I'm from the Lopez community land trust on Lopez island WA and we have 11 straw bale houses that were built with earthen walls under natural plaster. We had some moisture problems within the straw bales caused by inadequate roof overhangs and poor flashing around windows and doors that has all been fixed. But now we are seeing some high moisture readings 20-25% even though we are certain that the outside plaster is not getting any direct contact from the rain. . . It seems it is either a problem with the transpiration rates between the plaster and the earthen or something else. Some of the walls are perfect with bale moisture at 12-15%. I am curious about the previous post which suggests there is some type of wicking which the plaster encourages as it breathes, if so, can the plaster be modified on our walls to help lower moisture readings. Thanks for your help. I really would love a copy of your book, btw, since I could make it available in our library for use by our residents!
Hi Mike...You must have moisture sensors in your straw bale walls, no? I've been told by others in the straw bale "business" that humidity levels as high as 20 to 25% won't cause harm...They say that's natural, they say. The important distinction here is, I think, that the straw bale moisture content cycles and will lower. If that's the case, I would suspect that there wouldn't be much trouble with mold forming. Now, I say this without any empirical data to back up my stance. That's the problem with so many issues in natural building. There is just not enough science to back up claims. The old idea that straw bale and earthen plaster walls breathes is one of the myths. Moisture moves very slowly and at extremely small quantities directly through plaster. Where most moisture enters is improper flashing and penetrations...that is, where electrical wires and plumbing penetrates a wall. To control the amount of moisture entering a wall we really need to pay close attention to moisture moving up through foundations, moisture entering through inadequate flashing, and through penetrations. Be sure to seal all penetrations.
I've been told and I've stated it in my various books on natural building that clay in earthen plasters is hygroscopic and will help draw moisture out of straw bales. If your walls are already plastered with earthen plaster, there's not much you can do. If straw bale walls are finished with cement plaster, that will trap moisture in the walls. In such instances, it's probably worth removing the cement plaster and replacing it with earthen plaster.