Drying both directions with Lime Plaster and Wood Fiber
It’s been a while since I bothered you with a question/discussion point…but we’re getting closer on a project in the high desert of California…I’ve pegged it as a “Mixed Dry” climate based on what I can glean from the BSC’s building guides…
And reading the guide and following Building Science Fight Club and other blogs, I’m wondering a couple of things.
First, the plaster expert is formulating a more European style lime plaster formula that can be applied directly to Steicodirect…a wood fiber exterior insulation designed to accept lime plaster. In the high desert (4500 ft) where our wet days and winter snow are followed by dry conditions the permeable Lime Plaster seems like it would work well…especially under substantial overhangs to shade the summer sun.
So what I’m planning is Lime Plaster…permeable Wood Fiber external insulation…a permeable WRB…and taped Sheathing.
inboard of the sheathing would be 2×6’s with batts or dense pack insulation…
then a second permeable air barrier on the inside and a service cavity and drywall with permeable paint. The plan would be to create a relatively airtight envelope that allowed drying in both directions…from the heated house out to the dry cold outside and from the sunbaked post-rain outside into the house.
Am I missing something? Principally…on the existing cabin we used Polyiso on the outside…so we put a dimpled housewrap between it and the sheathing to drain any water that got through the round the polyiso. Do we need to be cognizant of bulk water infiltration with the permeable exterior insulation? Or does the fact that the whole system should be vapor open allow us to use a permeable but flat (like self adhesive or applied) WRB?
Long question, my apologies…but I find when I talk about the desert I often have to double back to explain about falling snow and nighttime temps in the 20s.
Thanks so much for reading this…I look forward to the thoughts you all have on the construction.
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Bumping this. Also in 4B / high desert mixed dry environment and am considering a very similar wall assembly. Winters are cold (<25F) and summers are hot (100F+) with 20" rain/year. Also looking to ensure a permeable dry out-dry in design.
Did you ever plaster directly to your wood fiber exterior? What was your service cavity design?
From everyone I’ve spoken to (architect, contractor, vendor) t&g wood fiber insulation can function as a wrb as long as the t’s and g’s are intact. We will begin the installation of steico universal dry directly to our sheathing in a few weeks. Outside of that we will have 3/4” vertical battens and then 16mm thick horizontal shou sugi ban shiplap siding.
Our service cavity will be horizontal 2x3’s on edge (assuming we can source decent, straight ones), that will add support to intello membrane (our air barrier) that will in turn function as dense pack cellulose netting.
Wow...Mark B is got a great set-up. And on our end, Ryan Chivers at Limestrong has come up with a lime render formula he's very comfortable applying directly to the universal dry. He's issued an installation sheet if you go to his site, https://www.limestrongartisan.com/ it should be there. Two things to keep in mind...at least as far as I can tell. Working in 4B you've got more rain than we do in Joshua Tree. And when you apply the plaster directly to the wood fibre you of course has no rain screen on that side. I'm thinking I might spring for something like the Benjamin Obdyke Hydrogap...or even Tyvek stucco wrap if you're not trying to be super airtight. Otherwise...Mark B's totally there. Mark B., I'd only ask you how you're going to fasten the 2x3's to the studs given their on their short sides. I was thinking about pocket holes.
Screws, probably 4”.
I am now also looking into StoTherm ci Mineral, which is a one coat stucco applied to mesh over mineral wool system: https://www.stocorp.com/sto_systems/stotherm-ci-mineral/