Building Strawbale Walls – Part II

It’s been a while since the last shed update in April. I’ve been pretty busy over the last few months – installing the septic for the “real” house in May, then finishing up and plastering the strawbale shed in early June, followed by trenching and installing water lines, electrical, and gas to the “real” house pad (still in the process on the last two). While this post will be on the strawbale shed, I’m planning to post pictures soon about my other work this summer.

Once most of the wall meshing was completed, I installed concrete window sills on the two lower windows. The sills were cast in three pieces from Quickcrete 5000 using a set of custom molds. I applied window flashing membrane to the wood underneath the sills and then bedded them in a urethane construction glue. Just in case it needs it, I also applied a bead of glue along the outer edges where the sill runs under the plaster to help prevent water intrusion underneath the plaster.

• 

• 

• 

• 

• 

• 

Shaping the door and lower window openings was the final task prior to plastering. My general approach was to staple mesh around the edges of the opening and then bend it back over itself, stuff the resulting void with straw, and then secure the other side of the shaping mesh piece to the existing mesh with zip ties.

Stuffing is a time consuming art. Because these windows are not shaped on either side of the outside only, but inside and the top as well, it takes some time of trial and error to stuff each segment so that it matches with the rest of the window. In the end it took about 8 hours to completely shape one window and about that long for the door.

• 

• 

• 

• 

• 

• 

• 

• 

• 

• 

• 

• 

By the end of May, I was finally ready to plaster the scratch coat. I plastered the walls using a mortar sprayer connected to a 20 CFM air compressor. Mom and Dad helped over the course of two days in early June. We used lime plaster from TransMineral USA mixed with cement sand in a 1:2 proportion. Dad did most of the mixing. I would scoop the plaster from a wheelbarrow into the sprayer and spray it onto the wall. Mom helped me trowel out the mortar once it was sprayed, leveling the high spots and filling in any areas that I missed. It started out slowly and then picked up once we got the process down. By the end we were moving fairly quickly. It was hard work but I think we all had at least some fun doing it!

The mortar sprayer worked very well and was definitely worth it, especially for the ceiling.

The plastering process had a steep learning curve. Part of the difficulty lies in the fact that it is not a common building material here in the US, so reliable information is scarce. Often, the information that is available is vague. Sometimes, the authors clearly do not understand what they are talking about; their terms are misapplied even though their methods work. Also, there are almost as many recipes and ways to apply and cure lime as there are plasters, most with varying results! Finding the correct sand is another confusing challenge. The shear breadth of the subject area and misinformation abounding online made this a difficult undertaking for me as a newbie. In the end the TransMineral instructions we used provided excellent results.

Plastering is definitely something I wouldn’t want to do everyday, but there is something satisfying about taking the plaster and applying it to the building. Although lime plaster is rarely used today due to the skill and conditions required to apply it, it’s high alkalinity (I got a small 2nd degree chemical burn from it), and the extended 10-day cure period between coats, it is a beautiful building material. While not as strong as concrete, it is lighter in weight and functions well in many applications. It possess some ability to self-heal small cracks. It breathes significantly better than concrete. Also, because the plaster is lime-based, it continues to cure slowly over time eventually turning into a calcium carbonate-sand matrix (similar to limestone but more sand).

• 

• 

• 

• 

• 

• 

• 

For curing, I covered the walls with sheet plastic and sprayed them down heavily twice a day with water for the first few days, then once a day for the remaining week. The plaster cured wonderfully to a light whitish-gray color and began to gain a lot of strength in the first two weeks (it takes 2 years to fully cure). It was much harder than I expected. Unfortunately, I used too much water during the curing process and in several places on the inside and one place on the outside, the straw began to mold slightly. Thankfully, I noticed before it proceeded too far and opened up the sheet plastic to increase the drying, but it smelled musty for a while afterwards.

With the scratch coat completed, the next steps for my shed are to complete and install the windows and then apply the next coat of plaster (brown coat). They have not been a priority with the other work going on this summer, but windows are coming along well now and I hope to have them completed in another few months.

• 

•