Light Straw Clay Construction (New Society Publishers, 2017) by Lydia Doleman is an essential how-to guide for those looking to start light straw clay projects. From the basics of LSC to project plans featuring LSC, Doleman gives readers everything needed to start out on their own successful projects. In the following excerpt, she goes through the appropriate ways to use light straw clay in your projects.
Of the many natural wall systems to choose from, there are many reasons to choose a light straw clay wall system. Straw clay is highly compatible with framed wall systems because it is a non-load bearing material. Light straw clay can be infilled in nearly every wall framing system, be it timber framing, pole framing, conventional lumber framing, or framing specifically designed for straw clay infill.
LSC is also excellent retrofit insulation because preexisting walls can be furred out to any thickness. Furring out a wall simply involves adding stud material to the desired depth of wall. This can be done to the interior of a building or to the exterior. Using staggered studs or Larsen trusses also improves the insulation’s performance because it allows the creation of a continuous thermal envelope, virtually eliminating the thermal bridging that occurs in a conventionally framed building (where solid studs create breaks between insulated stud cavities).
Interior walls can be infilled with straw clay in buildings that have exterior wall systems of other materials. Interior walls can benefit from the soundproofing and thermal mass that straw clay provides, and they provide a seamless look because they take plaster as well as other natural wall systems. If done with good and consistent formwork and with attention to detail, the walls can be very flat, lending themselves to very smooth finish plaster, which leads to less “dusting” through the life of the wall.
LSC’s compatibility with conventional framing systems makes it easier to find contractors who can provide straightforward estimates for a project.
Wall systems or walls with lots of openings, like the south side of a passive solar building in the Northern Hemisphere, are highly compatible with straw clay, whereas cob, adobe, and straw bale are hard to work with around windows, doors, and other openings. It’s a somewhat common practice to design a building that takes advantage of the high R-value of straw bales for the north, west, and east walls of a building, but use LSC in the south wall, which has the bulk of the glazing (windows).
One of the advantages light straw clay has over cob and adobe and other natural wall systems is that it slumps and sags very little while being installed, allowing an entire wall cavity to be filled in one work session. As long as tamping is consistent and there are not long periods of drying time between installations in the same wall/stud cavity, there is also very little shrinkage.
Many projects, particularly in urban areas, have to be carried out in limited space. When there is limited square footage to work with, the 18-inch to 24-inch width of straw bales or cob may rule these wall systems out because they eat into usable space. In urban areas and sites with limited space, straw clay can be an excellent choice to create thinner wall systems that are still highly insulative. LSC can be made to fill any wall width that can reasonably dry within the timeframe of most building seasons. Most LSC walls do not exceed a 12-inch thickness.
Straw clay is very fire resistant. Tests conducted by Joshua Thorton and John Straube found that, based on ASTM standards E 111 and E 84, LSC would very likely meet the conditions required for a fire-resistant period of four hours. They also reported that LSC is a “highly ductile material with the potential to absorb a fair amount of energy in the event of seismic activity.” (Thornton, Initial Material Characterization of Straw Light Clay, Canada Mortgage and Housing Corporation, 2005.)
As each piece of straw has been coated in clay and packed in the wall, there is very little that can actually combust. Although, the walls are breathable to vapor, the continuous wall envelope should not have open channels for sufficient quantities of oxygen to be present, which also helps LSC resist combustion. Like a lot of dense materials, it may only smolder.
According to architect Franz Volhard, one of the European leaders of earthen construction methods, his own fire tests of LSC demonstrated:
Light earth responds passively to the effects of flames, i.e. it does not contribute to the spread of fire.
• The formation of an “insulating” charred layer protects the surface of underlaying materials from direct exposure to the flame, which increases with flame duration.
• Neither smoke, nor fumes nor perceptible combustion gases were produced.
• No particles fell from the specimens which could have contributed to the spread of the fire.
• Compared with wood-wool magnesite-bonded panel, the fire behavior was better with less charring and no smoke development.
These results suggest that straw light earth could be classed as B1 “Not easily flammable.”
Given that the required formwork for LSC is usually very flat and rigid and resists bending under pressure (plywood with “strongback” reinforcement), it is easy to form flat walls into very rectilinear structures, be it craftsman-style bungalows or modernist buildings. Light straw clay can also be made into bricks. Once dry, they can be placed directly in wall cavities, stacked to insulate thick walls in wetter climates (where there wouldn’t be enough time in a building season for LSC infill to dry), or cut and fit in to odd spaces that need insulation. Curved walls and window openings can also be made with light straw clay, although they require more detailed planning and curved formwork.
