ECO-BUILDING – PART 11 – Preparations for Plastering a Straw Bale House

A job well planned and prepared is a job half done. A quality plaster job starts before the plaster is brought to the site. The condition of the substrate, or the base surface to be worked with, is very important – as the plaster can only be as strong as the substrate it is attached to. Straw generally makes a great substrate for a plaster, as so does a mesh – if any mesh is to be used.

The transition points of bale to wood must also be detailed, because they have their own rate of expansion. Because these two materials expand and contract at a varying rate, many cracks will appear if the transition points are not properly detailed with a plaster lath.
plaster and wood transition points

One method is to cover all wood with roofing felt to isolate the wood from the plaster. One can also use a plaster lath to give the plaster something to hang on to. The plaster should always have some surface to hang onto, and hang on its own without any structural support. This is even more important around doors and windows as well as at the intersections with the ceiling.

burlapOn exterior surfaces, all wood needs to be covered as described above and any large gaps need to be filled.
Materials used for doing the filling are cob, light straw clay, burlap, spray foam (PU-foam) or other suitable materials.

When the plaster is to be applied, the walls should be tight and solid. All wooden parts should be covered and the mesh should be attached firmly to the wood structures and/or sewn through the straw bale walls.

All holes and gaps should be filled firmly so that no deflation occurs. It is also good to keep the site clean of straw and other debris, so one can walk safely around the structure.

The floors should also be covered so that dropped plaster will not have the possibility to mar it.

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