Straw Building And The Principles Of Passive Solar Design: A Primer

“It all has to do with why did you build this building here? Why did you transport that oil
halfway across the earth?”
Pliny Fisk III

Vega House at Skyote Mountain, a strawbale home designed using passive solar principals

Vega House at Skyote Mountain, a strawbale home designed using passive solar principals

Sustainable Practice
To me, the most important issue before all of us – builders, contractors, homeowners, building designers, and architects – is how to build sustainably, and so this is the most compelling reason I can give to build with bales. Building with straw automatically results in pluses from the perspective of sustainability. But even the most well-intentioned strawbale project, if built without mindfulness of passive solar principles, can be a failure from the perspective of sustainable practice.

What Draws Folks to Build with Straw?
Many are the reasons to build with straw. For most folks, the most compelling reason is to reduce our consumption of forest products in the form of lumber. For others, it is the elusive prospect of building at a lower cost. For many others, the reason to build with straw comes down to aesthetics: the reassuring mass and thickness of 24” walls, the subtle irregularities of the wall as a handmade object, its reminiscence to vernacular architecture, the ability to shape, sculpt and carve those walls. Many are drawn to the inert properties of straw, resulting in less reliance on toxic materials e.g. fiberglass insulation, the hands-on immediacy of working with bales, or the reduction in heating costs resulting from the increased insulation of a straw bale wall.

What is Straw? (a perennial, seemingly silly, but essential Question…)
Straw is the by-product remaining behind after the harvest of certain agricultural grasses known as cereal grains. These include wheat, corn, rice, oats, rye, buckwheat, millet, sorghum (milo), barley, quinoa, amaranth, and triticale. The legislation which (currently) governs straw construction in my state (California SB332, February 2001) simply defines straw as “the dry stems of cereal grains left after the seed heads have been substantially removed”. Since it consists almost entirely of the cellulose stems remaining after the nutritious component has been removed, straw is relatively inert. So long as it is protected and kept dry, a strawbale wall should last just as long as any conventional, wood-framed wall.

Straw and Sustainability: The Big Picture
From the perspective of sustainability, the improved energy performance (= improved efficiency) of a strawbale-built home places it high on the sustainability index: reduced reliance on fossil fuels reduces production of greenhouse gases related to human activities, reducing our contribution to the carbon cycle, and beginning the long, slow turnaround away from human-induced global warming. Strawbale construction reduces our contribution to global warming in other ways. Agricultural activities in this country currently result in an enormous surplus of straw, a by-product of agriculture which was formerly burned outright, or is now (due to regulation) largely plowed under. But burned or buried, either solution results in the production of greenhouse gases. In the case of burning these have been largely carbon monoxide and carbon dioxide. If buried these are largely carbon dioxide and methane, byproducts of bacterial decomposition. Because it puts to use a waste material (agricultural by-product) and encapsulates it, building with straw reduces our contribution to the carbon cycle and to global warming: once “trapped” within a building’s walls, that straw cannot degrade, and is effectively removed as a source of greenhouse gases for the life of that building. Building with straw also saves trees. A wall built using the strategy of post-and-beam construction with strawbale infill can reduce the amount of lumber in that wall by over 40%. Using bales as the load-bearing component (“load-bearing straw construction”) can reduce the wall framing by as much as 90%. Saving trees makes sense spiritually. Moreover, from an ecological perspective, reducing the demands placed on our forests by the building industry also makes sense globally. Our deciduous forests represent a more effective carbon sink than do agricultural grasses (e.g. rice straw farming). In terms of global warming, every tree saved is that much more carbon captured as biomass, that much more greenhouse gas denied, and that much more oxygen contributed to the atmosphere via plant respiration.

Straw as a Function of Embodied Energy
Building with straw incorporates less embodied energy than a conventional (framed) wall. This pertains on a number of levels. Embodied energy, or “embedded energy,” is an assessment that includes the energy required to extract raw materials from nature, plus the energy used in primary and secondary manufacturing activities to provide a finished product. Thus, embodied energy is a measure of the real energy invested to produce a building material and bring it to a Site, accounting for the energy invested to: 1). produce a crop, 2). extract (=harvest) that raw material, 3). process it, and, 4). transport it to the jobsite. The energy invested to produce a hectare of grain is energy spent for foodstuffs: the building material produced (straw) is a byproduct of that process which, if not utilized as a building material, would otherwise be plowed under. Its embodied energy quotient is therefore “zero” from the perspective of its use as a construction material. To the extent that a forest grows without human intervention (an increasingly rare occurrence), the investment of energy resource to produce a unit area of timber also approaches zero. But inasmuch as forestry management and production have arisen as functions of the construction industry, timber production needs to be seen as a human-induced activity resulting in net embodied energy. Combined with the disruption induced by an essentially monoculture production, from an ecological perspective timber production must be increasingly regarded as a human-induced affront to any localized forest ecosystem. The harvesting and processing of timber is a relatively energy-intensive process, involving the felling large units (trees) on relatively irregular and difficult terrain, their transport to factories well-removed from the point of harvest, and subsequent intensive processing in the lumber mills. Intuition tells us that the harvesting and bailing of straw entails a lot less energy expenditure. Where I am writing this (the Central Coast of California), straw is a relatively indigenous material. Its source (the Central Valley) is far closer to us than the great timber-producing forests of Northern California, or those of Oregon, Washington or of British Columbia, farther north. For us, straw is a material near-at-hand whose intentional use will lower the embodied energy invested in the form of transportation costs. Overall it has been calculated that the embodied energy of a strawbale wall can be as little as 1/30th that of a conventional wood-framed wall.

