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OSB Exterior, finished with Lime Plaster.... But how?  RSS feed

 
Lauren Magnolia
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Hey there, Permies Universe!!
My husband and I are in the midst of a relatively unprecedented home-build. What started as a cob-house has become a timber/cob hybrid, subject to both building code and material availability. Our current inquiry is in regards to the exterior siding...
We plan to lime plaster/paint the exterior, which currently consists of OSB on balloon frame. Plan A is to wrap with tar paper, then chicken wire, then lime plaster with a coat of lime paint to finish.

Now for the Q&A...
1. Would a coat of lime paint on the OSB be enough for the plaster to adhere or is the tar felt & chicken wire lathe the best option?

2. What sort of options are available to us besides chicken wire, which is currently the best bang for our buck.
Has anyone any experience with "wood lathe"? I hear it's often found and recycled, but from where?

All input is greatly appreciated!!
 
Chadwick Holmes
Posts: 618
Location: Volant, PA
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Just lath, wood lathe is a tool that spins wood.

Lathe is sheet metal that has been expanded and has diamond openings about 3/8" by 1/8" or so, it does a much better job than chicken wire as it is designed fir this purpose. It is also not an expensive material.

Lime plaster is a good thing to have on wood from what I understand so you may be able to skip the tar paper, hopefully someone will confirm or deny that statement for us.....

Here is lathe

http://www.lowes.com/pd_11811-1278-LATH25R_1z0ud9q__?productId=50214505&pl=1

Hope this helps!
 
Lauren Magnolia
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I will recheck our local prices, thanks for link... That is cheaper than we'd previously seen it. I do hope someone has some experience with plastering over the OSB, we are concerned with insulation (though the interior will consist of foam board insulation and cob) as well as durability. We don't want the layer of plaster to pull away from the wood...
Thanks again!!!
 
Chadwick Holmes
Posts: 618
Location: Volant, PA
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Yeah, so you staple up the lath and that holds the plaster, you are more likely to get cracking with chicken wire because the plaster is spread more between supports, the metal being the support. Chicken wire will rust out faster too as it is thinner material

The insulation value of plaster is small, you will want to plan that for inside the void in the wall structure,or under the plaster and the wire mesh you can use a closed cell foam.....more petro chemicals there....so you have to weigh how you feel about that too.
 
Lauren Magnolia
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I hear you on lath vs chicken wire... What we had in mind is a galvanized product i hopes of reducing rust, but we may well go with lath after all. Also, we had planned to use that pink, rigid foam board insulation and a layer of cob on the inside (as opposed to drywall, sheetrock, and that terrible yellow hell) Believe me, we have waded through our emotions about buying and using foam as well as OSB. In order to build to code as well as budget in a timely fashion, sacrifices have had to be made. Without a doubt, we are permaculturally beating the pants off the trailer this house is replacing.
One may only do what they can, and not what they can't...

My question today is, To tar paper or to lime paint the OSB (followed by metal mesh, plaster, paint)
Are you suggesting that this step is excessive? That we could lath onto blank OSB and continue from there? That seems to allow for untreated irregularities between wood and wire...

Thank you again for your generous participation in this, our dream It honestly means the world to us!
 
Chadwick Holmes
Posts: 618
Location: Volant, PA
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forest garden fungi goat trees wofati woodworking
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Oh, please understand no judgment on foam, I was in construction as a young man, I am resonable for so much Ick ....I could never ride a high horse!!

Lime is a very interesting material it actually keeps wood from rotting from what I have heard, but this is the only material in this sandwich that I cannot trust my knowledge on, so that's why I said I hope a lime person can confirm that for us.....

Good luck on your project, that's awesome!
 
Lauren Magnolia
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Thank you!
 
William Bronson
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Location: Cincinnati, Ohio,Price Hill 45205
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Your question got me interested in lime plasters. My research led me to this:
http://www.greenbuildingadvisor.com/blogs/dept/musings/install-stucco-right-include-air-gap

It discusses Portland based stucco, but I think it would apply.
 
