Misting Straw Bales with Water Before Plastering

Written by Andrew Morrison

Misting Straw Bales with Water Before Plastering

rain falling on the groundI 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.

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4 Responses

  1. At 2:55 PM, Southern Sally said…

    Nice blog site! This is my first time perusin’ this site and I like what I’m seeing.
    I am wonderin’ if there is a more official way of testing the walls for moisture content rather than going by sense of smell alone. It scares me to think that there could be mold growth happening in the walls without my knowing it…I don’t have the best sense of smell!
    Let me know…Southern Sally

  2. At 10:56 PM, Andrew Morrison said…

    Follow your nose…It always knows! I remember that from my childhood: Toucan Sam and Fruit Loops! Yes Sally, you can definitely use more accurate methods of moisture identification. A small hole can be drilled into the existing plaster and moisture meter sensors can be shoved into the bale. In fact, some people actually install the meters in the wall while they build with a monitoring station in the utility room so they can watch the moisture for the life of the home. In lieu of that, you can simply use a hand held meter with a probe long enough to reach the edge of the bale at the far side of the wall. Chances are the moisture levels will be high near the plaster for a while, due to the relatively large amount of water introduced into the wall during the plastering process. As long as the bales are under 20% moisture content, they cannot support mold growth, so even a wet wall will not allow mold to grow once it dries out. If you have high moisture levels and you smell mold, do what you can to dry the walls and retest the moisture often keeping track of any changes in a journal. Once the moisture levels are safely below 20%, you should notice the musty smell disappear in a week or so. Good luck.

  3. I learn more every time I come back here. I have recently heard of people mixing a plastic substance into the stucco for the exterior to make it water resistant and crack proof. I have read that strawbale walls need to “breath”, and i think this plastic substance would keep the exterior walls from breathing. What if i used papercrete on the interior walls? Would that let the walls breath while still protecting them?

  4. Hi Tom. Thanks for your message. Although your thinking makes a lot of sense at first glance, you would actually cause even more damage to the wall by trying to protect it this way. I would not recommend the elastomeric (plastic you mention in the exterior coat) as it will fully seal the wall. That might seem like a good idea, but it’s not. Although it will keep rain out, it won’t allow any moisture to escape the wall, and we KNOW that moisture will move into the wall from the interior (steam from cooking and showers, moisture from houseplants, breathing, etc.). If you apply an interior finish with higher permeability, It will allow more moisture to move into the wall. You would actually then end up with MORE moisture stuck behind the elastomeric and not more escaping through the more permeable interior. That’s because the moisture will load the wall from the inside out, due to the pressurization of the home. It’s actually a best practice to use interior and exterior plasters with the same permeability OR where the interior has a LOWER permeability than the exterior so that anything that does make it into the wall can be sure to escape as well. I hope that makes sense. Cheers.

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