John C Daley wrote:40 inches of rain will not be an issue.
Good design of the structure and its exposure to driven rain will deal with that.
Lime plasters on the outside are important.
"rain. This is perhaps the easiest form of water to deal with in a straw bale home.
Large overhangs, preferably 2′ or more, will direct the majority of the rain away from the walls by protecting them from direct contact.
With the application of gutters on the eaves, you can virtually eliminate rain from contacting the walls.
The gutters also give you the opportunity to collect your rain water for future use. [ see my signature for rainfall collection ]
If you have a multiple story building, be sure to add roof lines at each story. "
From ; https://www.thelaststraw.org/moisture-and-straw-bale-walls/
Primary consideration needs to be given to preventing liquid moisture from finding a way into the bales:
-ensure appropriate overhangs, splash protection, window and door detailing, etc.,
- and don’t forget to remember adequate protection for the bales during the actual construction process.
- Taking care in detailing window and door openings by design and implementation is very important
—providing a drip edge at the tops and bottoms of the openings, making sure that any “ledges” (such as at the bottom of a window opening) are sloped appropriately, and installing a sheet-type Moisture
Barrier such as tar paper, or felt, over areas above, beside, and below openings in the wall system before stuccoing
in consideration of any potential water intrusion that might occur at junctions of dissimilar materials.
- Some straw-bale builders in wetter climates feel that tar,paper isn’t sufficient, that it’s too easily punctured by the building elements (or builders)
—in which condition the tar,paper can potentially create a bigger problem than it solves—and recommend more flexible and self-healing membranes such as Bituthene or Ice & Water Shield (which is not intended to be an endorsement for WR Grace, which happens to be the manufacturer of both of those products).
It should be pointed out here that there is a profound difference between vertical water and horizontal water where straw bales are concerned.
Vertical water—such as driving rain hitting the sides of a straw-bale wall—is shown by experience to not penetrate very far.
Horizontal water, on the other hand, will seep from above into the middle of the bale wall, which is precisely the worst place for it to be."
Drip ledges should stand proud of any plaster to ensure water does not dribble down the plaster causing erosion.
A wider veranda with an extra shield can be installed where the rain is particularly fierce.
Good luck with it.