Bryant RedHawk
garden master
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Location: Vilonia, Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
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To install a plaster or stucco over wood, the wood first needs to be protected from moisture, just the moisture from the application of the plaster material will cause problems down the road from trapped moisture against the wood.
You do have options for doing this though.
You can use house wrap, I don't recommend the use of tar paper as a moisture barrier, that is way outdated except for roofs where asphalt/ fiberglass shingles are going over it, even then there are better options today.
If you don't want to use house wrap, you can use a water barrier paint.

Once you have the wood protected you install the metal lathe material which is the "tooth" for holding the plaster (stucco) to the wall.
The proper way to install this type of finish is : Base coat, put on thick enough to cover the lathe. This is then allowed to cure for a day.
Scratch coat, this is thinner than the base coat and you use a fine toothed trowel to apply it. This is then allowed to cure for a day.
Top coat, this is the finish coat, you use a smooth trowel and it is a thin coat of material usually with a little extra lime added for hardness.
When you are ready to apply the scratch coat you first wet the base coat with a brush loaded with water, this is shook onto the base coat not brushed like paint.
You use the same technique between the scratch coat and the finish coat. If the finish coat starts to stick to the trowel as you work it, apply water and continue. You can also dip the trowel in water.

Lime does not protect wood, Borax protects wood, lime is corrosive and will etch wood, over time this creates dry rot and is the main cause of wall failures on houses with a plaster or stucco finish.
 
Lauren Magnolia
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Thank you for mentioning Borax... Do you know any evidence of a similar paint to the lime paint often used with cob and other earthen plasters? The attraction was that we could mix it ourselves with materials on hand.

Another thought: using tar paper with firring strips (about a 2x4 ripped to 3x8ths on top, followed by a semi-permeable vapor barrier such as tyvek, lath, plaster, lime paint...

Thanks again, y'all
 
Bryant RedHawk
garden master
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Location: Vilonia, Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
224
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Lauren, Unfortunately I have not found anything that could be mixed from materials on hand that will work well. I wish I could and I am not giving up the search but so far I have not found any except for latex, but I don't have rubber trees to tap for that.

I use the lathe strips and tyveck house wrap, this gives you good protection over which you can put your plaster materials. Put the wrap on with a "slapper stapler" then with the lathe you want no more than 3/8" spacing between, this way there is enough for the base coat to anchor to.
The main reason they came up with the newer metal lathe material was for speed and less tendency to separate from the anchoring.
 
Brett Hammond
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Location: Maryland, USA
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It is important to only have ONE vapor barrier in a wall. Otherwise, moisture may get trapped between them, foster mold/rot and eventually cause your wall to fail. Most building codes require a single vapor barrier between conditioned and unconditioned spaces. Usually, it is on the interior side of the wall insulation to minimize condensation inside the wall in cold climates. In modern construction, the walls are built, unbacked insulation installed, and then a sheet of plastic is stapled up to the interior of the studs to create a vapor barrier before the drywall is installed.

In your case, if you want to use foam insulation, most types of foam are a vapor barrier. So I would not install any vapor barrier (ie. tar paper or waterproof paint) between your OSB and exterior plaster lath. If you feel you need to protect the wood, use house wrap which is breathable and will keep out liquid but will allow water vapor to pass so any trapped moisture will evaporate out.

I would recommend the metal lath over your OSB, because it is more dimensionally stable than wood lath strips as temperature and humidity change, you will get less cracking in your plaster. You may also want to consider a portland cement stucco plaster designed for exterior use. It is tricky to keep it from cracking while drying, but some good youtube videos will walk you through the entire process of attaching the metal lath to the house and plastering in layers. I think the trick is to keep it wet as it dries.

You may also want to consider the rock wool batt insulation sold by Home Depot which is only 50% more than fiberglass. Or they sell the denim stuff, but it is much more expensive. You can also order sheep wool online from manufacturers for about the same as the rock wool. And I think they took all the toxic stuff out of the fiberglass now, but not certain.

btw, the foil backed foam is only good to reflect radiant heat, so it doesn't make sense to pay extra for foil backed if it is tightly sandwiched between wall materials, and won't have the insulating R-value advertised unless it has space on the foil side. It is good for use up in your attic or basement crawl space.

Hope this helps. Best of luck!
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