Passive Solar Benefits Inherent to Strawbale Construction
Straw construction’s most obvious benefit to sustainability is of course the greatly enhanced R-values afforded by strawbale insulation, up to three times that of a conventional wood-framed wall insulated using fiberglass batts. Coupled with the reality that straw bale walls are nearly always plastered, inside and out, one sees that the most essential prerequisites to successful passive heating/cooling – insulation and thermal mass – are inherent characteristics of this technology, automatically conferred simply by building with straw. It should be noted that in order to be effective, the thermal mass should be positioned and oriented for direct solar gain: the sun’s rays must strike the plaster if it is to be effective as thermal mass.

...the building should adopt an elongated form, tending towards linear massing, and should be oriented on an east-west axis, no more than 20 degrees from true south...

…the building should adopt an elongated form, tending towards linear massing, and should be oriented on an east-west axis, no more than 20 degrees from true south…

Design of eaves is another element where strawbale construction inherently contributes to the solar solution. Generally, eaves are a necessary component to successful passive solar design, their projection carefully considered to control unwanted solar gain in summer, and admit more light and solar heat energy in winter. Eaves are an essential component of strawbale construction too, protecting the wall from excessive exposure to rainfall. Here on the Central Coast, where winter storms generally track from the south/ southeast, design of the eaves fulfills two obligations simultaneously in passive solar strawbale construction, protecting the walls and blocking unwanted heat gain in the fall, when the degree-days are at maximum.

Passive Solar Principles We All Should Practice
Sustainability is the fundamental reason to build with straw, and while many of the benefits of sustainability are inherent to building with straw, others aren’t. And as builders, if we are not mindful of applying the principles of passive solar design, we’ve really only delivered “half the goods” to the environment in terms of sustainable practice. The first two of these principles are grounded in the massing of a building, the physical form we choose to give our building. Ideally, the building should adopt an elongated form, tending towards linear massing, and should be oriented on an east-west axis. This results in the building having greater surface area exposed on its south flank – optimizing opportunities for passive solar gain – and maximizing other benefits, such as the ability to cross-ventilate the structure using wind energy, for example, as opposed to mechanical solutions (e.g. fans), and enhancing availability of natural daylighting into the interior of the building, thus reducing reliance on electricity as a light source. In the middle latitudes of the northern hemisphere, to optimize the potential for solar gain the building’s south flank should be oriented no more than 20 degrees from true south: the potential for vertical glazing to capture solar energy falls rapidly beyond this 20˚ range, and its reflective index rises. Of course, massing opportunities are greater when one has a “clean slate”, a new freestanding building as against the addition to an existing structure. And certainly many times we need to play the cards we’re dealt on many sites, particularly on small, urban lots whose orientation is less congenial to our goal. But the key is to keep the goal in mind, and apply it wherever possible. A canny approach to building design, at least in the more temperate zones, is to group minor rooms around a central, south-facing double-volume element. An open volume, so oriented and appropriately glazed, can function as an internal ventilator to the building. If it is made regular (without excessive irregularities) this space may function as an effective convection cell to distribute thermal energy to or from the various parts of the building. Daylighting is another key strategy to reduce our reliance on fossil fuels. While this is important in a residential application, this is particularly critical in our design of non-residential environments (e.g. offices). Design strategies which are mindful of solar orientation in their programming (the arrangement of uses/functions) within the building are part of a successful approach to sustainable development. Awareness of the horizontal and vertical distribution of rooms – placing rooms less needful of natural light, such as storage rooms towards the interior, or placing a garage on the northern flank of the structure, creates opportunities to daylight other rooms. This is entirely intuitive for most designers, yet it is surprising how often this principle is lost to other imperatives during the design process. Subtler is mindfulness of the horizontal and vertical distribution of rooms/uses as a function of thermal gradient. If mechanical means of heating, cooling, or distributing air are eschewed (the idealized goal of sustainable design), some rooms in a passively heated building will be naturally warmer, and others cooler. In summer, rooms along the south edge of such a structure will be warmer than those along the north edge, and given that heat rises, so will the upper story rooms. In winter the converse is true, when the most thermally comfortable rooms will be found in the uppermost, southerly quadrant of the building. This is a natural occurrence (in the sense of a naturally-occurring event), and a consequence deliberately accepted by any building owner who would practice a sustainable lifestyle. The design implication is that uses or functions are generally zoned in the passive solar house, taking into account that the occupant will migrate to the cooler rooms on a hot summer day, and to the warmer ones on a chilly day in winter.