Due to the nonstructural aspect of light straw clay, a superstructure must be erected prior to installing the wall insulation. Therefore, you need a dry work space, in most situations. There are various methods of infill, some of which leave the roof framing until after the walls are filled in. In wet climates, having a roof structure up allows more time for installing the LSC and is added insurance should a storm happen during an anticipated dry season. However, having a roofless building allows for significantly more airflow to help facilitate drying.
Other applications of light straw clay are ceilings and floors. LSC can be used between floor joists if permanent formwork is used, or by implementing what is referred to as “light earth reels” (Volhard) or “chorizo style” (they look like sausage rolls), where the straw clay is wrapped around a stick or bamboo rod and suspended between the floor joists. Adequate airflow must be provided, as it requires a much heavier clay slip to wind the straw and clay around a stick that will adhere while being suspended overhead. In addition to all the other advantages of LSC, the advantage of this method is that it takes plaster very well; plus, it can be very artistic.
If you live in an area with wind-driven rains, you might want to consider a rainscreen to protect your finished walls. Wind-driven rain does not rule out building with LSC, but it is best to plan for worst-case scenarios. This is also true for areas of high snow. A snow drift next to an earthen plaster or lime-plastered wall will cause problems. A rainscreen (even for a half wall) can be a smart choice to mitigate climatic issues.
If you are considering building a full-sized residence or something over 800 square feet, consider building a small shed or outbuilding first. It’s much more cost effective to work the kinks out on a small building than on a big one. There are so many little details in an LSC building. Things will be much more streamlined if you are familiar with its idiosyncrasies, so it’s best to start with a smaller project rather than jumping right into a big one. You may even discover that you don’t want to use LSC on the larger structure, or that you may want to use it in only particular parts of your building. Starting small will be much less stressful, too; you won’t feel the same time pressure about the myriad daily decisions you have to make on a construction project if you have gone through them once on a smaller project. Plus, you will have the added bonus of having the smaller structure as a place to put all your tools (or yourself!) while you work on the bigger one.
(Due to code constraints and permit costs, a lot of people choose to build smaller structures under inspectors’ radar. But be careful with this. Small “outbuildings” are often characterized by code as sheds, or non-inhabited agricultural buildings; often they are not required to meet code, but different municipalities have different requirements, so it is important to thoroughly look into your local building codes.)
Light straw clay is labor intensive. Even with a tumbler (more on that later), you will need a group of people to pack the straw into the wall cavities and move forms. It takes a minimum of five people to mix slip, mix straw clay, and pack it into walls. You could do this all solo; but it would take you several seasons for a whole house. But try turning this disadvantage into an advantage. Take the more-the-merrier approach, and turn your worksite into a work party! Remember that the labor can be done by practically anybody, and it’s relatively quiet and pleasant.
The rule of thumb with straw clay is that you need approximately one week per inch for dry time. For the average 12-inch-thick wall, that’s 12 weeks (more incentive to get the walls done quickly). During that time there are a lot of other parts of a project to complete, but especially in locales that have short building seasons and extreme weather, you must plan ahead for adequate drying time. With longer dry times, mold can be an issue. Little white fuzzy spots of mold sometimes appear, especially in areas with low air circulation or where an LSC mix with a higher proportion of clay was used, e.g., tops of stud cavities and repairs. (This light white mold is harmless.) If using a water thinner, like borax, in your slip, it can cause efflorescence, which is a buildup of salts that crystallize on the surface as they dry. These are harmless, and can be gently scraped off.
The amount of moisture dissipating from a straw clay wall system can sometimes overload an interior and cause surface molds to appear on wood, especially if the wood wasn’t completely dry, or it wasn’t treated/sealed. The use of fans and dehumidifiers is highly encouraged. Propane heaters are not recommended; they produce a fair amount of moisture in their heat production.
In systems where the formwork is permanent, there is less exposed surface area for the
LSC. In situations like this, or when dry time is short and/or humidity is high, care must be taken not to make too heavy a clay mix; you want to allow the LSC to dry well in the closed wall cavity. The use of water thinners to limit the amount of moisture in the slip is advised. Once again, the use of fans and dehumidifiers is key to facilitating the drying of an LSC wall system.
