A tremendous diversity of materials, techniques, and traditions have gone into building a home’s framework. Some approaches, like rammed earth, adobe, and straw-bale construction, are now well-established—if still unusual—alternatives to more mainstream techniques. Other materials are newer: blocks of aerated concrete, structural insulated panels that combine polystyrene or urethane foam insulation in a sandwich of oriented strand board, and hollow blocks formed with a mixture of cement and recycled polystyrene or wood fiber.
These materials all have their advocates, and they all have something to offer “green” building. But with the exception of structural insulated panels (SIPs), many of these products can be difficult to obtain or they require techniques unfamiliar to most contractors. For most residential builders in the United States, wood reigns supreme. So how can we build better and use this resource wisely?
Wood is one of the best-suited renewable materials used in construction. It has low embodied energy, natural durability, high strength, and low toxicity (when untreated). Ninety-five percent of single-family houses built in the United States today are “stick-framed” with dimensional lumber.
Many conventionally framed houses share a common problem that affects their “greenness.” There is often too much framing—oversized headers, too many studs placed too close together, and framing that gets in the way of insulation. Call it habit, tradition, or insurance on the part of the builder—the bottom line is that these and other wasteful practices make building envelopes more expensive and less efficient than they could be.
In the late 1970s, during an energy crisis that was likely a practice run for what’s to come, the National Association of Home Builders’ Research Foundation conducted studies to identify what structural configuration was necessary to maintain superior strength and yet allow the maximum insulation in wall cavities to improve energy efficiency. The result was called optimal value engineering (OVE). Research showed that as much as 20% of the framing material used in traditional construction could be removed without compromising structural integrity. Today, OVE is just referred to as advanced framing.
Advanced framing has two main benefits. First, it eliminates framing members that serve no structural purpose, thereby reducing waste and unnecessary cost. Second, it leaves more room for insulation and reduces cold spots from thermal bridging, making the house more comfortable and less expensive to heat and cool. Here are some of the basics:
Wood has attractive advantages as a building material, but the stupendous volume of material that goes into residential construction creates a variety of interrelated problems. Homebuilding consumes about 45% of the total lumber used yearly. This year, new residential home projects are predicted to consume 9.5 billion board feet.
One of the biggest controversies in the world of green building is how trees are forested. More than 95% of the original forests in the United States have been razed. Yet, according to the American Forest and Paper Association, there are more trees in the United States now than there were when the Pilgrims landed in Massachusetts nearly 400 years ago. On the surface, that seems like a good thing, but once we dig deeper into the issue, our views may be more tempered.
During the expansion of European–American culture, forests seemed endless and empires were built on what could be harvested. Replanting was unnecessary because there were so many trees yet to cut. As the resource became more depleted, tree farming became a good business practice and tree plantations began springing up across the country. Today, most “forests” are actually tree plantations.
It is only when we visit the tiny pockets of remaining ancient forests that we realize what a profound difference there is between a tree plantation and a natural forest ecology. It takes hundreds of years for a forest to regain its complex interconnections of life-forms and the inherent stability that goes along with them. Clear-cutting a forest only to replant with a single species destroys the forest ecosystem.
Lost habitat decimates natural animal populations. Because they are artificial environments with a lack of natural diversity, tree farms are not places where people want to backpack, hike, or fish. So people flock to our overrun national parks, now so overcrowded they are barely managing the flood of people looking for a glimpse of the real thing.
As concerns have mounted over how forests are managed, a number of programs have popped up to certify that lumber harvesting is causing no long-term damage to either the environment or the people who live nearby.
Lumber certified by the Forest Stewardship Council meets standards designed to protect the forest and the people who live there. The standards were adopted internationally in the mid-1990s and include strict requirements for how timber is harvested, how much can be cut, and how the forest ecology is to be protected. The program also mitigates problems when indigenous people are affected by lumber harvesting. FSC relies on a “chain of custody,” to track wood from the forest to the point of sale. Each forest, mill, distributor, and retailer must be certified to preserve the integrity of the lumber’s sustainability.
