Building with Strawbales
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Completing the walls of our Arizona Straw Bale home
We had our straw bale house plans drawn for a post and beam construction with the walls being straw bale infill. This means that even without the straw bales, the wooden shell walls, including window frames and door frames are in place, along with the roof. All this was 100% complete before we began stacking any bales.
The straw bales can either be laid on their base, which provides a wall thickness generally 18-22", or as in our case laid on their sides, which provides a wall thickness generally 14-18". Although bales on their sides produce a slightly thinner wall, the orientation of the straw within the bales still allows the bales to provide roughly the same R-Value.
When stacking the straw bales, we generally wanted to overlay them in a running bond formation (like brickwork on a wall) to help give the greatest strength to the wall. However, since the window and door frames were already in place, with uprights from floor to ceiling, we were fairly limited in how much running bond was actually accomplished. In the case of our structure, though, this is not a problem as the floor to ceiling wooden framework also provides strength to the walls. In a more traditional "load-bearing" structure the running bond is more of an issue since there is no internal floor to ceiling wooden framework.
During the straw bale house construction, the stacking of the full size straw bales generally went very quickly. Cutting of the custom bales and notching of bales took the most time. When a full size bale would not fit into an opening, a smaller bale had to be cut. This was a simple process where we used bale needles to sew baling twine into the bale, thus creating smaller bales. The bale needles had been created from chain link fencing material and functioned quite easily.
Once the straw bales were stacked into position, and all of the openings which were large enough to hold a bale or custom cut bale were filled, several gaps still remained. Generally, these were above window frames and all along the edge of both angled ends of the house. In these locations, we used metal lathe to create a basket, then we filled the baskets with loose straw, packing them in as tightly as possible. The earth plaster went directly over the metal lathe just as it went over the straw bales. This is the case with many straw bale houses.
Our next step was to "pin" the walls together to provide strength. We used bamboo, which was placed 2 to a bale on the inside and 2 to a bale on the outside. Using the bale needles and baling twine, these bamboo reeds were sewn into place, allowing proper overlap of the upper and lower boards of the wall, thus tying the bales into place against the framework of the house, tightly like a corset.
Earth plaster does not like to stick well to exposed wood or other smooth surfaces. So, the next step was to cover the wood posts with burlap, which had been soaked in clay slip. We created the clay slip by mixing water and pure clay. The clay was obtained from a nearby subdivision which was under construction. At the site, huge mounds of clay were being taken from where roads were being built. We dunked the burlap into a 5gal bucket of clay slip, and then the prepared burlap was stapled directly on the wood. This was a nasty, nasty job and I went through several staple guns before completing the project. But, our straw bale house plans called for it, so I had to do it.
As you might imagine, straw bale walls have a lot of small gaps or insets where the bales come together. To reduce the amount of plastering, we made cob, which is a mixture of straw, sand, clay and water and packed it into these gaps. The cob, being so thick, had to be made by hand in a wheelbarrow using a mortar hoe. Once applied, this looked like the mortar in between the bricks of a brick wall.
Once the burlap was up and the cob was on, the next step was to spray a clay slip over the entire walled structure, inside and out. The reason for this is the simple concept that earth likes to stick to earth. Spraying a clay mixture over everything makes applying the earth plaster easier. We used a texture sprayer which is normally used to spray texture onto sheetrock walls. Once this was complete, we were finally ready to put on the plaster.
The first coat of plaster was applied by hands smearing it into the straw bales. Working the plaster into the straw was critical. After the first coat went on, we came back and applied more to the major "low" spots on the wall to even things out a bit. The second and subsequent coats went on with trowel.
The first coat of earth plaster was 80% sand and 20% clay, water, with as much chopped straw added to it as it would hold and still be "smearable". The straw was put through a mulcher to shorten it. We got our dirt from the pile leftover after installation of our septic tank. We had a tremendous amount of rock in that soil, so we had to screen all the soil through 1/4 inch screen. Our soil was naturally 90% sand and 10% clay. So, we added some of the pure clay mentioned earlier. The plaster was mixed in a concrete mixer, which made things much easier than the mix by hand method.
The second coat was also mixed 80/20, but with less straw since it was to be troweled on. Also, this straw was double cut, or put through the mulcher twice to shorten it even more. And, this second coat was made with 4% lime water, instead of just plain water. The lime water was used to prepare the surface for the following lime coats. Just like earth likes to stick to earth, lime likes to stick to lime. We bought 85 gal trash cans and mixed 72 gal of water to 3 gal of hydrated lime. We used a bucket to dip out the appropriate amount of water for each batch made.
The picture below shows the exterior after the second coat which included lime water. The color of the plaster, which was much darker, began to lighten significantly. The final coat is virtually all white.
The cob, first coat and second coat using lime water was applied inside and out. We are leaving the inside walls at this stage. However, we expect to apply Venetian Plaster throughout the interior as our finish coat on the straw bale walls. We applied the next two coats of lime plaster on the exterior in March and April once the temperature stayed above freezing for at least a week at a time. The lime coats must dry slowly and if it freezes before it cures, a "blowout" will occur. I don't know what this is and I didn't want to find out....so we waited!
