Clay plaster & lath with batt insulation?
Heres my issue: i built a building.
Northern canada. Miserable cold. Alberta. Its pretty dry here.
Exterior wall:
I want to try clay plaster. I took a course where we were plastering a straw bale building. Excessive info about sealing plaster around windows/doors etc. I bought a bunch of fab bamboo fences…will make good lath. I also found some vapour permeable paint that works to paint dirt.
I figure i should put osb on the outside of my stud walls, then tar paper, then a WRB for an air space, more tar paper, then the bamboo, then 2/3 layers of clay/dirt/straw plaster, then wait a month of summer and then paint with the dirt paint.
-can i get away without the osb if my bamboo is solidly attached?
-can i get away without the WRB/air space because clay plaster causes less water problems than stucco?
– if i want to use roxul batt insulation in the wall, should i wait a month to put it in the wall till the outer wall is dry?
-On the interior? Poly? Osb & paint? ( i refuse to use drywall) what about interior bamboo and plaster or can i not plaster on both sides of batt insul because it would put too much moisture in the wall?
Do i need to be vigilant about air leaking through interior wall or can i relax on the interior because the exterior will be good.
Any advice would be super!
Meagan
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Replies
HI Meagan -
"-can i get away without the osb if my bamboo is solidly attached?" Northern Alberta where the wind blows as part of that cold? I bet structurally you need structural panels, such as OSB.
"-can i get away without the WRB/air space because clay plaster causes less water problems than stucco?" You can never get away without the weather-resistive barrier (WRB). Whether or not you need a free-draining space/ventilated rainscreen is typically linked to whether or not you have greater than 20 inches of precipitation annually. More than that, best practice is ventilated rainscreen; less than that and a face-sealed approach is ok. Personally, given the lack of elasticity in your cladding, I would go with rainscreen, not face-sealed.
"– if i want to use roxul batt insulation in the wall, should i wait a month to put it in the wall till the outer wall is dry?" If your outer wall includes the rainscreen space, you will get plenty of drying to the exterior, even while you insulate the interior. Plus, the moisture of construction for roxul batt is negligible and also air-permeable so plenty of drying all around.
HOWEVER: up to this point, no mention of continuous air control layer (even though your next question is about this in part. Have to say here: one continuous air control layer is mandatory, two is excellent. When it is super-cold, you can get convective looping within your building cavities (interstitial). In your climate, go for two.
"-On the interior? Poly? Osb & paint? ( i refuse to use drywall) what about interior bamboo and plaster or can i not plaster on both sides of batt insulation because it would put too much moisture in the wall?" I don't know what your building code says for interior vapor retarder in your climate. Our US code states that Class I (0.1 perms or less) or II (0.1 - 1.0 perms) would be required for your climate. I think your decision regarding Class I or II depends on what your plan on running your interior wintertime humidity at: keep it dry (20 - 30% RH at around 68F) go with Class II; keep it moist (35% RH or greater around 68F) and you should consider Class I.
Peter
Thank you muchly Peter!
Your answers are very smart-yes, wind is strong. i will use OSB. And WRB. And poly. And batt insul.
Thank you so much!
Meagan,
The answers you will get here will typically give y0u good advice from a building-science standpoint, but remember, to get it approved and live in it, your house needs to meet the building code in Alberta, and a lot of the things you are asking about aren't optional.
Its not a house malcolm, its a shop/shed.
(i actually built it because i wanted something to try natural plaster on) its technically a farm out building so it requires neither building permit nor approval. However, i want to do it well. I have built about a dozen farm sheds, each one much better built than the ones before. Theres nothing i enjoy more than learning new ways to build!
And there has been much discussion here on how poly is unnecessary, sometimes even harmful. On the east wall of my shed i actually have a perlite & clay freestanding dirt wall, which i am planning to both interior and exterior plaster without poly at all. (No one has ever suggested a fancy WRB air space when plastering straw bale walls. And im pretty sure i have the only 14" wide perlite wall on the face of the earth) However, i spent all last summer doing only 3/4s of the east wall doing perlite and think it is too much physical labour for the other three walls.
The north wall has exterior rigid, so its not getting poly either.
But for west and south, i am stuck mixing conventional osb & roxul ( i salvaged a bunch already) with alternative natural plaster (ie straw clay mud). And it is the amalgamation of conventional with caveman ways that makes me confused. Most people pick one or the other.
Hey Meagan, I’m trying thing bamboo lath and clay plaster technique for my interior walls and I was wondering if I could possibly chat with you a bit about getting started? My email is [email protected]. Thank you! -Courtney