Straw Bale and Building Science: Growing Up Together

The straw bale revival of the 1990s reintroduced builders to a pioneer building method that showed remarkable potential for building in a modern context. The nature of the basic components of a straw bale wall system – bales with plaster applied directly to bales – combined several obvious advantages over other wall systems while simultaneously raising some serious questions.

The advantages are familiar to TLS readers: low environmental impact, simplicity and well placed thermal mass. The serious questions and attempts to answer them were the lifeblood of TLS: What about moisture? Air tightness? Cold climates? Humid climates? Longevity of the straw in all these conditions?

There was a lot more going on in the construction world of the 1990s than the straw bale revival. The housing industry was recovering from multiple moisture-related disasters, many caused by overly airtight but under-ventilated homes and/or the use of un-vented “waterproof” exterior cladding systems. The research into these issues, among other factors, led to a rise in prominence of “building science” as a distinct engineering discipline. The straw bale revivalists were extremely lucky to have the attention of some of building science’s leading practitioners, in particular John Straube, who authored several key articles and studies about straw bale walls that were essential to answering the question of why bale walls were so resilient, despite some seemingly obvious moisture concerns.

Simply put, building science attempts to quantify the movement of heat and moisture through building assemblies and help designers and builders to make decisions that lead to structures that adequately deal with the moisture and temperature regimens of their particular climate and use.

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In order to build viable, durable homes and to answer the questions of all the naysayers, serious straw bale builders learned valuable lessons from building science, and were early adopters of the basic precepts of proper building design for moisture. In just a few short years, these principles have come to be well understood and well utilized by conscientious builders. Though some complex modeling and proofs were required to quantify them, they can be fairly simply stated and should be essential lessons for all straw bale builders:

      Keep moisture away from walls to the best degree possible

This was one of the first lessons expressed in the pages of TLS, often phrased as “giving the house a good hat and good boots. What seemed like colloquial common sense turns out to be an important lesson that conventional designers and builders all too often ignore: If rain is shed away from walls by properly designed roof overhangs and flashings, and if ground moisture and splash-back are minimized or eliminated, most moisture-related issues with walls can be easily mitigated.

By and large, straw bale builders have taken this to heart, and our buildings are able to withstand whatever the local climate can throw at them because we ensure that the level of overhang and flashing protection matches the severity of wind and precipitation each building site experiences.

This lesson is ignored at the peril of the building: Almost all the reported moisture issues that have negatively affected straw bale buildings in my experience have been a result of ignoring the need for overhangs, flashing at openings and/or foundation height and detailing.

The take-home lessons are several:

• Design and build overhangs that protect walls from prevailing winds and wind-driven precipitation. There is no one-size-fits-all formula for this. The “perfect” amount of overhang can vary from relatively small roof overhangs to full wrap-around porches. It is important to consider not just overhang width, but also roof height; multiple story buildings are generally not adequately protected by the roof overhang and may require skirt roofs, extended gables or other such strategies.

• Be sure that all wall openings have proper flashing above and below the opening. Water being shed by flashing should drip well clear of the wall plane. Any ledges or sills need to be properly detailed so that water does not end up entering the seam between the plaster finish and the opening.

• Properly moisture proof the seam between the foundation and the wall, to prevent moisture migration from the soil into the foundation and then into the wall. This can include sheet barriers, paints or the use of naturally moisture resistant material at this seam.

• Keep the walls an appropriate height above grade to avoid roof splash back from wetting the walls, and in snowy conditions from having snow pile against the walls. Proper gutters and/or grading materials that prevent splash back are the best options, as is a minimum 12-inch height difference between finished grade and the bottom of bale walls.

      Vapor permeable wall systems have distinct advantages in handling moisture loading within the wall system

Early naysayers of straw bale building were certain that moisture from within bale buildings (in cold climates) or from without (in hot climates) would be the death of all bale walls. After all, hadn’t thousands of well-insulated conventional homes experienced serious rot and mold issues due to vapor pressure driving moisture into the walls, causing condensation and all the failures associated with serious wetting? Conventional builders were learning to overcome these issues through the use of plastic vapor barriers on the warm side of the wall, preventing moisture from migrating through buildings materials and into wall cavities. Surely bale walls, with no barriers and an insulation material prone to rotting in the presence of moisture would fail at an alarming rate.

The fact that the early bale homes of the revival did not experience this issue was proof that something was going on that defied the common logic of the need for plastic vapour barriers. The answer was in the premise of vapor permeability, or the ability of certain materials (among them natural plasters and straw) to allow moisture to enter the material and slowly diffuse through the material itself to the drier side of the wall.

Not only did plaster and straw allow this process to successfuly occur, they happen to be excellent materials in this regard. Both materials offer vast amounts of moisture storage capability, able to adsorb (see sidebar) large quantities of moisture without any damage to the material, and the ability to quite freely allow this moisture to migrate under the right conditions, when the atmosphere on one side of the wall is drier than on the other.

Even in extreme climates, such as northern Canada, Alaska and Russia, where vapor drive in winter time can be very strong, bale homes are consistently able to show very low moisture content readings throughout the heating season.

