Hello Bob and Jim,
Your book looks fabulous! The "Details" are so important in strawbale building.
We built our 500 sqft strawbale house a couple of years ago. The interior and exterior walls have an inch and a half of earthen plaster on them and later we added three coats of lime wash made with well-aged lime putty to protect the walls while we decided on what to do for the the final coat.
Even though it's dry here in SW NM, we do get seasonal torrential monsoons with wind-driven rain.
We are now about to start in on the final coat of Aliz which will have clay, fine mica, kaolin, natural pigment and wheat paste. We've heard that wheat paste will make the final coat more water impermeable and more resistant to erosion, supposedly.
My question is , if the wheat paste creates greater water impermeability, will it also inhibit breathability of the walls? We definitely want our walls to breathe to prevent molds and other nasties growing in there. Plus, living in a breathing home feels soooo good.
An answer to this question will help us to breathe more easily as we go into the finishing of our walls!
Thank you for what you do and for sharing your experience with us.
I'm not familiar with wheat paste's water resistance characteristics--I have only ever added it (or any flour paste or gluten) to an interior finish clay plaster or alis to prevent dusting. In general, flour pastes or gluten are a relatively small portion of the mix. I'll guess that if it does inhibit water vapor movement it's not significant, and that if it offers some resistance to erosion from wind-driven rain, it's also not significant. Water soluble finishes like clay (earth) plaster or alis on an exposed exterior wall will need relatively frequent (by modern standards) maintenance. I'm also not familiar with how well an alis would stick to a lime wash. You might do some testing first--apply it to a section of one of your walls and watch it for a season, maybe spray it with a garden hose to simulate wind-driven rain. Another New Society Publishers book, Essential Natural Plasters, gets much further into the weeds on both interior and exterior wall finishes than the CASBA book's chapter on plastering straw bale walls--you might find more information there?
Location: SW New Mexico, 5300'elevation, 18" precip
Thank you for the information Jim. It seems that we received some inaccurate info from friends. So we'll be doing some more research and , as you suggested, test patches to see if the aliz sticks to the lime wash!
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