How does the dehydration effect work with natural lime based plasters over cob

I remember seeing a bit of Geoffs video where he talked about the ability of the finished plaster to actually pull moisture out of a strawbale wall.

Is that just because the finish breathes and they have lots of dry weather in australia, or is there some sort of molecular pump going on?

I remember he said they used a rough coat cob type plaster first and then finished it with lime plaster

Hi Bob...Here's the answer to this mystery. Clay is hygroscopic. That is, it absorbs moisture. I've been told by others that on a straw bale wall, a clay-based finish (earthen plaster) will draw moisture out of the straw bales, if any enters. As a scientist, I wonder about such things. They make sense, but I don't know if there is scientific proof of such. Dan

i'm just trying to understand the way this works, and what the limitations might be.

for instance if clay is exposed to 80% humidity and becomes very saturated would it at some point be so saturated that some of that moisture would be passed on to the straw bale which also has a tendency to absorb moisture.(although maybe not as much as the clay)
in a saturated atmosphere over time it would seem that some of that humidity would penetrate the clay and be absorbed by the straw

In the geoff lawton video he was speaking about a lime render over a cobb rough coat as if the interaction of the two renders acted together to pull moisture out of the straw bale-- would the lime plaster be even more hygroscopic than the clay providing sort of a double shield keeping humidity moving toward the outside of the wall

I'm sort of thinking out loud here, so under most situations there would be sufficient time when the atmosphere was dry enough to enable the two renders to rapidly dry the interior of the bale, while providing a resistance to penetration during the times when the atmospheric humidity was high trying to force it's way into the bale ?

There might be a slight effect as the lime cures, where it could grab some of its "water of hydration" from the surrounding cob. I have no evidence that this happens, or is a significant effect, however.

The most important factors seem to be good ventilation and the ability of coatings to breathe, however. As long as the right, breathable plasters are used over cob / bale, and the wall protected from incoming damp (weather / ground moisture), then the plasters generally shed any incoming moisture and continue to let interior moisture evaporate from the wall.
- Note that clay plasters shed moisture by two physical effects: any spray of water running off the fiber or straw, and the wetted clay of the surface swelling slightly to prevent further water penetration. The wall has to dry between wettings, which should happen pretty reliably if the building is in use (warm surfaces, good ventilation, air movement as daily temperatures fluctuate). The wall should be protected from wettings and damp as much as possible, of course, but the occasional weird sideways rainstorm is something most exterior finishes should handle as long as there's complete drying time in between.

- Lime plasters do not swell with water, but they are themselves more impervious to water than clay, and will remain structural even if relatively wet. They are used for situations where some extended damp conditions are expected, like lighthouses or farmyards.

- I do not know the dynamics with straw-bale in such conditions; structural bale must remain below about 13% moisture at the core (18%? 15%?) in order not to rot, so it remains structural. This is one reason I would be tempted to use a straw-clay infill material rather than strawbale in very damp climates. But there are plenty of others with more experience out there documenting these details - I believe there's a Straw Bale Association that is doing great work documenting wall core dryness vs. surface wettings and so on.

Any sealed coatings (such as cement stucco) will tend to trap dampness in the wall instead. cement stucco over older earthen walls has caused numerous structural failures in walls that had previously stood firm for decades or centuries. I would expect it to have a similar effect on strawbale, except in climates where damp is truly absent (e.g. the Southwest, where adobe was historically built on bare earth with no foundation or footings.)

-Erica W

thanks Erica, that makes sense to me and helps explain the mechanisms at work