Lime Plaster on Wood-Fiber Insulation
Hey all,
a second question for me…I have been chasing an answer in the outside world and realize there may be some knowledge here that can help me.
For our house in the high desert we’re hoping to use wood fibre external insulation. In my research I discovered that a few of the European companies that makes Wood fiber produce sheets that are dense enough…and specifically designed…to take a lime plaster. There is no need to apply lath…only to embed fiberglass mesh. Global Wholesale in Maryland is a supplier of one of these…but no one imports the specific Lime Plaster systems they use in the UK or EU. So I’ve been talking to plaster suppliers and have a couple of lines on pure-lime plasters that should work nices.
But interestingly, Steico UK, the British arm of the company that supplies the sheets actually REFUSED to recommend any plasters for the American Market, citing EIFS/STO lawsuits. I looked them up and they seem to revolve around Portland cement based systems applied without a rainscreen of any kind to vent the air behind the product. I guess in the Northwest they’ve had trouble with moisture and the attendant rot and mold.
There seems to be no problem with the Lime-based plasters (or Renders as they call them in the UK) and the wood fiber board overseas. So I’m thinking the problem is with the cement.
Does anyone know if lime plaster is breathable enough, when applied to wood fiber that there would be ample permeability? Or should I be thinking of putting something (even tyvek stucco wrap) behind the insulation to provide some air movement?
I’d love to know your thoughts.
thanks
Mark
Yucca Valley
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
EIFS is a good system but controlling bulk moisture is critical. I had an EIFS house, and I took extra steps to keep the wall cavities dry. If I was doing that house today, I would use a traditional hard stucco system.
Haven’t a clue about render over wood fiber.
True lime-based plaster without polymer additives has some permeability, yes. I would think at a minimum a drainage layer behind the wood fiber insulation is still a good idea, say two layers of grade D building paper, but I'm not familiar with common practice in your climate. I've heard that the "desert" is about the only place face-sealed stucco has "survived" without a drainage layer, likely on low exposure single-story homes with good overhangs. If the sheathing is OSB I would want to drain it but I always err conservative on water management.
475 can bring in Gutex's version of this system if you contact them.
Jason! thanks...you're advice makes a lot of sense. And John, FYI, 475 has some amazing stuff, but the recommend, rightly...that if you use Gutex you wrap it with the whole conventional system of a rainscreen with incorporated fiberglass lath. Gutex doesn't have a product in their line that is specifically designed for direct application of Plaster. Outfits in the UK that do this consistently, do so only with lines like Steico or Pavatex...which are designed specifically to take plaster directly. That system is great...but only adds to the expense of the already expensive insulation. But if that's the way we go...then we don't need the more expensive "plaster ready" SteicoProtect product.
When I price it out...it may make more sense just to wrap it in paper and lath...but I always assume labor is going to be the biggest expense in insulation...but in this case I might be dead wrong!
What I need to to is stop complaining and get a sample of the SteicoProtect and figure out what's so magical about it that the Brits seem to slather it with Lime plaster and call it a day.
https://gutex.de/en/construction/details/timber-frame-construction-insulated-render-facade/
Is this not the application you are looking for? Not trying to push the product, just saying it is available and there is a North American supplier.
Look at the California Building Code & Strawbale appendix for info on lime plaster & application directly to straw.
https://up.codes/viewer/california/irc-2018/chapter/S/strawbale-construction#S
Deleted
Thanks Tim,
knowing the county, I think this is great advice...especially with a product they're not going to be familiar with. If I can point to these guidelines as acceptable for strawbale, they'll most likely be fine for a more stable substrate. Good thinking! Thanks!
John,
You're right about the Gutex...it's super solid and the guys at 475 have always been insanely helpful and clear. What I need to do...when I price this stuff out is find out if they'll sell the Thermowall without their dedicated Gutex render...which is cement based. Thermowall is a system...and I don't know if they'd break it up. But it's definitely worth a try.
It's advertised on 475's website...but like I said earlier (you may have jumped on later) Lucas at 475, who's their building science guru out west, suggested I use their Multitherm 40 with a rainscreen and lath. So who knows..I just want to use lime plaster...and apply it directly if those darn brits can do it without a problem.
But I'll definitely loop them in one more time!
Hi, I didn't look at you location- Yucca Valley you have an expert in Joshua Tree. Janet & George at strongarmconstruction.com/ give them a call for help.
Tim
Tim! Amazing. I just spoke to Janet about stuff like this...got her name through someone else and she steered me toward a mutual friend! I love how small a world this is!
Check out Straub's article on lime plaster permeability. The study was written for strawbale construction but may have some good information for you. Adding cement to lime plaster improves it's strength but reduces its permeability.
https://www.google.com/url?sa=t&source=web&rct=j&url=https://tallerconco.org/wp-content/uploads/2017/05/Straube_Moisture_Tests.pdf&ved=2ahUKEwjR1t_PjcfsAhVOs54KHfp5AxQQFjAAegQIAhAB&usg=AOvVaw2HRMSFB8ufxcsvsrj3orUN
Thanks Kent! I've been reading up on the building science site on plaster ... and the wicked combination of cement and impermeability...I noticed on the code material that Tim sent, Building code specifies that stucco should be applied directly to the strawbale. It makes sense in terms of not wanted to disrupt the permeability. I'll check out the article. I'm hopeful in the desert with our substantial overhangs and arid climate that we stand a chance of holding up to the elements. If any of you are curious...here's the cabin we built already...it's our shelter while we plan the house.
Hi Mark,
Were you able to execute the lime plaster over wood-fiber idea? I'm up in Bishop and am considering the same thing. But, like you, have had trouble finding information and products specific to the US market.
Cheers,
Ben.
Ben!
Great to hear from you. I'm in the middle of something but I'll check back in the AM and let you know everything I learned. We got a working product we developed...about the time I ran out of money! I'm building conventionally now...and I kind of hate myself for it!
But I'll have names and numbers tomorrow. Love Bishop. Once drove an hour to see a movie there 'cause it was the nearest theater!
more tomorrow.
Mark