Take-away lessons include:

• Don’t insert sheet barriers into a bale wall system. Their presence is actually more likely to cause moisture issues than prevent them, by trapping migrating moisture behind an impermeable barrier. If barriers are being used, ensure that they have a perm rating that is at least the equal of the bales/plaster.

• Use permeable plasters. These include clay/earthen plasters at the high end of the permeability scale, lime plasters, and lime-cement plasters at the lower end of the scale. Straight cement plasters are not permeable enough to allow proper moisture migration. The use of at least 25% lime content with a cement plaster will provide sufficient permeability. Definitely avoid acrylic-based plasters, which have very low perm ratings.

• Don’t use impermeable finishes/paints on permeable walls. If all the basic wall materials are good and permeable, applying a typical latex or oil paint to the surface can negate the permeability and cause issues. Ensure that all finishes are at least as permeable as the plaster.

•  Different permeability rates on the inner and outer side of the wall don’t seem to be problematic. Though conventional wisdom says that the warm side of the wall should be less permeable than the cold side, the sheer number of bale homes in cold climates with high perm clay plaster on the interior and lower perm lime-cement plaster on the exterior indicates that this type of imbalance is not problematic.

      Air tightness is important, and “breathability” is a misleading term

This is a lesson that doesn’t seem to have been as widely understood or accepted. Early in the straw bale revival it was said that bale walls didn’t experience moisture problems because they were “breathable.” The term “breathable” was used as an equivalent to “vapor permeable” and understood as such it is not problematic. However, plenty of bale enthusiasts took this to mean that the walls provided an actual air exchange; that somehow fresh air from outside would make its way inside and provide adequate ventilation for the building. This is certainly not the case.

Building science has definitely shown us that making a building as airtight as possible is a key part of making the building very energy efficient. Leaks anywhere in the wall system (or anywhere else in the building enclosure) allow warm air to cross through the building to the colder side, resulting in higher energy usage to keep the building tempered.

Bale buildings have the air tightness advantage of using large expanses of uninterrupted plaster as the main air barrier on the interior and exterior of the walls. As such, it is relatively easy to create a continuous barrier and to inspect and maintain the integrity of that barrier. Bale buildings have the disadvantage of using plasters that shrink as they cure, leaving inevitable gaps at all seams where the plaster intersects with other materials. The gaps that open up where plaster meets ceilings, windows, doors, exposed framing, electrical boxes and other dissimilar materials can be significant, and lead to bale buildings often scoring very low on air tightness tests. While many professional builders have created air fin details to adequately handle all these interruptions to the plaster (with some bale homes able to meet the strict air tightness standards of PassiveHouse), these are not widely adopted by bale builders.

The idea of creating airtight buildings has been met with a great deal of resistance from straw bale enthusiasts, resulting as it does in the need for mechanical ventilation. Many cling to the mistaken notion that their “breathable” walls will reliably allow enough fresh air into the building to keep indoor conditions healthy and pleasant. In most cases, it is gross air leakage around the perimeter of the plaster that allows for this exchange, and this type of leakage also increases the amount of moisture moving into the walls, increases the potential of water leakage on the exterior and allows insects into the walls. For maximum energy efficiency, it is imperative to build air tight and then ventilate appropriately; controlling the amount and quality of incoming air is a much better ventilation strategy than allowing leaks through cracks.

Take away lessons include:

• Consider every seam between the plaster finish and other materials and figure out ways to prevent leakage when the plaster separates from the seam when it cures. At the very least, these seams can be caulked, but this is the least long-lasting and effective (not to mention least environmentally friendly) way to handle air tightness. Air fins that use permeable sheet barriers (like housewrap) or solid materials (like Homesote or other board style products) are a more permanent and effective solution. Every seam should be thoroughly considered at the design stage, and clear details and instructions given on plans for builders to follow.

• Perform a blower door test on the building after the body coat(s) of plaster have been applied and cured/dried, and before the finish coat is applied. This will show where there are leaks that were not properly addressed, and these can be fixed.

• Perform another blower door test after the finish coat has been applied, to ensure that all issues have been dealt with, and to fix any remaining small leaks.

      We don’t know how much ventilation is enough ventilation

One of the lessons we have not yet learned from building science is how much mechanical ventilation is actually necessary for a relatively (or extremely) airtight straw bale house, and this would be a valuable lesson to figure out.

Current mechanical ventilation systems prescribed by building codes are designed to operate in homes that have little or no capacity for dealing with moisture (ie, no permeable wall system and few or no materials in the home that can act as moisture “sponges,” taking in moisture when humidity levels are high and releasing moisture when humidity levels are low), and homes that have significant amounts of poisonous off-gassing from materials within the home.

Most bale homes have a significantly different moisture profile than conventional homes. Permeable walls with a vast moisture storage capacity are very different than thin, painted wallboard over a plastic vapor barrier. Many bale homes also feature other interior finishes that are similarly good at acting as moisture sinks, like exposed wood, cob, interior clay plasters and earthen floors. Compared to many homes that have virtually no surfaces that are moisture permeable, bale homes can regulate large swings in humidity levels without mechanical assistance. In combination with a reduction in or elimination of toxic materials throughout the home, many bale homes do not require the amount of ventilation demanded by code.

