Green building expert Chris Magwood compares wood framing systems to cob walls, compressed earth blocks, and straw bale walls in pursuit of a sustainable home building practice.
In the sustainable home building industry, many prospective homeowners want to make environmentally friendly choices, but struggle to make sense of a building industry that labels every house framing option as “green.” For environmentally conscious builders, like me, sustainable construction encompasses many factors, including ecosystem impacts, carbon footprint, material waste, energy efficiency, cost, durability and indoor air quality. Ultimately, those sustainability considerations must mesh with practical concerns, such as labor, building compliance, and material sourcing and availability. One measurement that helps builders and future homeowners compare various materials’ environmental impact, embodied energy (EE), totals the amount of energy invested in a product during sourcing, shipping and installation. Further, when sustainable home builders include the energy associated with a material’s use and disposal, or that product’s “downstream,” they have completed a life cycle impact assessment. Now, no house framing system — neither cob, compressed earth blocks, straw bale, nor wood frame — scores top marks in all criteria, and embodied energy and life cycle analyses differ based on site-specific circumstances, so homebuilders must determine their priorities and make choices that best reflect those goals. One size never fits all in green building.
House framing systems are often the first thing an owner-builder wants to discuss. And no wonder: Walls support the windows and doors that physically define a space, play a crucial role in a home’s energy efficiency, and dominate a building’s aesthetic.
To help homeowners refine their ideas into buildable homes, I created these charts: Wall-Framing Systems and Conventional vs. Improved Wood Framing. The charts compare five house framing systems systems according to 11 criteria, and show how each system rates relative to the others. Many people assume that alternative wall systems (such as straw bale, cob and compressed earth block) are more sustainable choices, but if you study this chart, you’ll see that wood-framed walls perform well when paired with improved materials.
In the 20 years since the resurgence of interest in straw bale building, the number of houses raised using this type of so-called alternative construction has grown quickly. Straw bale building is one of the few alternatives with comprehensive code language, and the International Residential Code incorporated straw bale walls in 2015 — a fast ascent from fringe following to mainstream acceptance.
In straw bale building, workers stack bound, rectangular bales of leftover stalks from grain crops (such as wheat, rice, oats, barley and even hemp) to form walls (see photo). The exposed surfaces require a coating of clay, lime or cement-lime plaster to provide external structure, seal and finish. Straw bale walls easily adapt to many aesthetic possibilities — the material is equally at home in straight, square buildings as it is in round or curvy structures.
Accurately rating straw bale walls can be difficult because numerous building systems incorporate straw bales, but differ greatly in process and materials. For example, simple, load-bearing straw bale wall systems score well when builders choose minimal window and door framing, a simple wooden top plate, and site-made earthen plaster (see Wall-Framing Systems chart). However, when straw bale builders use complex framing systems with plaster mesh and cement-based plasters, they significantly raise environmental impacts, along with financial and labor input.
Finally, because straw bale systems typically pair lower-cost materials with higher labor needs than conventional building methods, costs vary widely depending on the source and price of that labor.
Cob building has ancient roots, and most cultures have engaged in some form of this building style. Cob is a mixture of clay-rich soil with sand and natural fiber, most often straw or grasses. The wetted blend forms a dense, sculptable medium that builders hand-form into walls (see photo). This simple process works well with a wide range of materials and in a wide variety of climates. Finally, cob builders often apply a protective layer of clay-based plaster to the finished walls.
Although cob currently lacks explicit building code language in North America, hence the material’s low scores in the categories of building-code-compliance and labor-input, some smaller cob structures are legally permissible because of size exemptions. Cob also scores poorly in energy efficiency, thus the need for additional insulation in all but the most temperate climates.
To address efficiency concerns, homebuilders should incorporate other insulating materials into a cob wall, either in the form of a double-wall construction with insulation in the center, or some type of exterior or interior insulation and cladding. Insulation choices can dramatically affect a cob wall’s ratings, increasing both the cost and environmental impact.
Constructing an energy-efficient cob home is certainly possible, however, as is lowering labor requirements by using mechanical equipment. But, improvements in those areas will detract from the low-cost, low-impact simplicity that draws many builders to cob construction in the first place.
Of all the alternative wall-framing systems, cob has the least amount of data available to support building-permit applications, though some prominent cob homes have set a promising precedent.
