I received an email this morning from some folks in New Zealand. They are wondering if it is okay to leave the plaster off of the outside of a house if they wrap it with metal siding instead. I have attached their email below along with my response. In addition to my original response, I have thought of more to say on the subject. The additional information is noted here as well.
Hello there. We are currently in discussions with our architect and local council regarding building a straw bale home in New Zealand. I have your DVD. The site for our house is too exposed for a plastered straw bale home so we want to construct a covered straw bale home- using corrugated iron on the outside over the straw bales- do we have to plaster the outside of the strawbale home even though it’s covered and won’t be exposed to the elements?
There are two things you need to consider in order to wrap the exterior of the house in the manner you describe. The first is that you will have to place at least a slip coat of plaster on the bales before you add the metal. This is not to protect against the elements but to reduce the fire risk. Without any plaster, the bales remain “hairy” and could channel flame spread. The other consideration is the breathibility of the assembly once you place the metal over the bales.
It seems there might a tendency for the metal to condense and drop water into the bales. Also, any moisture moving from the inside to the outside of the structure would be trapped behind the metal. You might consider using a product like a mortar net (www.mortarnet.com) to separate the bales from the metal and to give a potentially drainable skin between the bales and the metal. Good luck.
More to Say:
What I didn’t ask before is exactly why you think the site is too exposed for plastered bales. That is a crucial piece of understanding that I believe needs to be addressed. The use of the corrugated metal will ultimately work; however, it presents a lot of extra work in order to make it functional. Plaster can be very suitable to many environments as long as it is applied properly and protected with smart design details. So, I guess I would have suggested that Adam use plaster unless he is absolutely sure he cannot do so successfully.