I have had several clients of my consulting business call me after they started to plaster their straw bale house. The calls come with a similar voice over the phone: panic (sometimes mild and others extreme)! “My house is smelling musty.” That pretty much sums it up and you can imagine the concern. “Do I need to tear my walls out and start again?”
No. You don’t. In all of the cases where this has happened, the clients had several things in common. First of all, they were all working with the brown coat to some capacity. Second, they had sprayed a lot of water on the scratch coat to insure proper bonding of the brown. Third, and perhaps most importantly, the weather was moist and no heaters were used during the curing process.
It is good practice to allow the plaster as long as possible to dry and cure; however, not at the risk of saturating your bales with water. It is essential that the walls be sprayed down well enough for the brown coat to bond. Spray the walls the night before you plaster until the walls no longer accept water. In other words, until it runs freely down the walls. Spray them again in the morning before you start mixing your plaster to the same measure. That is all you need. Once you have finished plastering, keep the walls moist by all means. Do not over dampen your walls. Allow them to slowly cure for two days or so, and work them as necessary to stop cracks from breaking out. After a couple days, let them be and allow them to dry naturally. After a week or so, start introducing heat into the house to dry out any excess water in the walls. The best source, other than natural heat, is electric because gas heat will only increase the moisture in the air and thus the walls.
If you get musty walls, use the heaters to dry them out and open windows if the weather permits. It is not the end of the world and you will survive the trauma! That’s the good news.