Building Your Concrete Home: Part 1
By: CAROLE MCMICHAEL
For years, stick-built has been the overwhelming building method of choice; even today, it comes in at about 85 percent of the market. Still, magazine and book racks in building supply stores are bulging with information on other methods home buyers can consider in deciding which one best suits their needs, tastes and budget.
In the last eight years, home buyers and builders have increasingly chosen concrete for their home. Even in this area, they have a choice of materials. Concrete masonry block, ICFs (insulating concrete forms) and removable forming systems (cast-in-place) are the top three choices. Precast/prestressed, tilt-up and autoclaved/aerated concrete systems are some other options, but less popular for residential building.
"About 1994," said Jim Niehoff, residential promotion manager for Portland Cement Association, "things started to pick up in a significant way nationally. PCA estimates that for 2001, the market share for all concrete building methods run about 14.6 percent. Of course, concrete masonry block has always been around. With the strongest showing, it's market share is hovering right around the 9.5 percent mark. The ICFs market is also very good. Estimates for 2001 for ICFs alone are approximately 2.5 percent.
"Cast-in-place systems are pulling in about 1.7 percent of the market, but are expected to capture a more significant share in the next couple of years, based on the work of the Concrete Homes Council, which is part of the Concrete Foundations Association (CFA). There is a network of concrete contractors who are familiar with the concrete forms building methods below grade; they just have to be convinced to take it above ground. The skill is in place.
"What would make a difference? First, the council needs to help contractors see that they could sell these houses — that there would be public demand. Second, it needs to show them that concrete works for production builders. Removable form systems lend themselves very well to larger scale housing developments where you can go into the entire development, lay out the form work and basically pour the walls, then tear the forms down and the next day set up the forms on the slab next door."
Taking a Closer Look
To most people, concrete masonry block means the original gray block. According to Harry Junk, residential market manager for NCMA (National Concrete Masonry Association), a lot of new types of masonry forms are being produced today, referred to as architectural units. They are cut and shaped differently and offer a variety of textures, surfaces and colors.
Specifically, masonry systems offer: interior insulated block; exterior insulated block; in-block or cavity insulated block; pre-insulated block; and mortarless insulated block. Each type has different advantages.
Dennis Graber, an engineer at NCMA, noted that the exterior insulated block is the most energy efficient. With rigid foam insulation placed on the outside of the block walls, it provides an uninterrupted surface that can become super-insulated just by increasing the thickness of the insulation. All the concrete mass is on the inside, so temperature swings between heat and cold are moderated. Because it requires an exterior finish, it is more expensive than other types of block.
In-block insulation, which fills inner cavities with rigid foam, loose fill or expanding sprayed foam, provides a concrete surface on both sides. This eliminates the cost of additional exterior surfacing and produces a more durable inner wall surface.
"If builders choose in-block insulation, they will have to deal with thermal bridging," Graber said. "However, webs can be cut down to improve insulation and R factor."
Mortarless insulated blocks are dry-stacked, placing foam inserts in block cavities. Instead of mortar, a coat of bonding cement is applied inside and out. The resultant flat surface in high quality block creates a very attractive finished look, as well as producing a barrier to moisture. The flat surface also lends itself to attaching economical traditional finishing materials.
Pre-insulated block offers a respectable R-value (up to 20), which makes this a choice in the North. Besides reducing the size of the inner webs, insulation can be mixed into the concrete block during manufacturing. These blocks also are particularly easy to cut because they are lighter than conventional blocks.
Interior insulated block is a good choice if the home buyers plan to use a traditional drywall interior finish anyway. "This type of block is the one most likely chosen for building a residence," Graber said. "and generally, it is the least expensive."
"Florida is the area where masonry block is strongest," Junk said.
"Somewhere in the neighborhood of 80 to 85 percent of homes use masonry. That includes anything from tract type homes up to multimillion dollar high-end custom homes."
There are other choices the home buyer will make if he chooses masonry block; specifically, the color and texture of the block. Mineral oxide pigments can be mixed into the concrete and the mortar. Color is also affected by cement color, aggregate color and the amount of water added. Mixing different concrete colors can create a variegated or marbled effect. Texture options include split-face, fluted, ribbed, scored, ground face, sandblasted, striated and glazed finishes.
"The surface look is limited only by your imagination," Junk said, "but there are codes, such as ASTMC 90, that set standards for properties or performance.
Insulating concrete forms, which are left in place, come in three basic styles: pre-formed interlocking blocks, panels, or plank systems, usually connected with plastic ties. The rigid foam forms are filled with concrete onsite. Interior and exterior finishes are applied to the foam. As with masonry, ICFs come in a variety of shapes and component parts. The factory-manufactured blocks typically range from 8 inches x 16 inches to 16 inches x 4 feet. They include interlocking edges to simplify assembly.
Panels are larger units, ranging from 1 foot x 8 feet to 4 feet x 12 feet with flat edges that use separate connectors. They offer the advantage of assembly into wall units prior to delivery.
