The New Strawbale Home: A Coat That Breathes

A Coat That Breathes

The classic, time-proven, straw bale wall consists of bales with a plaster “skin” applied to both faces. A thick coat of plaster serves multiple functions. Protecting the exterior of straw bale walls from wind, rain, and abrasion, it also seals bales from birds, rodents and insects that view them as an attractive home. Inside, plaster provides a smooth finish to bale walls while adding thermal mass. Common choices for exterior plasters include cement/ lime, lime, and clay, while interior plasters are often lime, clay, or a commercial gypsum or Structolite product. Consider your climate when deciding which type of plaster to use, as an appropriate design and finish will require minimal maintenance.

It's important to plaster both sides of a straw bale wall to seal out oxygen for fire resistance. For example, if an interior wall were “furred out “ with studs to attach drywall without plastering it first, the resulting space would act as a chimney in case of a fire. A well-sealed bale wall is also critical for energy efficiency, as even minor gaps will allow air infiltration through the bale, reducing its effective R-value.

Micaceous clay plaster makes a luscious interior finish.

A cement/ lime stucco retains much of the strength of cement and vapor permeability of lime.

Breathability—more accurately known as “vapor permeability”—of plasters is also a consideration. While structurally stronger, cement stucco is relatively less permeable than lime or earthen plasters. It is known to absorb moisture from rain, as well as the ground, and wick it into adjacent wood or straw. As long as bales or wood can dry out in time through the stucco, there is little cause for concern. But elastomeric coatings—which are often recommended by contractors because they tend to eliminate small cracks in the stucco—can actually trap moisture in the wall and should be avoided. (In climates with driving rain, a siloxane coating on stucco has been tested to repel moisture without affecting permeability.)

Clay plasters are the most permeable, with the added advantage of being hydrophilic, which means “water loving". Clay readily absorbs moisture and holds on to it, actually wicking moisture away from straw or wood. This accounts for the preservative effect clay has on these materials, making them much less vulnerable to moisture issues. Welsh cob houses and German timber-frame/ straw-clay walls provide historical examples of this effect; many such buildings are still occupied after centuries of use. Earthen plasters on straw bale walls may also have this kind of long-term beneficial effect.

Conventional builders often believe they should wrap a straw bale house in an air or sheet barrier (like Tyvek), as this is a standard part of stick-frame construction. This is because stud-framed wall systems covered with gypsum board (also known as drywall or Sheetrock) on the inside and oriented strand board on the outside tend to have many minor cracks where they are joined, which allows air filtration that compromises comfort and energy efficiency. “House wrap” seals these air gaps and, in theory, repels rain while allowing the smaller water molecule to escape. However, testing results from Canada and elsewhere indicate this product may not live up to all manufacturer's claims.

In a straw bale home, house wrap is not only unneccesary—as a well-detailed plaster eliminates air infiltration—but can be a critical mistake. Such a barrier will prevent the plaster skin from “keying in” or bonding with the straw bales, reducing the wall's structural strength. Worse yet, as warm, moist air migrates from living areas to the colder exterior through the straw bale wall, it will tend to condense on the inside of the exterior housewrap. This liquid moisture will now dry out slowly, and when the temperature warms, it can produce conditions ripe for fungal growth.

Conventional wood frame buildings also typically employ a sheet-type vapor barrier on the interior to prevent this warm, moist air from infiltrating through surface finishes, into the walls and attic. This practice, however, may lead to a false sense of security, according to some studies. The modern life of a family of four can easily generate 18 gallons of moisture vapor per week into the household air through cooking, bathing, and washing. An interior sheet barrier can actually serve as a vapor funnel through any rip or penetration, concentrating moisture at this gap. These small air leaks can create big problems by concentrating moisture vapor in only a few places; when it condenses, the resulting moisture can be significant. In cold climates, the problem can be severe, often resulting in the deterioration of the roof or wall materials of conventional homes

In a straw bale home, the same principles apply. Plaster finishes, whether earth or cement-based, will offer an adequate air barrier to vapor moving into a wall or ceiling from the interior, as long as potential cracks and crevices are sealed to prevent gross air leakage. Gaps in interior finishes around electrical outlets, overhead fixtures, window and door frames, plumbing etc., should be well detailed to reduce or eliminate vapor infiltration. The best strategy for managing indoor moisture is to install mechanical ventilation at the source— in bathrooms, the kitchen and laundry room. Unheated attics and roof systems should also be vented. Evidence suggests that clay plasters, which can absorb and store large quantities of moisture, can beneficially moderate excess humidity in the home.

Ongoing home maintenance begins with observation. If you have concerns about specific areas, it's wise to install moisture meters or to periodically use a moisture probe in critical spots. Peace of mind is worth the effort.