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

Eco-Building on the Cheap: Foundation & Framing

We wanted to build a shop to do projects, plus we needed a place for our Balance of Systems (BOS) for our off-grid equipment. Other than our chicken coup, this was our first building attempt together. We wanted it to be functional but didn't have a huge amount of cash to spend on it. We also wanted to use local products as much as possible and started with a gravel bag foundation. The gravel came from our friend's gravel pit...the sandbags, unfortunately, had to be obtained through an online store.

Gravel Bag Foundation...

• Filled sandbags with gravel to about 3/4 full, tamped, then used pieces of wire sewn through the top of each bag to hold together.
• First tier of bags were laid flat on the leveled ground, butted up against each other (end to end) to form our 18 x 18 shop foundation.
• Two strips of barbed wire were afixed on top of the first level of bags around the entire perimeter. This is supposed to help hold the next level of bags in place.
• Second tier of bags were laid on top of the first tier, over the barbed wire and staggered from the first level so that the seams were offset between the two layers. 
• Covered the entire bag configuration with chicken wire and used fence staples to secure to the bags.
• Used cement in various places on top of the bags to help level the bag foundation in preparation for the sill.
• Foundation was cobbed (2 coats) and a thick cement coat was added to complete the gravel bag foundation after we finished framing the shop.

Now, normally with a bag foundation you would also have made a gravel trench that went below the frostline with your bag founation on top. This would prevent damage to your foundation and structure if the ground heaved during extreme temperature changes. We chose not to do this, only because much of our earth here already contains a fair amount of gravel and rock. Hopefully, it won't be an issue for us!

Framing the Shop...

• Obtained wood locally from a small family business.
• Created a sill with 2 x 8's and drove 3/8" rebar, 3 feet long through the sill into the bag foundation to secure the sill to the foundation.
• Used 6 x 6's for corners and doors & doubled up 2 x 6's for posts on about 4.5' centers. 
• Used 2 x 4's for horizontal support and tying the posts together.
• Researched high wind load roofs and, from recommendations, decided to build gabled roof ends which are supposed to be excellent in hurricane prone locations.
• Constructed an 11/12 roof pitch - about 42 degrees - ideal conditions for our latitude for winter sun conditions if we decide to add solar electric or solar hot water later.
• Built rafters on 2' centers and used hurricane straps as extra security. 
• Bought used metal for roofing, also obtained locally.  
• Designed the overhang on the south side to keep out summer sun and let the winter sun in. This allows us to take advantage of the low sun from mid-Oct to mid-April and helps heat the interior of the shop.
• Used Board and Batten system: 1 x 6s placed and secured vertically from top horizontal beam to bottom (sill); 1 x 3s placed and secured vertically over the gaps between the 2 x 6s. I love the look and feel of it!

Shop in it's "almost complete" form. Total spent: about $2000

Up Next: The Fun Part...

Straw Clay Infill (for insulation & thermal mass).