The Advantages and Appropriate Use of Light Straw Clay

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.

2×4 interior walls are infilled with LSC to aid in sound dampening and creating a sense of privacy for this small 800-square foot three bedroom home. Photo credit: Lydia Doleman

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, doing away with 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 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″ to 24″ 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″ thickness.

Rounded corners are achievable with LSC wall systems by using rounded forms. Photo credit: Lydia Doleman

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 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.” (Franz Volhard, Light Earth Building: A Handbook for Building with Wood and Earth, 2016, p. 225.)