Kinark Sustainable Living Centre

Walls

We visually interact with the walls of a building more than any other component, and for that reason the walls are often the defining element of a structure. They are also the building component for which there are the most options, including sustainable options. So how to wrap the building?

Design Intent

1. To create beautiful, long-lasting walls with natural materials
2. To showcase several sustainable wall material options
3. To provide high levels of thermal insulation
4. To challenge the “flat wall” aesthetic

Materials and Methods

The majority of the exterior walls of this building are made from bales of hemp straw. This agricultural by-product allows for the creation of thick, thermally efficient and structurally sound walls by stacking the rectangular bales in a manner similar to concrete blocks. The bales are coated on the inner and outer faces with a runny clay slip, followed by a thick coat of clay, sand and straw (approx. 1 part clay, 1 part sand, 1 part chopped straw). Once dry, this clay plaster has a strength comparable to concrete, excellent moisture-handling properties and negligible environmental impacts. A finish coat of fine sand, clay and cattail fluff (1 part clay, 2-3 parts sand, some cattail fluff) gives the wall a smooth finish, and a coating of a silicate-based mineral paint helps seal the wall to water without being impermeable to the necessary flow of moisture in and out of the wall.

The rounded south wall is made with conventional “stick framing” and uses a clay-dipped straw as the insulation in place of manufactured insulations. The same plaster and paint coatings are applied as on the bale walls.

The high clerestory wall is also conventionally stick framed, and uses local sheep’s wool as the insulation between the studs.

The cordwood arch around the main door represents another sustainable method of wall construction. Although used mainly decoratively on this building, mortared cordwood (in this case mortared with clay/sand and stuffed with sheep’s wool in the cavities) can be used to create entire exterior wall systems.

Questions

Won’t straw bale walls burn down easily?

Both the Canadian and American governments have conducted full-scale fire tests on straw bale walls and all tests show that they perform far better than conventional wall systems in a fire. Because the bales are densely packed, there is not enough oxygen trapped in the wall to support combustion. Coupled with the fire-proof plasters on both sides, these walls pass a 2-hour commercial fire test with flying colours.

Won’t straw bale walls rot?

The composition of straw and wood is nearly identical; you can think of each straw as a little tree! This means they have similar characteristics when it comes to handling moisture. In any conditions in which a wooden wall will last a long time, so will a bale wall. Expose either to continuous moisture, and you’ll have rot. The hundreds of years of lessons learned to keep wood walls dry are transferable to straw bale walls, and incidences of moisture problems are equally rare.

What is the R-value of a straw bale wall?

Laboratory tests put straw bale wall R-values between R-28 to R-40. Real world testing tends to produce higher figures. Either way, the thickness of the walls makes for above-average levels of insulation.

How strong is a straw bale wall?

In laboratory testing, straw bale walls have shown themselves to be at least as strong and in some ways stronger than comparable conventional methods. Their compressive, shear and in-plane strengths are such that they can be used in any residential construction scenario in place of any conventional material.

Won’t the clay plasters melt in the rain?

Clay plasters can handle quite a bit of exposure to rain and snow. The nature of clay is such that wetting on the outside layer causes the clay particles to swell and close their pores, making further, deeper wetting unlikely. What clay plasters cannot handle well is constant erosion; a steady drip will affect clay plasters long before cement plasters.

Can you use cement plasters on straw bale walls?

The majority of straw bale buildings do use cement-based plasters. While there are many technical debates about which kind of plaster is better, either can be used successfully. We chose clay for this building because the high amount of energy that goes into making cement plasters does not compare favourably with the very low embodied energy in the clay plaster.