Cordwood building makes use of wood resources unsuitable for log or milled-lumber construction. Builders stack short pieces of round or split wood lengthwise across a wall, [as the length of the pieces determines the wall’s thickness (see photo).] For structural support, a bed of mortar — made from clay, lime or cement — forms a structural matrix around the wood pieces. The mortar reinforces the wood on the inner and outer edges of the wall, leaving inner-wall cavities around the wood that can be filled with insulation.
Like straw bale building, cordwood walls net a wide range of scores in the chart below, depending on decisions builders make before and during construction. While wood sourcing has some effect on environmental impact, a cordwood builder’s mortar and insulation material choices are the largest influencing factor on sustainability.
Cordwood is prone to air leakage, which greatly affects its energy efficiency. Providing an adequate air seal at the many joints between wood and mortar can be difficult, but any leaks can negate insulation improvements added to the walls. Coating both sides of the walls in plaster (or another sheathing) will improve thermal performance levels significantly, but will change the aesthetic and add to the costs.
While cordwood walls aren’t formally recognized by any building codes, historical and modern precedents contribute to the availability of permits in many jurisdictions.
Contemporary compressed earth blocks (CEBs) use mechanical- or hydraulic-powered presses to tamp soil with low clay content into large bricks, drawing on the ancient, rammed earth building techniques. In many cases, builders bring block-making machinery to the building site to use the site’s soil as the basis for the blocks; however, some commercial CEB producers form blocks at a central plant for shipping. Once formed, compressed earth blocks are laid up like concrete blocks, using clay- or lime-based mortar to bond the blocks together.
Both CEBs and rammed earth walls have wide ranging environmental impacts, primarily determined by the amount of cement used. As with all earthen building systems, CEB and rammed earth walls have little inherent insulation value. Insulation strategies include double-wall construction with insulation in the center (which will double the amount of blocks or rammed earth required), or an interior or exterior insulation layer with a finished surface to protect it; however, such strategies raise costs and environmental impacts.
Availability of the materials and specialized labor for CEBs and rammed earth ranges widely by region in North America. Codes in the United States tend to approve this style of construction where it has historical roots, such as in New Mexico.
Most homeowners are familiar with conventional wood-stud construction, in which 2-by-4 or 2-by-6 lumber forms a stud-frame wall. Builders fill the stud cavities with insulation, and then clad the exterior and interior surfaces with sheathing material. Another wood frame construction method uses double-stud walls to increase wall depth and insulation values.
Conventional wood framing can be a surprisingly eco-friendly construction style, especially if builders use sustainably harvested lumber. Learn more about locating sustainably harvested lumber online by reading Find Sustainably Harvested Wood. Wood frame walls use a relatively small amount of material to achieve suitable structural strength and meet code requirements.
From an environmental point of view, the insulation and cladding materials that accompany conventional frame construction stack up poorly in terms of embodied energy and carbon costs. Fortunately, materials with much lower impacts are available, allowing an improved wood frame wall system (see Conventional vs. Improved Wood Framing).
Improved wood frame models outfit code-approved structural systems with fairly accessible and affordable components to keep environmental impact low and energy efficiency high.
As the chart on Page 54 demonstrates, no magic building system combines the lowest possible environmental impacts with the highest energy efficiency, lowest cost, best availability of materials, lowest labor inputs, and easiest building code acceptance. If one did, we’d all be building that way! While a simple cob cottage may meet the needs of one owner, a straw bale home might better suit another.
The low- and high-impact ranges of each category on the Wall Framing Systems comparison chart also show how seemingly small decisions made at the planning stages can result in significant environmental repercussions in relation to a structure’s embodied energy, toxicity and energy efficiency.
Finally, we should understand the original architects of frame houses as sustainable builders when they made economical use of a natural material. The Conventional vs. Improved Wood Framing chart spells out the options that you can take to dramatically decrease the embodied energy costs of traditional wood framing systems. Improved wood framing can combine low-impact, low-cost materials to build modern, highly energy-efficient and eco-friendly homes using conventional, code-approved framing techniques.
Above all, the successful homebuilder must identify priorities, and make informed choices based on specific data. Whether you choose a cob cottage or an eco-framed home, know what you want from the outset, prioritize your goals, and make decisions that closely align with your objectives. If we hope to make choices that are better for the environment, green builders must drop preconceived notions and embrace facts.
Chris Magwood founded and directs the Endeavour Centre to promote sustainable building in Ontario. Magwood is the author of Making Better Buildings, and of titles on straw bale construction.
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