Posted by Jeffrey | Posted in The Playhouse | Posted on 23-06-2012
Tags: aprovecho, earthen plaster, hempcrete, light straw clay, natural building, playhouse, slip chip, straw bale, strawbale, timber frame
Location: Aprovecho, Cottage Grove, OR
Date: August 2011 – July 2012
This project was the main build in Aprovecho’s ‘Sustainable Shelter Series’.
The playhouse is 150 square feet, small enough to avoid costly permits in Oregon. It features a variety of different natural building materials and techniques to both educate and be a model of what is possible.
I had the pleasure of working on this structure for the best part of a year. Spending time at Aprovecho is always a joy, being surrounded by so many varied but inspired people.
The structure, stove, most walls and base floor pour of this build were completed on the seven week Shelter Series. Subsequent plastering and finishing were completed over the fall & spring practicum and times in between where I was work trading.
Here is a little rundown on how the playhouse was made.
The building features a timber framed Japanese wind brace structure. This alternative to knee bracing is an elegant way to provide sheer strength to the structure.
Four different wall systems were then installed.
Straw-bale for the North wall where most insulation is needed. The bales were stacked on top of one another like bricks, then pinned together using bamboo. The gaps between bales are then stuffed. This system is great as the walls go up really fast, the downside is that it takes longer to prepare the wall for plaster. Straw has an R value of 2.5 per inch.
Light straw clay was used for the East and West walls. Light straw clay is loose straw coated in clay slip to prevent rotting and burning. The mixture is then tamped into removable forms to create solid wall systems that can be plastered without further preparation. Light straw clay has an R value of 1 -> 2 per inch depending on how much clay is used and how much it is tamped.
Chip Slip was used in the Southern wall. This wall system is created from wood chips coated in clay slip, again to prevent rotting and burning. The slip chip system needs a form which will stay on the wall permanently, we used reed matting for this. Chip Slip has an R value of about 1 per inch.
Hempcrete was the final wall system used. Hempcrete is made from hemp stalks coated in lime, lightly tamped into a removable form.
For the East, South and West walls, a Larson truss system was used to eliminate thermal bridging (Wood transfers heat far better than insulation, if you have a timber spanning from the inside of the building to the outside, then this is a source of heat loss. The Larson truss avoid this).
Once the walls were in, the straw-bale was base plastered immediately. We used a clay/sand/straw plaster.
The light straw clay walls need to dry before plastering, this takes around 1 week per inch of wall. While you wait, it’s a great time to get the roof on!
A standard metal roof was installed on the North roof pitch, this will be hooked up to a rain water catchment tank. On the South side a green living roof will be installed.
The final part to our building was the earthen floor. After layers of drainage rock and insulating volcanic rock, we poured a 4 inch base floor. This was again mixed from sand, clay and straw. This floor will provide lots of thermal mass, sucking the hot air out of the room in the summer and storing heat in the winter.
This was a fantastic project to be involved with, it showcased natural building in Oregon’s best thinking.