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).
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.
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.
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.
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.
A quick overview:
An in depth talk on slip straw wall systems by Michael Smith:
Mixing Slip Straw by Michael Smith:
- Light Straw Clay Handout, Ben Waldman
- Light Straw Clay Workshop Slides, Ben Waldman
- All About Larsen Truss System, from Green Building Adviser.
- 2015 International Residential Code Appendix AS (Light Straw Clay Construction)
- Book: The Art of Natural Building by Joseph Kennedy
- Book: Graphic Guide to Frame Construction by Robert Thallon
- Web / Book: Eco Nest Homes. Stock plans, wrote the current code, book available for purchase on slip straw and infill systems.
- 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.