Render question

[maineoven]

I have read the archived posts on stucco and render but still have a few questions. Thanks for any help.

I have a bunch more firings before I put the 2 render coats on my vermicrete layer. The vermicrete has formed some large expansion cracks that open and close with temp changes. I plan on using surface bonding cement with the liquid additive to form the stucco layer which I will later paint with stucco masonry paint.

I read that you would first fill in the cracks with SBC before the first layer goes on. If I cover the cracks when the oven is cool, I don't understand how the cracks won't bust through the stucco layers when heated up. How do you handle the expansion cracks so they don't crack the stucco?

Also, how long should you wait after the first coat before putting the finishing stucco coat on the oven?
Thanks

[david s]

Re: Render question

Don't worry about the vermicrete layer cracks. The layer is not structural. The only reason to add cement, and therefore water is so the stuff will stand up vertically on its own. If adding vermicrete to an enclosure style oven it is better to add the vermiculite dry. The problem you now have is that you have to eliminate the water otherwise the steam pressure can crack the outer stucco.the dryly vermicrete is slightly elastic anyway so won't cause a problem so long as it's dry. Try placing some plastic sheet when firing, over the vermicrete to test if water condenses under it to test if all the water is eliminated from the vermicrete.

[maineoven]

Re: Render question

Thank you David, but even if the vermicrete layer is dried out, won't the cracks that already exist continue to expand and contract as the oven is fired? If the cracks go up and down based on when the oven is hot/cold, then how do you prevent the stucco layer from cracking also?

[Tscarborough]

Re: Render question

As he said, the vermicrete is not structural, i.e. it will not mirror through the render/stucco. Do not fill the cracks with SBC (that is for structural, non thermal crack repair), fill them with either loose vermiculite or a very high ratio vermicrete (like 8 or 10 to 1).

[maineoven]

Re: Render question

I understand now, Thanks