Cellulose + clay plaster for round pole timber frame
I am building a round pole timber frame using a professional designer in Southern Wisconsin (whole trees) for a small house. For the wall assembly I originally planned straw bale + plaster but after discussions and consideration I wanted more height with no second story overhangs, and no moisture problems down the road. So I’ve decided to use 10-12″ dense pack cellulose as an in-fill between timbers.
I plan to use horizontal girts on both sides of timbers, and use dense-pack cellulose with netting on both sides of girts to hold it in. On interior I’d like to use wood lath + clay plaster because it will serve as an air barrier and we have clay so it’s embodied energy is low. And it’s cheap and beautiful. I know it’s not common. On the outside of exterior girts I want to fir out a bit and put vertical wood siding panels, with some weather resistant paint or finish on back and end grain.
What I want to know is do I absolutely have to sheath this? It may be in the code, and I know reused foam is cheap and can double as a WRB, but I sense that sheathing isn’t structurally necessary in this design. Secondly, even if condensation builds on inside of the siding it’s firred out and can drain. Regarding fire risk, cellulose is resistant. I think a WRB would probably be a good idea with cellulose behind wood siding, but I can’t think of a good way to attach house wrap besides on the underside of the wood siding itself which is logistically impossible I think, so back to foam. I’m really trying to avoid more materials than absolutely necessary from a simplistic perspective, and especially foam. But it’s my first and maybe only build so I’m probably naive. I suppose I could use lath + plaster on exterior of cellulose and put firring strips in the plaster for the wood siding, but that seems pretty labor intensive. I don’t want a plaster exterior, just don’t want to deal with the maintenance if I can use wood siding.
Any suggestions welcome!
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Replies
Hi Corky.
I'm generally reluctant to comment on what to do with atypical assemblies like this, because I'd have to make a lot of guesses. I'm sure others will be here to give you some input. I always recommend working with a local professional with experience building good houses in your climate and with the type of construction you are using. In the mean time, I'll just point out a few things.
First, if you don't want moisture issues, add roof overhangs to your design. The most common source of moisture problems is water, moistly rain and snow melt. Roof overhangs keep a lot of water way from and therefore out of your walls. Along these lines, you absolutely need a WRB!
Second, the clay plaster may be an airtight material, many things are, but it's the details that actually keep air out. So, before getting to committed to that as your air barrier, it may be a good idea to determine how you are going to detail electrical boxes, the top and bottom of the plaster, the transition at rough openings, the transition from the floor or foundation, and the transition to the ceiling or roof, etc.
Whether or not you need structural sheathing depends on how your timber frame is engineered, not a question that someone can definitively answer here without seeing your plans, and potentially even then, it may need to be an engineer. But you may be asking about continuous insulation? which depends on your climate zone and local code codes.
If you end up with structural or rigid foam sheathing, you could use that as your air barrier instead of the plaster inside. It will be much more straightforward to get the details right.
There are a couple of good details here for a house without sheathing that you can reuse:
https://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm
That has interior drywall as the primary air barrier.
Having lived in houses with plaster, I can't see that ever being air tight, you would need a dedicated air barrier layer. Something like a well detailed vapor barrier or a smart membrane such as Intello Plus.