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Light Straw Clay Workshop Resources

SlipCrew

What is Light Straw Clay (LSC)?

LSC is a combination of clay slip and straw that can be used as insulation in a wall system. Straw is coated with clay slip but not flooded and then tamped into formwork. The formwork is removed, the slip straw allowed to dry, and then plastered.

Also known as light clay, straw clay, slip-straw, leichtlehm (Germany).

Clay Slip

Making clay slip is easy. Find a suitable clay soil, add water, and mix until a paint-like consistency has been reached. Some tips:

  • Presoaking your clay will make it significantly easier to mix.
  • Screening your clay before and after will it easier to mix your clay slip and your slip-straw. (1/4″ screen is sufficient)
  • Making lots of smaller quantities will ensure a more uniform consistency.

Straw

A wider stalk straw such as wheat straw is ideal. The wider stalk holds more air and will be more insulative. Ensure your straw is not moldy, wet or smelly before you mix it. Moldy straw can cause serious problems. Buy more than you think you’ll need.

Mixing

There are a variety of techniques. Find what works best for you. On the ground using pitchforks, on a raised table mixing by hand, or through a homemade bicycled powered machine are just a few of the ways to get your straw covered in clay slip. Don’t be afraid to experiment.

Ensure that all of your straw now looks “dirty”. Less clay = higher R-value, less bonding. More clay = more bonding, lower value.

Installation

There are a variety of framing systems in which you can install light straw clay. The more effective ones (Larsen Truss, Double Stud Wall, Post & Beam) help limit or eliminate thermal bridging. If you don’t have experience with framing, seek help. Most wall you’ll want to install light straw clay in are structural.

Strong form work is critical. Use 1/2″ OSB or thicker and screw it in liberally. After placing a couple of heaping handfuls of your mixture into the form work, tamp it. Consider this while tamping: the denser your wall is the less room for air and the lower the insulative value. Not enough tamping and your wall will have voids and won’t be nearly as strong.

Remove the forms immediately and allow it to dry.

Videos

A quick overview:

An in depth talk on slip straw wall systems by Michael Smith:

Mixing Slip Straw by Michael Smith:

More Resources

Web:

Books:

Material Vendors:

  • Wardle Feed & Pet Supply. 7610 W 42nd Avenue, Wheat Ridge, CO 80033. Phone: (303) 424-6455. Usually lots of strawbales available.
  • Pioneer Sand. 7608 Highway 93, Arvada, CO 80403. Phone (303) 279-4748. Best screened clay in town, sold by the ton.
  • Stone Leaf Pottery. 5891 Nolan St # B Arvada, CO 80003-5592. Phone: (303) 463-8081. Bagged clay, good for very small projects or precision work.
  • Extra Building Materials. 400 W 53rd Pl Denver, CO 80216. Phone: (303) 296-8090. Used lumber and other miscellaneous building materials.
  • Burlap can usually be found at your local coffee roaster.
  • For bulk, cheap straw look on Craigslist.

Light Straw Clay Workshop Report

Slip straw is a great beginning natural building technique because it is quick to learn, simple, Light Straw Clayand can be used in new buildings or to add non-toxic insulation to existing buildings. Ben Waldman, contractor and natural builder, taught a slip straw workshop at the GrowHaus in Denver last month.
Workshop participants included permaculture students, landscapers, farmers, and people who wanted to explore options for building projects they have coming up.

Mixing Clay SlipSlip straw is made by combining fiber (preferably wheat straw) with a runny clay slip. The hollow shaft of the straw creates air pockets, while the clay binds the fiber together and adds thermal properties.

Once the straw and clay slip have been mixed to the right consistency, the mixture is added into a wall’s framing and then tamped down.

Slip straw is not load bearing on its own, but is meant to be used as an infill for framed walls. Different framing designs were discussed, along with their advantages and disadvantages for combination with this technique.

Once the straw has been fully tamped into the wall, it will be finished with a clay or lime plaster. Clay is able to control moisture on its own, which helps protect infill of the wall against mold or Mixing Plasterrot.

Clay plaster is another huge topic that will be taught at a future workshop, but students in this class got to have some hands-on introduction.

Burlap coated in clay slip was added over wood framing members to help bind the plaster to the wall.

Students left the workshop having gained firsthand experience with natural materials, knowledge of how to achieve the right mixtures for slip-straw and the steps involved in applying this method for better wall insulation, and access to additional resources.