FSC protects existing forests from conversion to single-species tree farms, and ensures that the lumber you are buying is sustainably harvested.
In all, there are more than 50 different forest certification systems in the world, each claiming to monitor a certain amount of acreage. Others in North America include the Sustainable Forestry Initiative (135 million acres), the logging industry’s American Tree Farm System (3.15 million acres), and the Canadian Standards Association (170 million acres). A new program, the European Programme for the Endorsement of Forest Certification (PEFC), is being developed to compare systematically all the certification programs globally so there is a common reference for wood procurement. The number and variety of these programs can be confusing. But the programs are at least a step in the right direction, and the benefits of specifying FSC-certified lumber will continue to make new converts among home builders.
Using engineered lumber, which uses chips and small pieces of solid wood glued or laminated together, is another way to make sure that wood resources are not wasted—and engineered products often result in a better house. Once the exclusive domain of commercial buildings and high-end houses, engineered lumber is now commonplace in conventional construction. Two things make this family of products green: engineered products use wood fiber more efficiently than solid sawn lumber, reducing waste; and they can be made from wood species that quickly regenerate themselves.
For example, I-joists are made partly of wood chips and use only half the fiber to perform the same structural function as solid timber. And they do the job better, with longer allowable spans than solid timber of the same width. In addition, they can be made from aspen or other species that regenerate: These trees are really the fruit that springs from an underground root system, undamaged when a tree is harvested.
Finger-jointed studs and glue-lams are another type of engineered lumber. Both are made from solid pieces of defect-free wood glued together to make larger pieces. Finger-jointed studs have an important advantage over sawn lumber in that they are straight right from the factory—and do not warp afterward. Conventional studs are milled from fast-growing farm trees, famous for twisting, splitting, warping, and checking, making it increasingly difficult to frame a straight wall, which can cause all kinds of problems down the line.
Glue-lams are 2 by 4s or 2 by 6s that are glued together face-to-face to create a beam. Laminated veneer lumber is like plywood but in the dimensions of beams. Parallel laminated lumber is made from long strips of trees that are glued together lengthwise to create beams stronger than the tree itself. These replace large-dimension timbers used for long spans—over a garage door, for instance. Increasingly popular, they save old-growth trees and do the job better, stay straighter, and are typically less expensive.
All engineered wood uses a binder to hold the wood or wood fibers together, which can affect indoor air quality. For instance, urea-formaldehyde is a water-soluble glue that is used for indoor products such as particleboard and medium-density fiberboard. The binder off-gasses (releases) formaldehyde for up to five years and is a major contributor to poor indoor air quality. Phenolic resin off-gasses a mere 4% of the formaldehyde that urea does.
In timber-framing, or post-and-beam construction, carpenters become craftspeople, using mortise and tenon and dovetail joints, with wood pegs holding the large pieces of wood firmly together.
Timber-framing provides the structure for the building, and insulation is then added between the posts. SIPs, straw bales, and more conventional insulation can be used to seal the envelope. One advantage is that the frame can be erected quickly to get the building under cover and the infill added once the weather protection is in place.
Since big timbers are the cornerstone for this construction technique, one of the important considerations is where the wood comes from. Timbers should be chosen with care—local FSC certified, salvaged, or milled from standing dead trees are the best choices. Otherwise, there is the possibility that the large-dimension lumber has been taken from old-growth stands and/or clear-cuts.
Another issue is the amount of wood that goes into a timber-frame house. There’s a lot of it, certainly more than would be used for a conventionally framed house or one that makes significant use of engineered lumber. Because a key element of green building is an efficient use of materials, there is a sustainability argument to be made that timber-framing is inherently a worse choice than a house built with advanced framing techniques.
SIPs are a sandwich of insulating foam and oriented strand board that can be used for floors, walls, and roofs. SIPs are a green building product on every level. They combine insulation and structure in a single product. Houses built with SIPs are efficient, comfortable, and have virtually no air infiltration. Heating and cooling costs are typically half or less of a conventionally built and framed house.
