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kebwi 02-07-2010 12:00 AM

Using hardibacker as vermicrete form
This question was addressed in another thread, some concerns came up, then the thread kind of fizzled before I could get clarification on the details.

My enclosure will consist of poured, formed vermicrete, not a hand-molded vermicrete dome. The structure will have square vertical sides. I had originally intended to form the pour with wood, then perhaps wrap the vermicrete structure in chickenwire after it cured before applying SBC.

However I am now strongly leaning toward walling the vermicrete off with hardibacker to strengthen the sides and support the square corners (along the top edge).

I can think of two ways of doing this:
  • Form with wood, pour and cure the vermicrete, then mortar the hardibacker against it.
  • Form the pour with hardibacker and be done with it.

Clearly, I was planning on the second method. However, I'm not at all convinced that merely pouring vermicrete into a hardibacker form will force the two to adhere to one another. When this came up before, it was suggested I mortar the hardibacker to the vermicrete, but I don't understand how to do that if the hardibacker comprises the form. Should I actually slather the inside the hardibacker with mortar immediately before pouring the vermicrete in? The seems a little odd.

Any thoughts on this?


kebwi 02-09-2010 07:42 AM

Re: Using hardibacker as vermicrete form
I'm still trying to determine whether pouring vermicrete into a hardibacker form will cause the two to properly adhere? I'm guessing not, even if I thoroughly wet down the inside face of the hardibacker. Correct?

Tscarborough 02-09-2010 08:23 AM

Re: Using hardibacker as vermicrete form
1 Attachment(s)
It will adhere just fine, but the vermicucrete has almost no strength in shear, so it will just pull away some of the vermicurete if the hardi board moves.

Here is that cross section I promised you.

kebwi 02-09-2010 08:36 AM

Re: Using hardibacker as vermicrete form
Okay, with the exception that my dome-formed insulation is InsWool HP Bulk and not vermicrete, that is exactly what I was planning on doing. I guess I didn't realize that the surface bonding cement would be strong enough to encase the hardibacker against the vermicrete (it just seems so thin). That's why I was considering locking the hardi to the vermicrete with bolts and large fender washers or tie plates incased deep in the vermicrete, or wrapping the hardi/concrete-block/vermicrete structure in chickenwire before stuccoing it.

So, I should just go ahead and pour the vermicrete behind the hardibacker and then not waste any time getting the surface bonding cement on because the hardi won't actually be very strongly held in place following the pour? But it will hold strong and long enough to get the whole thing enclosed?

I'm just asking that it won't immediately fall away so I don't even have time to work on it. I need to wait a day or two for the vermicrete to thoroughly harden so I can place bricks on it, stuff like that.

On another note, will the SBC wrap up a 20" vertical of hardi? I read somewhere that the hardi might be too smooth for SBC or stucco to poperly stick too. I was consider wrapping the wall in chicken-wire or expanded diamond lathe to give the SBC a matrix. Should that be unnecessary?

Thanks again.

Tscarborough 02-09-2010 08:52 AM

Re: Using hardibacker as vermicrete form
It will stick fine, no chicken wire needed.

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