As many of you know I maintain an “Ask the Experts” opportunity at www.greenhomebuilding.com for people to post specific questions that they might have regarding various aspects of natural building and sustainable architecture. I have been doing this for about two decades and some of the experts have been helping me with this for about that long. The various questions and answers are eventually posted on the website for people to learn from.
As an example, here is a recent question that I received:
“We live in northeastern Oklahoma and I am planning to build a 1 story 40’x40′ house on a conventional concrete stem wall foundation. We have twelve acres and have a hillside covered in beautiful sandstone. I’ve experimented with building slipform stonewalls on my land and have had some conventional framing construction experience the past couple years and feel pretty confident in that. My question is, could I build the stud framing of the exterior first, then infill with light clay straw (for a 6 inch wall) let it dry completely, then do slipform stone (8 inch thick) on the outside of the light clay straw stud walls (using just one side of the slipform built up against the light clay straw wall as the other side of the form)? I would build the roof onto the slipform stone walls. I’d run electric in conduit inside the LCS, then do a lime plaster on the interior with the stone on the outside. Our climate is driest in end of June – September, so I’d plan to start the LCS in fill July 1st.”
Michael G, Smith, who answers questions regarding cob and light straw/clay answered:
This is an interesting and well thought-out idea. I have no doubt that the wall system you describe is technically feasible to build. Do you understand that for reasons of thermal efficiency it is is preferable to have the insulation (LCS) on the outside of the wall and the thermal mass (stone) on the inside? Yet from the standpoint of weatherproofing and durability and possibly aesthetics I can understand why you’d want the reverse configuration that you described here.
My only major concern is that it is typically recommended that straw-clay walls be protected with vapor-permeable coatings on both the interior and exterior surfaces. Lime plaster qualifies as vapor permeable, but slipform stone-and-concrete does not. The concern is not only about the initial drying (which you have addressed by the building sequence you described) but also about long-term durability. If any moisture makes it into the walls over time, it needs to be able to evaporate out quickly, before the straw starts to decompose. There are several scenarios to consider:
1) Water getting into the base of the wall from flooding or roof splash combined with poor waterproofing detailing.
2) Water getting into the top of the wall from a leaky roof.
3) Water getting into the wall from leaky pipes.
4) Water getting into the wall from exterior weather such as wind-driven rain or roof splash, especially around windows, doors, or other material transitions.
5) Water passing into the wall from vapor in the living space driven by heat differential during the heating season, which then condenses on the much cooler thermal mass of the stone wall.
Any of these things could happen over the long run, especially if the building is going to be in use for 100 years or more. There are things you can do to reduce the likelihood of each. The scenario that seems most concerning in the short term is #5. The air pressure that can drive moist air through the walls of a building is proportional to the difference in temperature between inside and outside. During your cold winters, warm moist air from inside the building will be driven through every crack by this temperature differential. When that water vapor comes in contact with the much colder, uninsulated exterior stone wall, it is likely to condense into liquid water. There are a few things you could do to prevent this:
1) Pay very close attention to reducing the number and size of cracks by following the air barrier recommendations spelled out in “The Natural Building Companion” by Racusin and McArleton.
2) Reverse the wall assembly so that the insulation is on the outside would reduce the likelihood of condensation inside your wall system where the moisture can’t easily escape.
3) Create a vented airspace between the straw clay and the stone portions of the wall would essentially solve the problem. I’m not quite sure how you would achieve this in practice, but it is worth considering.
If you want more specialized guidance on this question, I would recommend hiring Jacob Racusin (author of the book listed above) for a consultation. You can contact him through his company at newframeworks.org