When building with straw, there is no substitute for good design, impeccable detailing, and generous roof overhangs. If, however, your goal is to use earthen plasters but would like some added durability and rain shedding capabilities, a vapour-permeable surface can be added.
In this video we test two different products that are often sited in the literature for this purpose. The first is siloxane, the other is a potassium silicate paint which is sometimes known as waterglass. Both are vapour permeable. Here are the results.
Moisture Properties of Plaster and Stucco for Straw bale Buildings – this bulletin from the Canadian Mortgage and Housing Corporation (CMHC) highlights several tests whereby various coatings, including siloxane and linseed oil, were tested for their water-shedding ablilities. The testing finds: “Both the siloxane and elastomeric treatments reduced the water absorption dramatically and had little effect on the vapour permeance. The siloxane was much more effective.”
While we agree that siloxane improves water-shedding capabilities, the video above clearly demonstrates that potassium silicate-based finishes are even more effective than siloxane at standing up to driving water.