacks. However, for major loads, we use steel columns resting on footing to eliminate settling (and to reduce the effect of wood shrinkage).
Once the top floor/ceiling is finished, the roof is framed, allowing extra height and overhang to receive the 8-inch masonry walls. The opening headers are basically “skeleton” framed and temporarily supported with stiff knees until the loads are transferred to the masonry.
Next, we install windows and doors. Because we have no sheathing on the framed walls, we apply strips of plywood (4 to 5 inches wide) around the openings prior to our head-and-seal flashing. This eliminates ordering windows with nonstandard jamb sizes. The installed windows and doors are then sealed into the wood framing. Finally we begin the masonry. Our walls are typically 8-inch CMUs, with vertical #4 rebar on each side of all openings, within 8 inches of a corner, under any load points, and at 4 feet on center. Cores with rebar are grouted full, ensuring that about half the cores are filled with concrete and steel. The rest are filled with expanding foam insulation.
Our masons apply flashing, rope weep holes, and an approved moisture barrier house wrap to the frame wall as it’s built. There is typically an inch of weep space between the wood framing and the masonry. This protects the windows during the masonry construction. We leave a gap of 4 inches between the block and window and door flanges to apply a trim product later. Door and window headers are constructed with PowerSteel lintels or CastCrete concrete lintels. A horizontal bond beam is constructed at each floor line, and the framed floor system is bolted to the bond beam using approved anchor bolts. The bond beams are grouted with two continuous horizontal #4 rebar. In the case where the bond beams cross our window and door openings, we use pour-through bond beam blocks so that the headers and the continuous bond beam are placed at the same time.
We cantilever the attic floor out 4 inches with a treated 2×4 plate attached to the bottom of the floor system. An anchor bolt is dropped down from this plate, and the top bond beam is grouted around the bolt. The rafters are attached to this floor system with metal ties. After the concrete sets, the anchor bolts are tightened from the inside.
With this system, house settling because of shrinking wood members is greatly minimized, and there are few callbacks for settling cracks and door adjustments. The exterior finish is typically stucco or stone. We recently developed engineering for composite walls using a combination of 4-inch concrete masonry with brick and 4-inch concrete block and natural stone.