Sustainable Building Design and Construction 2008

Wall systems

The wall systems of the MPAC building are one of its most unique features.

The building is based on upon eight load-bearing columns made of large, round, hemp-straw bales. These 4x4 foot round bales are very tightly compacted when they are made, and stacked three high they make for very stable and strong columns.

The straight walls sections between the round bales are infilled with regular, rectangular hemp-straw bales. Both the round and square bales come from Moorcroft Hemp Farms www.hemphomes.ca , located just a few kilometres north of the building site.

All the bale wall sections are covered with a slip coat of clay. The round bales had the slip coat sprayed onto the surface, while the square bales had their two exposed faces dipped into a clay slip prior to installation. All the bale walls are then plastered with a clay/sand/straw plaster inside and out.  All the clay for the plasters was found on-site, while digging the trenches for the geothermal heating system.

Clay plastered straw bale walls are one of the most environmentally friendly building systems available. The combination of locally-grown, minimally-processed straw and locally harvested clay means that the embodied energy in the wall system is only a tiny fraction of any other wall system. Combined with excellent thermal performance over the lifetime of the building, this system saves energy in every possible way, and is comparable in time and cost to any conventional building system.

There are lots of places to go for information on straw bale building. For starters, try The Last Straw Journal at www.thelaststraw.org. In Ontario, start with the Ontario Straw Bale Building Coalition, www.osbbc.ca.

The three "wings" of the MPAC building are all based on conventional framing practices. We use double 2x4 walls, which allow us to use the minimal amount of wood (which is all locally harvested and milled) to create walls of adequate thickness for good thermal performance. By spacing the two frame walls apart from each other, we minimize the thermal bridging effect that is the key weakness in most frame wall systems.

We have used three different insulation types for the three wings.

One is infilled with hempcrete, which is a site mixed combination of ground hemp, water and hydrated lime. This materials creates a light weight, rigid, insulative fill in the wall. We use slip forms on the frame walls to pack in the hempcrete. This is a similar mix and process to the one we used for the foundation grade beams.

The second wall is infilled with light clay/straw. For this material, we blend chopped hemp straw with a small amount of clay slip (a runny mixture of clay and water). This material is then packed into slip forms, resulting in an insulation with very similar properties to the hempcrete.

The third wall is insulated with cotton batt insulation. These batts are similar to fiberglass or Roxul batts, but are made from non-toxic, recycled denim fabric. They are installed in the same way as other batts, and have comparable R-values and fire ratings.