For the first lime coat, which was coat number 3, we screened concrete sand through a window screen. We hydrated the lime per normal instructions and let set up for several days. We also mixed the "subdivision" clay into a wet mixture and had it set up for a few days. We then mixed these three components in the following ratio: 1/2 part clay, 1 part lime, 2 parts sand. We added water to give it a nice smooth-able texture. The second lime coat which went on the next day consisted of 60 grit silica sand and lime. The following ratio was used: 2 parts silica, 1 part lime, sufficient water for texture.
Since the lime coats needed to dry slowly, we covered the walls with tarps and hung a misting system to provide 24 hour mist to the wall. Additionally, we had to spray the walls since we live in such an arid climate. Notice in the picture below, the mist falling. Both of these coats went on extremely thin. We left them misted and covered for 3-5 days. It takes approximately 30 days for the full chemical reaction to occur which has the lime taking carbon from the air and calcifying into a mortar like covering.
So, living with tarps hanging around our house was part of the process and is normal when doing any straw bale house construction. In fact, many straw bale houses have tarps hanging on them for months while the owners finish the project. My wife and I are a little more "driven" than most, so we got finished fairly quickly.
The picture below is showing the second lime coat on, but still drying. In this picture, the lime has been up for about 4 days. It is drying nicely, but we still have to keep an eye on it to make sure it doesn't dry too quickly.
By the first of May, our house was fully covered with the lime coats on all four sides. After the 30 day curing period, we did have some cracking, but nothing major. However, the cracks needed to be repaired before the monsoons arrived in July. So, in late May, we "spot" applied an additional lime coat to the cracks to fill them in.
Once October rolls around, we'll apply a lime wash and colored pigment. The lime wash (1 part water, 2 parts lime) will further fill in the hairline cracks and the colored pigment, ochre purchased from earthpigments.com will return the exterior color to the original deep color we had with the first coat of earth plaster. Also, we'll add methyl cellulose as a binder and drying retardant. The mixture will be applied with a heavy masonry brush. Hopefully, with the misters, the tarps and the methyl cellulose, the limewash will dry slowly enough to prevent any cracking.
The most important thing at this point is that whenever it rains, we don't care!! This is very liberating as we bit our nails all throughout the construction hoping rain would not undo our work. You have to go through what we did to fully appreciate this feeling....bring on the rain!!!
'width' is a duplicate attribute name. Line 1, position 36.
'width' is a duplicate attribute name. Line 1, position 36.
The straw bales can either be laid on their base, which provides a wall thickness generally 18-22", or as in our case laid on their sides, which provides a wall thickness generally 14-18". Although bales on their sides produce a slightly thinner wall, the orientation of the straw within the bales still allows the bales to provide roughly the same R-Value.
When stacking the straw bales, we generally wanted to overlay them in a running bond formation (like brickwork on a wall) to help give the greatest strength to the wall. However, since the window and door frames were already in place, with uprights from floor to ceiling, we were fairly limited in how much running bond was actually accomplished. In the case of our structure, though, this is not a problem as the floor to ceiling wooden framework also provides strength to the walls. In a more traditional "load-bearing" structure the running bond is more of an issue since there is no internal floor to ceiling wooden framework.
During the straw bale house construction, the stacking of the full size straw bales generally went very quickly. Cutting of the custom bales and notching of bales took the most time. When a full size bale would not fit into an opening, a smaller bale had to be cut. This was a simple process where we used bale needles to sew baling twine into the bale, thus creating smaller bales. The bale needles had been created from chain link fencing material and functioned quite easily.
Once the straw bales were stacked into position, and all of the openings which were large enough to hold a bale or custom cut bale were filled, several gaps still remained. Generally, these were above window frames and all along the edge of both angled ends of the house. In these locations, we used metal lathe to create a basket, then we filled the baskets with loose straw, packing them in as tightly as possible. The earth plaster went directly over the metal lathe just as it went over the straw bales. This is the case with many straw bale houses.
Our next step was to "pin" the walls together to provide strength. We used bamboo, which was placed 2 to a bale on the inside and 2 to a bale on the outside. Using the bale needles and baling twine, these bamboo reeds were sewn into place, allowing proper overlap of the upper and lower boards of the wall, thus tying the bales into place against the framework of the house, tightly like a corset.
Earth plaster does not like to stick well to exposed wood or other smooth surfaces. So, the next step was to cover the wood posts with burlap, which had been soaked in clay slip. We created the clay slip by mixing water and pure clay. The clay was obtained from a nearby subdivision which was under construction. At the site, huge mounds of clay were being taken from where roads were being built. We dunked the burlap into a 5gal bucket of clay slip, and then the prepared burlap was stapled directly on the wood. This was a nasty, nasty job and I went through several staple guns before completing the project. But, our straw bale house plans called for it, so I had to do it.
