Eco Building: Fun with Straw-Clay Infill

Feeling like we accomplished the task of building our shop with a Gravel Bag Foundation and Timber Frame Construction, next we needed to provide some sort of insulation. We researched straw-clay building and found that it provides both insulation and thermal mass, plus it's totally natural and we could get everything we needed locally.

Straw-clay building has been used by Europeans for centuries with lots of success. Usually, straw-clay is completed by using forms between upright studs and the forms are packed with a straw-clay mixture. The forms are removed and the walls are left to dry before putting a protective coat on, such as natural plaster, which also lets the walls continue to "breathe" - very important in straw-clay construction, as this will prevent mold from forming and destroying the walls.

Our system was basically the same, however, we already had the "shell" of our shop completed with board and batten construction. We decided to try straw-clay infill where we used form boards between our upright studs on the inside wall and filled with straw-clay mixture. Normally, both walls would be exposed after removing the forms, but we just had the one inside wall exposed. We live in a semi-arid place with very low humidity. I definitely wouldn't recommend trying this in a wet, humid area.

Our Straw-Clay Recipe:

You will need:

• Lots of good Straw (not old or moldy)
• Lots of Clay
• Water
• Buckets (we used 5 gallon buckets)
• Wheel barrel or cement mixer to mix your clay and water.
• Tarp for mixing everything together on.
• Wire - We fastened wire in various places with screws from one stud to another - just to provide additional wall strength. 

Recipe:

Mix 2 buckets water and 1 bucket clay in wheel barrel or cement mixer to create a clay slurry. 

Pour over 3 buckets packed straw. 

“Fluff” the slurry and straw together to produce a light mixture, not too heavy. We mixed the straw and clay slurry together by hand.

Pack into your form boards which should already be in place.

Wait for about 1/2 an hour, remove form boards and let your walls dry.

This can be a time-consuming task, but friends can really help the process go quicker…and it can be a lot of fun working together. The smiles in the picture are genuine. It was truly a fun experience and I think everyone enjoyed it. The beers afterward were a bonus too!

Here, we're mixing the straw with the clay-slurry.  

Next, packing the straw-clay mixture into our walls between the walls and form boards. It's a dirty job, but someones gotta do it!

We spent about 8 hours on this particular day and we got about half of our shop walls (18' x 18') completed.

We waited about an hour before removing the form boards and viola!  - beautiful, natural walls that provide insulation plus some thermal mass. We waited about 2 months, to ensure the straw clay was completely cured, before protecting our walls with natural plaster.

Up Next: Natural Plaster