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grant fullmer wrote:I have found hemp hurd in Idaho for a decent price
Jim Reiland wrote:Hi Emily,
Thanks for the clarifications. Either wall system should be protected from wind-driven rain by either or both adequate roof overhangs and a suitable plaster.
Note that both straw-clay and hempcrete could be plastered, but they could also be sided if you were concerned about wind-driven rain. (Addendum. Straw bale walls can also be sided, but it's best if the exterior bale surface is plastered first. At the very least a scratch coat provides an air barrier, resists fire, insects, and rodents. Adding a brown coat is better as it supplies a better air barrier. Adding siding to a straw bale wall requires some planning. One method is to let 2x ledgers or posts into the bales on whatever centers the siding requires, usually 2', and make a note of where they are. Once the plaster covers the wall secure furring strips through the plaster to the ledgers or posts, then secure the siding to the furring strips. This results in a "proper" rain screen with an air gap (the thickness of the furring strips). When stacking the bales against plywood sheathing--a wall system that works great in some climates plastering the exterior isn't possible, and may not be critical as the plywood takes on some of the function of the plaster. Add siding to the exterior surface of the plywood according to your area's best practice. Rain screens are a great practice, but may not be absolutely necessary in some climates. Be sure to install insect screen at the top and bottom of the wall so it's not an open space for critters to occupy.)
Clay plasters are lower cost and user friendly but can erode from the wall if hit by too much rain. Sealing them with some silicate based product or linseed oil helps, but also makes it difficult to fix as a repair plaster won't stick as easily to the treated plaster.
Lime plasters are more durable but they aren't water repellent so much as water reservoirs. In natural wall assemblies like straw bale, straw-clay, or hempcrete walls exterior plasters function to absorb liquid moisture (wind-driven rain), then release it as water vapor. This mechanism can fail if there's too much rain and not enough dry time between rainfall events. When that happens the water soaks into whatever substrate the plaster is on--straw bales, straw clay, hempcrete--where it stays until exterior conditions pull the moisture back out in the form of water vapor.
How long that takes depends on exterior conditions--warm and dry summer weather is best for evaporating water from the walls, though very cold winter conditions help to prevent microbes from becoming active. I have seen straw bale and straw-clay walls that were wet on the exterior surface survive for months in S. Oregon winters without apparent damage because it was too cold for the microbes to start eating. Some of the repair work I did on straw bale structures in S. Oregon involved only the lower few feet of the wall where the plaster absorbed liquid water from rain splash and roofs without gutters.
It costs more, but larger overhangs protect walls better.
RE your comment about "adequate dry time." The handed-down wisdom in the straw-clay building world of "one week of dry time per 1" of wall thickness" really depends on optimal drying conditions. I have tested straw-clay walls many months after they were placed and found readings well over 20% moisture content. It's possible that the measuring tools we use (moisture reading equipment designed for hay) isn't appropriate for a material like straw-clay, but lacking other reliable methods to evaluate the interior of a straw-clay wall we waited for the wall moisture content to come down before plastering either side. We began exterior plasters nine months after the walls were placed, and completed the interior plasters almost a year after the straw-clay was placed. We also didn't rely entirely on warm, dry weather to coax moisture from the walls. Over the winter the owner used a wood burning stove to raise the interior temperatures, and also ran dehumidifiers.
Jim
Many Hands Builders
Ben Brownell wrote:Protected clay is fine as others have stated. I'd be more concerned about having adequate insulation balanced with well positioned thermal mass to moderate cold outdoor temps, and a sturdy structural frame (heavy timber?) to support significant snow loads if that's a feature in your area.
Aaron Yarbrough wrote:: Clay is fine in light clay straw in almost any environment provided you have generous roof overhangs. Clay based plaster is probably fine provided the aforementioned generous roof overhangs but I think lime plaster is better.
The long response:
With significant roof overhangs an exterior clay based plaster should be fine. We did our whole exterior light clay straw house with earthen plaster and it's held up fine for the past five years. There is one fairly exposed corner with only two feet of roof overhang that I re-plastered a few months ago. Our climate is probably drier than yours but we still get ~32 inches of rain a year and when it usually comes in the form of blowing wind storms.
That being said since building our cabin I've switched to mostly using lime plasters for a few reasons. Note, these are anecdotal observations and maybe over time I've just become a better plasterer.
1) Lime based plaster dusts off less than clay plaster
2) I get less shrinkage with lime plaster than clay plaster.
3) Hairline cracks in lime plaster will heal themselves to an extent.
4) Once lime plaster cures it is pretty hard wearing
5) If you're adding a tint to your plaster it's easier to maintain a consistent color.
Regarding cost and causticness
A lime plaster is mostly sand (~80%) so a 50 pound bag of type S hydrated lime (~$20 in my area) will go a long way. You could probably plaster the interior and exterior of a 200 sqft house with less than six bags. I've found that contact with hydrated lime mostly just dries out my skin. I'll typically wear light gloves and safety glasses when I'm plastering.
Mike Harris wrote:Hi Emily,
I'm not an expert -- more an enthusiastic enthusiast.
But the way I understand it is that clay walls will be just fine in your region if you take the necessary precautions.
If you're able to stop your walls from coming into contact with driving rain, you should be good. Which means your walls should be raised high enough off the ground to account for splashback, and your roof overhangs should be deep enough to protect the walls from rain.
What I'm concerned about is when you say the walls will be 'sealed'. Sealed with what exactly?
Clay walls are problematic when they're sealed in such a way that prevents water vapour from exiting the wall -- like with concrete or plastic paint. The moisture then becomes trapped in the wall leading to failure. Many heritage building are destroyed by using the incorrect materials for renovation.
If you're sealing your walls with something breathable -- like mineral paints or a natural plaster -- the vapour will be able to pass through this barrier and exit the wall, leaving it dry.
Brandon Charles wrote:The only place in the US that I know of where you can go set up shop and stay indefinitely is Slab City. If anyone knows of another spot that is similarly free of policing, feel free to chime in.