Sorry to be late to this discussion--just back after a long absence.
I stopped counting the number of straw bale buildings I had a hand in working on--gotta be approaching one-hundred or so--and the only ones with rodent problems were a handful of older buildings (i.e. built well before our current level of best practice evolved) that had been poorly detailed at the foundation and at joints where exterior plaster met different materials like timber frames, top plates, etc.--all places where rodents could gain access. On one
project we were called in specifically to address rodent issues,
mice had gained access by scurrying up the walls and into poorly detailed soffit vents--nothing to do with the bale walls themselves.
I agree with John--rodent control shouldn't be a concern when the wall is properly clad--whether the finish is plaster or siding. But a well applied plaster--whether just a scratch coat or the entire three-coat regime--goes beyond appearances.
I depend on the exterior plaster as my primary air barrier. Doing this well doesn't just eliminate entry routes for critters of all kinds, it keeps air from streaming through the walls. My secondary air barrier--probably as important as the first--is the interior plaster layer. We're careful about how we flash windows and doors and seal any wall perforations, and about secondary stops behind joints where plaster meets framing so as these joints appear to open and close, they are still sealed. This attention to detail doesn't take that much extra effort, and performs very well on the blower-door tests we need to pass. We're generally testing below 1 or 2 ACH (air changes per hour), which compares favorably to the average Oregon home built to minimum code, which is testing at 7 ACH.
So if you don't plaster the exterior, you need to do a really, really good job on the interior plasters.
An initial coat of plaster also makes the wall surface less likely to ignite
should embers or flame from a wild fire happen along, or as likely, someone working near the wall with an angle grinder or welding torch. And there is some evidence that even a single layer of exterior plaster--a scratch coat--offers some moisture management benefits in that it absorbs ambient moisture from high humidity and re-releases it to the space between the bale wall and the backside of the siding before it can pass into the straw. Not that the straw couldn't handle the seasonal moisture fluctuation--it could, so long as the moisture level stays below 20%.
There's yet another option for some of us who live in temperate areas like S. Oregon and most of California. And this method may have application in other regions--it's worth exploring. That's to stack the bales against 1/2" plywood sheathing. Has to be plywood as the glue in OSB makes it less vapor permeable. Even then, 1/2" plywood is at the very edge of not vapor permeable
enough, so it may not work in all locations. But where it can work, it solves a number of problems. Although 1/2" of plywood isn't as good a fire barrier as 1" of lime or clay plaster, it is a rated fire barrier, which at least protects the bale wall from flash burning. The plywood may play a structural role (shear), it can definitely be used to exclude critters, and if supported with thoughtfully placed framing, can be clad with plaster or siding, and either of these over a rain screen gap.
We didn't go into a lot of detail on this topic in CASBA's book
Straw Bale Building Details: An Illustrated Guide for Design and Construction, but there are a couple of applicable details there.
Jim Reiland
Many Hands Builders