Hi Jay,
Your questions are extremely relevant, and at the forefront of what most natural/sustainable builders are thinking about these days.
It has certainly been my experience that the bale buildings I've been involved with have not experienced any moisture issues. That's upwards of 50 buildings in Ontario, dating back now 20 years. So that's good.
However, having moisture "problems" like mold is not the same thing as having well balanced humidity levels inside the building. To some degree, bale buildings (and other natural wall systems) do a great job of handling interior humidity levels by acting like giant "sponges" with a huge amount of accessible storage capacity for moisture. So when a building is being loaded with moisture (boiling pasta, showering), this humidity can be absorbed by the walls (and other natural materials in the building, like
wood ceilings). Most conventional homes experience moisture problems quickly because few or even no surfaces in the house are able to absorb humidity (latex paints, sealed floors, etc). The ability of natural wall systems to allow this moisture to migrate in either direction (back into the building, or out the atmosphere) means that even high levels of humidity tend to get re-dispersed quickly. All good things!
As we all start to make our buildings more air-tight (a necessity for cold/extreme climates if you care about
energy efficiency), we eliminate leaks in the building that are "fast" routes for excessive moisture to move through. This is a good thing, but it means that the walls become the only medium for storage/exchange. From all experience, they seem to do this quite well even in fairly air tight buildings. However, higher than average humidity in the building can mean seeing condensation on windows in cold weather and a sense of "dampness" if levels rise too high (above 70-75% RH). Wall permeability may not be able to keep indoor RH at or below these levels, even though the walls themselves are not experiencing any issues that would cause failure.
So the question becomes, how much and what type of air/humidity exchange is needed. And
the answer is: Nobody knows! Some research has gone into creating ventilation standards for conventional buildings, and that's what's been enshrined in codes. However, these standards assume non-permeable walls, no moisture-open surfaces in the building, and a fairly high degree of material off-gassing. This means that code requirements for ventilation are many times more than is necessary for natural homes. But how much more? And how to deal with codes when looking to install "alternative" systems for ventilation.
I do believe that mechanical ventilation is a good idea, in any kind of house. Fresh air is good. Balanced humidity is good. So a system to help provide those conditions is a good thing. For some reason, people in the natural building world tend to be dead set against this, claiming that "having machines help the house to breathe" is anathema. However, many of these same people will have a refrigerator, a computer, a mechanical heating system, a pumped
solar hot water system... in other words, use machines to make their homes comfortable. A Heat Recovery Ventilator (HRV) is actually a very simple machine, with only a fan and no other moving parts, and can be extremely effective in keeping indoor air humidity balanced, and providing tempered fresh air. They can be controlled automatically, or manually for those who are off-grid or don't like the machines to make decisions. A small unit isn't very expensive.
There are simpler systems that are more "natural".
Solar air heaters can draw in fresh outside air and bring it into the house pre-heated. Earth tube ventilation is also a possibility (though it can have moisture issues of its own). Fresh air intakes near wood-burning devices can draw outside air and deposit it near the heat source... In other words, there are multiple strategies to get fresh, tempered air into the building in a cold climate. Strategies abound! Having these strategies accepted by code officials can be more difficult. If you're choosing to use an "alternative" system, you will likely need an HVAC engineer to design the system (or run calculations on the system you have designed). Be sure the permeability/porosity of interior surfaces is being considered, as this storage capacity will greatly affect the amount of air that needs to be moved.
Code officials tend to dislike systems that require manual input to function, or systems that can be altered in the normal course of building occupancy. For example, you might have a house with all-porous materials inside. Great. But if the next homeowner slaps a coat of latex all over that, the ventilation assumptions go out the window and the system may not work. However, simple solar hot air collectors can be an automated system, as can other small, fan-based systems.
At this point in my building career, I would not ever opt for no ventilation at all. It just makes a building much more comfortable. However, in my current straw bale home, we run the HRV relatively infrequently, especially now that construction moisture is finally starting to subside (after almost 2 years!). But it is great to have it. Given how little we use it, we could definitely have gotten away with a smaller, alternative system. However, prices for HRVs are getting so competitive, it would be just as expensive to build my own collectors, buy fans, a PV panel to run the fans, and the controllers. Both would work.
When cladding over a natural wall system, we only use "rain screen" cladding systems, in which a strapping material is used to create open vertical channels between the siding and wall, and these are open (though screened against critters) at the top and bottom of the wall, allowing for air to circulate and move moisture. It may (don't have any studies to show) slow down moisture moving to the outside, but it definitely drastically reduces the amount of moisture that gets on/into the wall from precipitation.
For us, choosing a cladding is a very design-dependent decision. Plaster finishes are definitely vulnerable to bad weather. In some designs (one story, good overhangs, etc), these factors can be mitigated and I feel fine going with plaster. But many designs/locations don't suit finished plaster surfaces on the exterior, and many owners don't want to maintain exterior plaster. Many owners and builders will choose a high embodied energy cement-based plaster to help withstand these conditions. The impacts of this decision on the environment can be worse than adding a layer of sustainably harvested wood cladding.
The book I wrote is an attempt to help people figure out the balance they'd like to achieve when considering questions like cement-based plaster vs. wood siding.
Long answer!
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