With SIPs, some initial cost savings come from rapid enclosure of the envelope, especially when building in cold weather. Once the frame is up, you can work in an insulated space—and that means jackets come off and productivity goes up. When the job is finished, you have an airtight, highly insulated envelope that also helps cut noise inside. Moisture and mold issues inside wall cavities are thwarted because these units are completely sealed.
However, observe several cautions when building with SIPs. First, make sure to protect the oriented strand board on the faces of the panels from water. The OSB gives the panels structural strength. If it gets wet and stays wet, it loses its integrity. Once they’re up, careful flashing is also key.
The second issue is running electrical cable. SIPs are made with channels in the foam to accommodate wiring. When ordering, it is imperative to provide an electrical plan so the manufacturer knows where to run these chases. Plumbing should never be run in an exterior wall, so typically there are no chases provided for that purpose.
There also is a concern that insects may burrow into the foam. According to the Structural Insulated Panel Association (SIPA), termites won’t feed on the foam core but they may nest there. As a result, manufacturers offer borate-treated panels and the SIPA suggests termites can also be discouraged by a steel mesh barrier placed around the perimeter of the house and at foundation penetrations.
Rigid foam insulation is a vital component of structural insulated panels, and some people wonder whether using a petroleum-based product like that can really be green. The amount of oil used to make insulating foam (which is primarily air) is very small. Compared to the energy used in a home over its lifetime, the small amount of petroleum used in its construction is miniscule. If formaldehyde is a concern, ask the SIPs supplier whether the skins can be made from formaldehyde-free OSB.
Autoclaved aerated concrete (AAC) blocks take the place of a number of components used in standard stick-frame construction, replacing wood, insulation, house wrap, and drywall. The result is a house that’s fireproof, mold-proof, insect-resistant, hypoallergenic, sound-absorptive, and engineered to withstand hurricanes and earthquakes.
Aluminum powder added to a mix of sand, lime, water, and cement creates a five-fold increase in volume while trapping insulating air bubbles. It’s hardened in a mold and then processed in an autoclave to produce blocks in a variety of configurations. Blocks can be cut on-site with a specialized hand saw or band saw and laid up somewhat like conventional concrete block.
Walls made with AAC block have far less air infiltration than conventional 2-by-4 construction, with insulating values for the 8-inch-thick block as high as R-21.
Insulated concrete forms include Rastra, which is made mostly of recycled polystyrene with some cement to form a material the company calls “Thastyron.” It’s made into hollow-core blocks that are relatively lightweight, can be cut with ordinary handsaws, and glued into place. Once stacked into walls, the blocks are reinforced with steel and filled with concrete. The company reports that 10-inch-thick walls have an R-value of 36.
Fiber-cement blocks, such as Durisol or Faswall, are another option. These hollow-core blocks are made from mineralized wood shavings and Portland cement, stacked, and then filled with reinforcing steel and concrete. Manufacturers claim that the material is noncombustible, sound-absorptive, dimensionally stable, and boasts a 78% recycled content.
Products like these share many green benefits and give builders interested in exploring new avenues considerable leeway. They are not as widely available as wood building products and may make the most sense in areas with local or regional suppliers. One more consideration: There is a learning curve in working with new materials, but converts will tell you it’s well worth it. These products may have a higher embodied energy than wood framing, due to their use of concrete and other manufactured materials, as well as shipping. But they typically offer higher energy efficiency than a conventionally built wood-framed structure by reducing air infiltration and increasing thermal mass and insulation.
Building better doesn’t have to be difficult. Here are some tips to keep in mind:
David Johnston’s work in green building has been embraced by homeowners, building professionals, and sustainability advocates around the world. In 2007, he was named International Sustainability Pioneer by the European business community. Johnston’s previous book, Green Remodeling, is a recognized guide to green remodeling.
Scott Gibson is a contributing editor to Fine Homebuilding magazine, and a freelance writer and editor.
This article was adapted from Green from the Ground Up (The Taunton Press, 2008).