…group minor rooms around a central, south-facing double-volume element…properly glazed…an effective convection cell to distribute thermal energy to or from the various parts of the building.

…group minor rooms around a central, south-facing double-volume element…properly glazed…an effective convection cell to distribute thermal energy to or from the various parts of the building.

The Principle of Thermal Mass
Thermal mass was touched upon earlier, and it is an absolutely essential component if the passive solar building is to be successful. Anyone who designs buildings in California has at least a passing familiarity with this concept, with regulators assigning thermal mass “bonus points” and mitigating these against “negative points”, such as excessive glazing. Solar mass is the “battery” in the solar heating/ solar cooling picture. Materials having the highest specific heat make the best candidates, and these include any of the denser finish materials utilized in construction: exposed concrete, plaster, stone, and tile being the most common. Gypsum board has some value, although its utility is limited – neither its density nor its specific heat are quite as high, and it is installed in relatively thin layers, limiting its overall contribution as mass. Water is one of the optimal performers, and water storage features (trombe walls, etc,) feature prominently in the more exotic solar solutions. Since water is the natural enemy of any strawbale wall, water storage should be very carefully considered and detailed in any building utilizing straw as a component. Horizontal mass is generally more effective than vertical mass: a horizontal floor surface has far more exposure to solar gain than any vertical wall in general. So greatest reliance should be placed on horizontal surfaces in the “thermal battery” equation. But don’t rule out vertical surfaces out of hand. These can be oriented by the careful designer in such a way as to “see” the sun directly, by careful placement, interior to the south-facing exterior wall, or by splaying them planimetrically in such a way as to pick up solar gain. In any event, vertical planes surfaced with thermally massive materials will inevitably accommodate thermal gain and loss, even if indirectly by convection, and will thus generally contribute to the energy performance of a building. Heat gain collected in the day can be used to keep the building warm during the colder night – again, to be most effective, the mass must be positioned to optimize its exposure to solar gain. Conversely, if allowed to cool at night by ventilating the building to the cool night air, its “stored coolness” can be used to mitigate excess heat gain on a hot summer day. This relates back to the subject of massing strategies: a long, narrow building is better situated to ventilate its thermal mass at night, simply because once the windows are opened, air can pass through it more readily and thoroughly compared to a more centralized form, one whose surface area to volume ratio is lower. More subtly, the color of the exposed thermal mass has a role to play in its effectiveness. Dark thermal surfaces (massive) intended to absorb and then release solar heat energy should be dark, to optimize their gain. Non-massive finish surfaces should generally be light-colored, as these will reflect light, facilitating natural daylighting.

Other Considerations
If photocells and or solar hot water collectors are part of the sustainable design solution, south-facing roof surfaces should be pitched to optimize photovoltaic gain. The optimal angle for this purpose is a function of the latitude of the building; at our latitude (Santa Cruz), this angle approximates a 9:12 (33.7˚) roof pitch. Finally, appropriate ancillary devices can be an important part of the sustainability package. These include canny exterior shading devices such as strategically placed trellises planted with deciduous vines. They will limit solar gain in summer and fall, when they are fullest, and admit light and solar gain in winter, after they have dropped their leaves. Projected shading devices above windows and retractable external thermal shutters can limit heat gain and restrict heat loss, respectively. Attic mounted air-to-air heat exchangers can recapture 70-90% of outgoing heat or cold.

Conclusion
Human activities continue to vastly outstrip the ability of this planet to sustain life. This is not a new observation, but an old one recognized and disseminated on the first Earth Day in 1970. Each of us, by our actions and decisions, has within us the potential to affect sustainable practice. Straw building is but one of many possibilities towards this end, and mindful adherence to basic solar principles “completes the picture” in terms of implementing this one solution to the present, ongoing, and now chronic dilemma facing all of us, but most particularly residents of the United States, as citizens of planet earth. My hope is that this primer serves as another building block in the transformation from casual to directed consciousness, and (after thirty years, yet again) towards a sustainable future.

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