Slightly smaller than panel systems, plank systems range from 8 inches x 4 feet to 12 inches x 8 feet. They use ties and usually are not pre-assembled into wall units.
An ICF's cavity, which will hold the concrete, affects the thickness of the finished concrete expanse. The grid wall produces varied thickness in a waffle pattern. Flat wall design, as the name implies, produces a continuous, even thickness of concrete. Post and beam designs, which produce widely spaced vertical and horizontal columns of concrete enclosed in foam, are another option. All the systems available have been engineered, code-accepted and proven in the field.
The foams, which are inert and non-toxic, are either pure foams or expanded polystyrene(EPS)-cement composites. The pure foams can be EPS, extruded polystyrene (XPS) or polyurethanes. EPS uses small plastic beads that are fused. XPS has the same makeup but comes out as a continuous mass and is available in sheet form. The latter also tends to be more expensive with a higher insulating value. Polyurethane is a mixture of other ingredients, and is more expensive than EPS and XPS, with a still higher insulating value. The cement-foam composite mixes EPS beads and Portland cement. It offers greater strength and perhaps durability, but it is harder to cut and shape. PCA suggests that builders should not use an ICF based purely on the type of foam used.
"The market for ICFs is fantastic," said Joe Lyman, executive director of the ICFA. "We believe that very soon demand will outstrip supply. Currently, there are about 20 major players out of a total of around 70 ICF manufacturers. The upper and lower quarters of the country where you have major temperature swings see the greatest concentration of ICF building, but there are also significant pockets in the Midwest where there are tornados, such as Missouri, Kansas, Arkansas and Oklahoma."
Unlike the ICFs, the removable forms, usually made of aluminum, are reusable up to about 3,000 times. Rigid foam is placed in between the forms, with concrete poured outside the foam. The forms are held together with ties and strengthened with steel rebar before the pour. They are removed once the concrete cures.
"We promote three methods of construction," said Ed Sauter, executive director of the CFA. "Outside walls only; outside and interior walls; and outside, inside and decks (floor and ceiling). The latter, of course, is the safest (from storms) and most efficient from an energy perspective. Within these three methods, there are variations depending on the system used: insulation on the outside; insulation in the middle (sandwich wall); or insulation on the inside (not used often).
"There are also variations based on the forming systems used, which include the tunnel forms for casting walls, floors and ceilings at the same time, smooth aluminum or wood forms, forms with patterns imparted to the walls and forms with form liners.
"The beauty of this building method is that there are fewer operations and trades involved. Depending on the system used, at least the interior walls are completed and ready for finishing and painting when the concrete contractor walks off the job. In the case of the sandwich wall, both interior and exterior walls are ready to finish."
The Selling Points
Although there are differences in the degree of benefits among the concrete building methods, they all share some incredible advantages over wood.
Concrete's disaster-resistance features cover wind storms, fires, floods, tornados and hurricanes. Depending on the prevalent disaster(s) in different parts of the country, there is no region that cannot benefit from building with concrete. The protection is not just in terms of investment in a house, but in the lives of people who live in one.
Concrete's energy efficiency is another significant benefit. Compared to wood, all the concrete building methods provide superior insulation, tighter construction and greater thermal mass, which improves control of heating and cooling changes. This all translates into smaller HVAC units and reduced monthly utility bills. Homes built in cooler climates can produce greater savings on heating; those built in warmer climates, greater savings on air conditioning. Also, in general, the larger the house, the greater the savings. Beyond utility costs, maintenance costs are also lower and the concrete house will last considerably longer than stick-built ones.
The environmental and health benefits connected to building in concrete are also noteworthy. Wood might be a renewable resource, but not at the rate we are consuming it, especially compared to concrete. Concrete doesn't put out noxious fumes to ignite allergies, but stick-built homes often use sealants and glues that help make houses "sick." Add improved air quality to the reduction of noise pollution (a key reason for the increased use of concrete in multifamily home developments), and it is easy to see why homes built with concrete are increasingly the recipients of Green awards.
The bottom line for these concrete building methods is that higher initial costs are outweighed by the long-term savings, and as concrete housing grows, the initial cost gap is closing.
"When compared to the cost of wood, the figures we hear from the builders for masonry," Junk said, "are from the same cost to a maximum of 5 percent more. An average of 2 percent to 3 percent more is a good figure to use. This is a base cost that can be returned to the owner in three to four years with energy and insurance savings.
"I think the momentum is going to continue to build," Niehoff said. "Four or five years ago, taking ICFs as an example, people who came to the building shows might have understood the basic concept but maybe had never heard of concrete homes going up in their area. Now, they want much more specific details — the brand names, the different types, who can build one for them, what the benefits and drawbacks are. In other words, they are more educated on the topic.
"I see this trend continuing. The growth of the market will depend on getting the word out to the home buyers and the builders."
(For Part 2 in this series, see the article in the August/September 2002 issue of Concrete Homes.)