As you might imagine, straw bale walls have a lot of small gaps or insets where the bales come together. To reduce the amount of plastering, we made cob, which is a mixture of straw, sand, clay and water and packed it into these gaps. The cob, being so thick, had to be made by hand in a wheelbarrow using a mortar hoe. Once applied, this looked like the mortar in between the bricks of a brick wall.
Once the burlap was up and the cob was on, the next step was to spray a clay slip over the entire walled structure, inside and out. The reason for this is the simple concept that earth likes to stick to earth. Spraying a clay mixture over everything makes applying the earth plaster easier. We used a texture sprayer which is normally used to spray texture onto sheetrock walls. Once this was complete, we were finally ready to put on the plaster.
The first coat of plaster was applied by hands smearing it into the straw bales. Working the plaster into the straw was critical. After the first coat went on, we came back and applied more to the major "low" spots on the wall to even things out a bit. The second and subsequent coats went on with trowel.
The first coat of earth plaster was 80% sand and 20% clay, water, with as much chopped straw added to it as it would hold and still be "smearable". The straw was put through a mulcher to shorten it. We got our dirt from the pile leftover after installation of our septic tank. We had a tremendous amount of rock in that soil, so we had to screen all the soil through 1/4 inch screen. Our soil was naturally 90% sand and 10% clay. So, we added some of the pure clay mentioned earlier. The plaster was mixed in a concrete mixer, which made things much easier than the mix by hand method.
The second coat was also mixed 80/20, but with less straw since it was to be troweled on. Also, this straw was double cut, or put through the mulcher twice to shorten it even more. And, this second coat was made with 4% lime water, instead of just plain water. The lime water was used to prepare the surface for the following lime coats. Just like earth likes to stick to earth, lime likes to stick to lime. We bought 85 gal trash cans and mixed 72 gal of water to 3 gal of hydrated lime. We used a bucket to dip out the appropriate amount of water for each batch made.
The picture below shows the exterior after the second coat which included lime water. The color of the plaster, which was much darker, began to lighten significantly. The final coat is virtually all white.
The cob, first coat and second coat using lime water was applied inside and out. We are leaving the inside walls at this stage. However, we expect to apply Venetian Plaster throughout the interior as our finish coat on the straw bale walls. We applied the next two coats of lime plaster on the exterior in March and April once the temperature stayed above freezing for at least a week at a time. The lime coats must dry slowly and if it freezes before it cures, a "blowout" will occur. I don't know what this is and I didn't want to find out....so we waited!
For the first lime coat, which was coat number 3, we screened concrete sand through a window screen. We hydrated the lime per normal instructions and let set up for several days. We also mixed the "subdivision" clay into a wet mixture and had it set up for a few days. We then mixed these three components in the following ratio: 1/2 part clay, 1 part lime, 2 parts sand. We added water to give it a nice smooth-able texture. The second lime coat which went on the next day consisted of 60 grit silica sand and lime. The following ratio was used: 2 parts silica, 1 part lime, sufficient water for texture.
Since the lime coats needed to dry slowly, we covered the walls with tarps and hung a misting system to provide 24 hour mist to the wall. Additionally, we had to spray the walls since we live in such an arid climate. Notice in the picture below, the mist falling. Both of these coats went on extremely thin. We left them misted and covered for 3-5 days. It takes approximately 30 days for the full chemical reaction to occur which has the lime taking carbon from the air and calcifying into a mortar like covering.
So, living with tarps hanging around our house was part of the process and is normal when doing any straw bale house construction. In fact, many straw bale houses have tarps hanging on them for months while the owners finish the project. My wife and I are a little more "driven" than most, so we got finished fairly quickly.
The picture below is showing the second lime coat on, but still drying. In this picture, the lime has been up for about 4 days. It is drying nicely, but we still have to keep an eye on it to make sure it doesn't dry too quickly.
By the first of May, our house was fully covered with the lime coats on all four sides. After the 30 day curing period, we did have some cracking, but nothing major. However, the cracks needed to be repaired before the monsoons arrived in July. So, in late May, we "spot" applied an additional lime coat to the cracks to fill them in.
Once October rolls around, we'll apply a lime wash and colored pigment. The lime wash (1 part water, 2 parts lime) will further fill in the hairline cracks and the colored pigment, ochre purchased from earthpigments.com will return the exterior color to the original deep color we had with the first coat of earth plaster. Also, we'll add methyl cellulose as a binder and drying retardant. The mixture will be applied with a heavy masonry brush. Hopefully, with the misters, the tarps and the methyl cellulose, the limewash will dry slowly enough to prevent any cracking.
The most important thing at this point is that whenever it rains, we don't care!! This is very liberating as we bit our nails all throughout the construction hoping rain would not undo our work. You have to go through what we did to fully appreciate this feeling....bring on the rain!!!
'width' is a duplicate attribute name. Line 1, position 36.
'width' is a duplicate attribute name. Line 1, position 36.
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