There are some situations that are not right for straw clay. You should consider other options if you:
• live in an area with an excessively short building season (one where wet weather is followed closely by freezing weather)
• have no access to materials for building the supporting structure
• have limited access to straw or clay
• have no labor pool
• live in a building with preexisting termite or mold issues
• have a major straw allergy.
These are some of the reasons you might want to explore a different wall system. Note, however, that some innovative builders have implemented the use of straw clay bricks for short-season building. The increase in surface area greatly decreases dry times, and the bricks are light and stackable.
Some regions in North America do not have a code for the use of straw clay as an insulating wall system. This could make a permitted project challenging because you would have to work with the local building department for permission to build this way. Also, some banks may not grant a loan for an unconventional wall system; and some insurance companies will not insure an unconventional project.
The most obvious property of LSC is its insulative ability. Insulation levels are rated by their “R-value,” which is the measure of a material’s resistance to heat flow. A higher R-value means better thermal performance. The R-value of LSC walls is mostly contingent on the density to which it is packed and the amount of clay used. Good tests have been done on the R-value of LSC by the Design Coalition, Douglas Piltingsrud, Franz Volhard, and others.
The R-value of a wall assembly will also depend to some extent on what type of wall system is in place. If building a structure and not using split stud or Larsen trusses, the thermal envelope will not be continuous; the insulating quality of the entire wall system is diminished because there will be thermal bridging through the studs.
Given the broad range of densities achievable with LSC, it is important to achieve the correct one to meet the desired R-value for a wall system. Field experiments are required to find the ratios that will yield your desired density.
The following is a guideline recipe by Douglas Piltingsrud (designcoalition.org):
Following is a basic formula for making a wall density of 13 pounds/feet to the third (pcf) with a thermal resistance of R-1.69 per inch. A 12-inch (30 cm) thick wall with this density will yield an insulation value of R-20. At a 15-inch (38 cm) wall thickness, the R-value would be 25.4.
Prior to commencing the LSC wall infill, it is important to figure out what quantities of clay are needed to achieve the wall density you are looking for.
Build a minimum of three boxes that have one cubic foot in their interior. Use screws to put it together on at least one side, so you can remove a side of the box to assess compaction and remove the sample.
Using gallon buckets or a similar scalable vessel, and starting with the slip formula above for an optimal mix of 13 pounds/feet to the third, make some test batches. If you are using site subsoil with an unknown percentage of clay instead of bagged clay, then make a number of batches using different amounts of subsoil and ranging from a heavy slip to very light slip, then mix each of the slips with 6.7 pounds of straw, and tamp the LSC into the forms. Three forms can give a very good range. Label your forms, so you’ll remember which mix has which amount of subsoil. Remove one side of the form after tamping to make sure compaction is suitable. Remove the straw clay sample and let dry. Expedite this process with heat and air movement (sun is great too, but protect from rain) to dry the samples. Once dry, you can weigh the samples to find the achieved densities.
You can then evaluate both the variability of the density of the mix based on the amount of subsoil in each sample and the variability of the density of the sample related to the level of tamping.
An optimal mix will yield a dry sample of 13 pounds/feet to the third, which is R-1.69 per inch. If you have a lighter sample, you will have a higher R-value, but you may be losing the necessary binder by having too little subsoil in your slip or insufficient tamping. If you have a heavier sample, then you will have a lower R-value, which means that the subsoil quantity is higher than optimal and/or you are compacting the mixture too much. With testing and practice, it is possible to achieve an optimal density of mix and to get a feel for the correct level of tamping. Other desired densities and R-values can be achieved using the same process and varying the amount of slip and compaction.
No testing has been done yet on the acoustic qualities of an LSC wall system. However, Franz Volhard calculates sound reduction by “using the values of other massive building elements with a corresponding mass per unit area.” He adds: “Earth building materials add mass to a timber frame structure and it is possible to achieve good sound insulation using a simple, single-skin construction. Compared with other massive wall infill materials — all earth materials and light earth in particular are softer and more elastic. Sound vibrations are softened and attenuated. As a heavy but soft building material, earth therefore offers excellent sound insulating properties.”
Given the variability in density, the potential for split-stud construction and the variety of skins that are compatible with light straw clay, the potential for sound absorption is high.
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