Straw Bale Building – the Devil is in the Details

When it comes to building with straw, success will depend on your attention to the specifics.

Recently, we’ve been hired to work on several straw bale projects – a single family home outside of Saskatoon, SK and a detached backyard office pod in rural Alberta. For the first project we were brought in to do the bale work in an already erected timber frame, later applying earthen plasters to the interior and exterior of the home once the bales were up. For the second project we were hired to do a hands-on consultation, whereby we taught the owner how to make and apply earthen plasters to bales that were already in place.

In both cases, decisions that would affect the finishing of the buildings – how the plasters would end at the bottom of the walls, how the plasters would make tight connections to the framing and meet the windows, whether or not there was sufficient overhang to adequately protect the plasters from driving rain, etc. – had been made well in advance of us being hired for the project.

These decisions made our work difficult and in both cases the homeowner would have benefited greatly from our advice had we been involved in the design phase.

There is much to think about when designing a building but if you do not have a clear idea of how the various materials are going to intersect, and how those intersections will be detailed, it behooves you to engage the trades that will be doing the actual work. If you do not, the quality, performance and durability will suffer. For brevity, I’ll highlight several critical places that should not be overlooked when marrying straw bales and plasters to the other essential building elements.  These include: plaster stops, designing around the bale module, and detailing windows.

                    

The first critical detail is what we call a plaster stop. This is a clearly defined place for the plasters to start and end. Plasters stops acts as both a depth gauge and a support. Plasters should be carried on the foundation, so the plaster should come to rest here. At the top of the wall, the plaster stop is often a piece of wood or similar material that makes abutting the ceiling material to easy and uncomplicated. In both instances because the plasters did not have clearly defined places to land, less than ideal solutions were adapted to make up for this design omission.

                      Straw bale with timber frame

Another nuisance that added time and cost, while compromising the performance of the building envelope, was the fact that the dimensions of the walls (height and width) were decided upon independent of the bales that were to make up the walls. This presents two problems: 1) more bales will have to be split and tied to fill the cavity, which creates unnecessary work, 2) the top course of bales, which if the height is matched to the bales can be inserted whole, will have to be cut and stuffed at the top – a practice that is generally less air tight. Designing around a bale module makes for a tighter building, while also saving labour and cost.

                     Window in straw bale building with self repairing tape, and sisal rope prep for plaster

Finally, another area of oversight concerns windows and, in particular, how the plasters will finish to them. Rain sheeting off windows is particularly hard on plasters, especially earthen ones, and care must be exercised here. Window flashings (with drip edges), and an exterior window sill that can safely move water away from the wall once it terminates at the bottom of the window, are essential elements of good design.

As mentioned before, these are just a few of the many issues that are often overlooked by those unfamiliar with the unique needs of natural materials. Having worked on these projects and observed first-hand just how much time and energy goes into remedying these oversights, we encourage our clients to involve us early on to work as part of the design team – with the other trades, their designer, engineer, and the building department – to create a complete plan. It’s money well spent.


How We Can Help

Dirt Craft offers a wide-range of services to those wishing to build an earth home and we are happy to work with designers, architects, engineers, building officials, contractors, etc. to make sure the details are taken care of from the start. For more details visit our consulting page here.

 

 

 

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