At the same time, it is not wise to have a relatively airtight home with no ventilation whatsoever, especially if wood stoves, masonry heaters, pellet stoves or other combustion devices are used in the home. If the home is closed tightly against the weather for days at a time, there is a need to ventilate the home.

Take away lessons include:

• If you are serious about energy efficiency, be serious about air tightness. If your climate is moderate, this may not be so important, but if you live in a place where you are using energy to temper your indoor conditions on a regular basis, then reducing that energy usage is closely tied to air tightness.

• Provide an airtight straw bale home with some kind of mechanical ventilation. While many will balk at using mechanical ventilation (I did for many years), it is a good idea. If you use a mechanical heating system, a refrigerator or any other piece of mechanical equipment, then there is no point in being a purist and avoiding mechanical ventilation. The best units are very energy efficient and work very well.

• You probably don’t need to ventilate as much as codes require. Apply good judgment to the usage of your ventilation equipment. Many systems are triggered by humidistats, and will only turn on when indoor humidity levels are high. Monitor humidity levels and adjust the units to keep the building in the range that you desire.

• Passive or non-conventional ventilation may very well be adequate for straw bale homes. Solar hot air collectors, earth tubes, thermal chimneys and other such methods of encouraging air exchange are valid options to explore. While each comes with limitations, they can also be made to function adequately. Such strategies will be easier to implement if we can figure out what are the true ventilation needs of a bale home. Adequate monitoring coupled with homemade systems can be viable.

      Rain screen cladding is a reasonable option for straw bale homes

There are many circumstances in which it makes sense for a straw bale building to have a final finish that is not plaster. The aesthetic of a plastered straw bale wall is so intimately associated with straw bale building that it often seems like the choice to clad a bale building in any other way is some kind of defeat or is not a “real” bale building.

The truth is, there are many reasons for using an exterior cladding in conjunction with bale walls. The additional weather protection added by a rain screen cladding of any type – wood, brick, stone, metal or even rain screen plaster – removes the key concern of protection from precipitation. With rain screen cladding, bale buildings become viable in nearly any climatic condition and on any site.

The aesthetic of finished plaster is not universally loved. Many people who may otherwise be attracted to the positive qualities of bale buildings are dissuaded by the “need” for a plaster finish. In many parts of the world, plaster materials, equipment and especially the labor required to create a durable, beautiful, long-lasting plaster finish are not available or are cost prohibitive. Vernacularly appropriate claddings are always available, and can increase the appeal of bale buildings where such claddings are the norm.

Plaster finishes are also the most maintenance-heavy finish. Cracking of plasters, while it can be greatly minimized, is inevitable. For many, this is fine and acceptable. For a great many others, it is not. Hairline cracks hidden behind rain screen cladding are not a visual distraction to owners (or future buyers), and are much less likely to be vulnerable to moisture infiltration.

While the additional material used to make the cladding system may seem intensive, consider that the cladding can also reduce the need for extreme overhangs/porches, the use of high-embodied energy lime or cement-lime plasters in favour of earthen plasters and can greatly extend the lifetime of the plaster and the bales.

Take away lessons include:

• Bale walls behind cladding should still be properly plastered and the plaster should include proper air fin details. The plaster coating is still the wall’s main air barrier, fire protection, insect/rodent protection and structural element. However, plaster behind cladding does not need to look great and can often be comprised of a single, thick body coat, which can reduce labor time significantly.

• The cladding should be a proper rain screen, which means there should be an adequate vertical ventilation space (at least ¾-inch) between the face of the plaster and the backside of the cladding. The ventilation space should not be horizontal, as there will be inadequate natural circulation due to convection in a horizontal space. This vertical ventilation space should be free of blockage from the bottom of the wall to the top, and include some form of screen or mesh to prevent insects from moving into the space.

• The strapping used to create the ventilation space and provide attachment for the cladding should be applied over the plaster or, if embedded in the plaster the strapping should have adequate air fin detailing to prevent large leakage losses.

• Placement of windows, doors and all associated flashing will change significantly depending on the type of cladding used. Ensure that all detailing is suited to the intended cladding at the design phase and is carried out appropriately on site.

Recent gatherings of straw bale builders have had the lessons of building science front and center. Few bale builders came to construction from within the conventional industry, and it is interesting that a group of largely uninformed amateurs have, in a relatively short time, come to embrace the cutting edge of building science and the understanding and improvements it brings to design and building practice. It is important that we work as hard to spread these important lessons as it was to spread the lessons of low embodied energy and energy efficiency at the beginning of the straw bale revival. If we can make exemplary buildings that embody best practices and hit the highest targets of energy efficiency and occupant health, that will help to spread the rest of the straw bale message.

Chris Magwood is the Founder and Executive Director of The Endeavour Centre, a not-for-profit sustainable building school based in Peterborough, Canada.  His new book, Making Better Buildings is due out soon and you can read a review of it in this issue.

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