Hi Jay, Yes, thick wall construction can be "penalized" because building departments look at a structure's
footprint when calculating it's size--which determines both the permit cost and the eventual property tax assessment. And thicker walls = less space inside, a disadvantage for thick wall construction that can be partially offset by designing window seating (to replace free-standing furniture like couches), locate a potted plant, etc. At least one county in California (Sonoma County I think) takes a more progressive approach, measuring the building's interior dimensions and then adding 6" for the exterior wall as if it were built with a conventional framing-insulation system. If more building departments took that approach it would provide incentive for all thicker wall construction, most of which exceeds current code requirements for insulation value.
As for moderating humidity, that's something unique to thick interior clay and lime plasters. Most straw bale buildings have at least a 1" thick, and usually thicker plaster on the inside. In addition to being a thermal storage battery, absorbing heat and releasing it, these plasters have an enormous ability to absorb and release moisture from occupant activities like cooking, showering, and breathing. The plastered walls moderate humidity fluctuations. The straw bales themselves are able to absorb and store quite a bit of
water vapor too, but it's important that moisture levels inside the walls not exceed 18% or so--the level at which microbes become active. That's why the plasters must be vapor permeable (what many term "breathable) so any water vapor that makes it all the way through the plaster will be able to either come back out, or pass through the other side of the wall, driven by temperature and vapor pressure.
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