Will I be using an acrylic rubber "non-toxic" breathable roof sealant? The Cob Dome experiment

When I bought those Hurricane Ties a couple days ago - or yesterday? Anyway I was pondering how to make sure the roof is waterproof and yet breathable. Silicone is too expensive but then I saw the claim of acrylic being breathable - and about half the price of silicone. I had previously made a silicone coated canvas - and it was water proof, much easier than "linseed oil" that does not dry properly without petroleum or mineral oil spirits...

So anyway....

Suddenly I just discovered - after searching cob roofs on youtube - sure enough. Someone did the EXACT precisely same thing I was pondering!!

elastomeric coating (Maxi-Stretch) to the dome.  The manufacturer (Ames Research) pre-colored the polymer to a light tan shade.

I decided to continue to move away from a traditional lime plaster exterior rendering due to the fact that all the natural builders here in Arivaca have had no long-term success with keeping the lime plaster from separating from a cob base wall.  This is due to the daily 40-50 degF temperature swingshere, that cause lime plaster to expand and contract at a different rate than the underlying cob wall, thus essentially separting itself from the cob wall.

Here's the vid

  I applied to coats of liquid rubber (Blue Max), followed by 3-coats of elastomeric.   This brings the weather-proofing to seven total coats for the dome.

Please note that this process of appling a painted-on rubber coating onto cob (straw/clay) is EXPERIMENTAL, with the thinking that it cannot be any more of a maintenance burden that having to replace a lime plaster rendering every two years as has been the case with other adobe building I have constructed.

Also note that these high-tech coating are non-toxic and safe for water-harvesting.

So it's non-toxic - which really means (in my view) it's not caustic or not reactionary.

 Polyacrylic elastomers, also called ACM rubbers, are synthetic rubbers composed of acrylic monomers.

 http://cobstudio.blogspot.com/

 So he also mixed in "barn lime" - calcium carbonate - with his hydrated lime... just as I was planning to do. He says it works more like clay as a "binder." Not as a filler or like sand - as aggregate for strength. Yes that makes sense now that I think of my own experiment using the barn lime on my entrance "road."

So a film was made of his Cob Dome!

 So before watching that mini-documentary... I will read this pdf on WHAT acrylic rubber really "is."



yeah that didn't tell me much...

Polyacrylate rubber (ACM) is formed through copolymerization of acrylic esters with monomers.

It should not be used in temperatures below −10 °C (14 °F).

oops!!

So that kind of rules this stuff out for me.

It would probably crack or something strange...

yeah I'm not really interesting in watching the doc - I just did a "speed" review - without the play on. Just skim ahead on the visuals as stills....

What I was interesting in was just confirming that my "idea" was not totally crazy - in fact someone has done it on a "grand" scale.

So that kind of leaves me back to the drawing board. It's not a big area. What's the low temperature that silicone can handle?

Silicone rubber is generally non-reactive, stable, and resistant to extreme environments and temperatures from −55 to 300 °C (−67 to 572 °F) while still maintaining its useful properties.

OK so I'll use that on canvas - as I had previously mentioned. I can get the cheaper canvas from Harbor Freight - their paint canvas - and figure out how to connect them. Then I can put more stuff over this also - another layer of lime or something....

Failures of Lime Plaster Over a Clay Base - StrawBale.com

https://www.strawbale.com/failures-of-lime-plaster-over-a-clay-base/

Use lime plaster OR clay but don't mix them on the same wall. ... some lime to the earthen plaster mix as a layer before a lime layer helps with it adhering. .... I suggest you take a rubber mallet to the walls you have completed and test them out.

So this is fascinating to me - as I mentioned before the new secret is to use lime that is hydrated or quick for reacting as mortar plus just straight calcium carbonate - plus HAIR as the binder.... and NO sand or clay....

We have a 10 year old cob, light straw clay, clay waddle barn that has a lime plaster over the straight clay cob. The lime plaster over all of these is working great! So…perhaps for these methods it’s different than for straw bale. I had also heard that adding some lime to the earthen plaster mix as a layer before a lime layer helps with it adhering. Likely it makes a difference if the wall is either still somewhat wet or is wetted before applying the lime plaster.

Andrew Morrison Mon, September 5, 2016 at 4:55 pm #

Great to hear Laurie. I find that lime over cob tends to work well if mechanically bonded. Perhaps whomever did the original build (you?) did a great job on that bonding for the entire barn. It is not impossible to do right, just very difficult with more failures than success from what I see.

 t can be really hard to successfully work with lime over clay. The main two reasons are that the two materials do not bond well by design (having different properties) and that the two materials move differently. In other words, the clay is soft and flexes UNDERNEATH the more rigid lime which causes the lime to delaminate from the clay. There is not much that I can offer in a repair as I just avoid the combination of the two materials entirely just to be safe.

 Wow this is what I was planning on doing! Good thing I discovered this....

it is possible to have success with lime over clay (plaster or base wall), but it’s not easy and most fail. It is primarily because the two mediums do not behave in the same manner and that causes separation. This would be true over plaster or base wall material.