Rammed earth and strawbale

rammed earth and straw bales, a great combination

 
   
 
 
 
   
         

Othona are a community with two sites, this one is on the Essex coast. The site is off grid and about 20m from the sea wall. The brief was to build a building which could operate winter and summer without central heating, just a wood burner to top up heat in the winter.

Othona believe that to experience community you must live in community, for at least a few nights in a row if not for longer. So people come for a long weekend at any time of year and the building is for sleeping and staying in. To be comfortable in heat or cold without central heating architects Zedfactory, started by Bill Dunster, designed a building which takes full advantage of the south face and solar gain which traps the available solar gain in thermally massive walls.

Most of the building was done by the community after a contractor had constructed the foundation and frame. We were asked to help with the rammed earth, mixes, formwork and organisation. The rammed earth forms the bulk of the east, north and west walls of the building. These thermally massive elements were topped with home made adobe blocks. After these were laid in place the whole thing was wrapped in straw bales. Click for more rammed earth and straw bale images.

Combining earth and straw bale is extremely easy, especially such a simple design. The main result is thermally massive

 

   
     
  Even in cold winter conditions south facing spaces collect heat by day in thermally massive walls and hold it with straw bale insulation  
     

walls which are highly insulated. These ensure a high level of comfort in summer and winter. Because the community don't use it continuously its ability to be comfortable even after a period where it is empty either in summer or winter is very important.

Building with volunteers means very low cost. The soil was sourced at site, bales were bought from a neighbouring farm. Working with volunteer labour is a bit unpredictable, how many, for how long? Since the community couldn't answer these  questions using commercial formwork, hiring it in, was out of the question. Instead we advised on how many used scaffold planks to buy. Once this was done we made up formwork which could remain in place if need be for months, as the total cost was around 300 for the boards. However the formwork was 20m long, easily enough to keep up to 30 people busy at any one time.

The technical aspects of tying straw bale to rammed earth physically, ensuring no gaps between the mass and the insulation, was done in two ways, both of which worked well. one was a simple wire tie through the walls where it was to be plastered over. The other method built strips of mesh into the wall which formed a horizontal tie into the bales.

One thing the community couldn't control was a planning restriction which meant the building had to be on 1.2m stilts against future flooding. So a contractor built a raised platform and erected a timber structure on top with a roof. This meant the community could finish the rest of the build themselves in their own time.

The community had planned to use as much material from site as they could. This started with a small group going to a range of different workshops to try to understand the different technologies they wanted to try, rammed earth, adobe and straw bale being the big three wall building techniques.

Because the roof was built before the walls the rammed earth could not be built to roof height. Adobe blocks were made to fill the gap between the top on the rammed earth and the under side of the roof, a very happy use of the two complimentary building systems.

     
       

    Rammed earth and adobe insulated with straw bales  

       

With the walls built and the bales in place the whole thing was lime plastered. This meant the walls being kept completely out of the weather, with no risk of physical abrasion. All the maintenance to the outside it to the render, a factor in the choice of materials for a community without deep pockets.    

       

    Second hand scaffold planks make cheap and effective formwork  

       

 

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