There’s some strange-looking stuff in the four townhouses under construction in this Washington, D.C. suburb. But the strange has a habit of becoming the familiar over time, and this may turn out to be another example.
The structures, called the 21st Century Townhouses, are a project of National Association of Home Builders Research Center, designed to test the value of alternatives to conventional lumber for building houses.
One of them is built of giant foam-and-board panels, another of a concrete system using styrofoam block frames nicknamed Legos by the research staff, another is framed almost completely in steel, and another is constructed of aerated concrete so lightweight it floats in water.
The controversy over timbering in the forests of the Northwest and recent fluctuations in lumber prices have increased the urgency of the home-building industry’s desire to find wood substitutes, but there are other concerns as well.
“As faster-growing (tree) species are planted, the quality of wood has decreased. It’s the No. 1 consumer complaint,” said Dan Ball, director of the project.
He also pointed out that the U.S. is behind other countries in using other building materials. “We’re unique in the world in having lumber cheap and available. We’ve not had a need to use other things.”
The industry fears that’s changing, and so the race is on to come up with lumber substitutes that will make sturdy, buildable houses acceptable to home buyers. Here’s a look at the main construction materials of the four houses:
– The lighweight concrete, called precast autoclaved aerated concrete, is permeated with thousands of tiny air bubbles, like foam, and strenghthened by a steam-and-pressure curing process called autoclaving.
The material’s manufacturers tout it as fire- and pest-resistant, acoustically efficient and having a high insulation value. It comes in slabs or blocks that are joined with a thin mortar.
One unusual property of the material, which has been used widely throughout the rest of the world for more than 50 years, is that it can be sawed and nailed and drilled like wood, making it convenient to use on a building site.
Getting masons to work with it was difficult at first, Ball said. “There was a learning curve,” he said. “They spent the first day and a half standing around complaining and writing comments on the walls.”
He added that a carpenter in his crew, who completed part of the job, did the best job of all, working as fast as the masons and with better quality. “He was used to making things into precise shapes,” said Ball.
He said that while lumber still is cheaper than lightweight concrete, the German-based company that supplied the product for the research house, Hebel, is building a plant near Atlanta, which will make the material more cost-competitive in the Southeast.
“When the workforce comes up to speed, costs will go down,” he added.
– Steel-frame houses have the same balloon-frame construction as a wood-frame house, with the wood studs, headers, trusses and joists being replaced by their steel counterparts. The framing members come in light-gauge C-shape components (actually more like an “E” with the middle line removed).
The members can be cut on site, just like lumber, but of course special steel cutting tools are needed. And steel members have to be fastened with screws, although manufacturers are trying to come up with nail-type fasteners, according to Ball.
Steel framing is universally used in office buildings, but is less desirable in northern states for residential use because it acts as a thermal bridge, transfering heat and cold, Ball pointed out. “You have to use a lot of extra insulation,” he said.
He said there is no evidence that steel-frame houses are more susceptible to lightning strikes than wood frame houses. But he did point out that care has to be taken with wiring, so it doesn’t come in contact with the steel framing. And buffers can be needed with water pipes, which create a racket vibrating against steel, he said.
– The I.C.E. (Insulating Concrete Effectively) blocks are polysterene frames that can be stacked up to provide stay-in-place forms that are then filled with concrete. The polystyrene foam has a metal web inside it to screw drywall into.
The system is cost-competitive with concrete wall poured in place, but has better insulting qualities because of the foam, Ball said. The system is used for ground-to-roof exterior walls of one townhouse as well as for the foundations of three of the houses.
– Structural insulated panels, which have been used here and there for homes since the 1950s, are made of a rigid plastic foam core sandwiched between panels most commonly made of oriented strand board (wood strands glued together), waferboard, plywood, sheet metal and gypsum board.
The load-bearing panels are commonly 4 1/2 inches thick and can be produced in a variety of standard sizes up to 8 feet by 24 feet. They can be cut on site or delivered in a package with the panels specifically designed for each wall, roof or other application with window and door openings already cut out.
The makers say the panels have a high insulation value and reduce air and moisture infiltration that comes from leaks in conventionally framed structures.
The four townhouses have a variety of other innovations, from steel roofs to plastic and gypsum moldings to a heat-recovery device that uses hot water coming from a shower drain to pre-heat water going into a hot water tank.
“A lot of these technologies will find some niche,” said Ball. “I would use three of the four (lumber alternatives) in my house, but I won’t say which three.”
