Insulating slab with Super Isol

[mrpbjnance]

I used the durolock cement board in place of the plywood and had it sitting on top of my blcok stand making it permanent. I had some 2x4 braces for temporary braces. I poured a 6" thick hearth which I put the Super Isol on Top. I carefully watched the durolock during the pour and there was no sag at all.

[dalucca2003]

So with the Durolock and of course rebar, there is no additional support under your hearth?

[maver]

support

the rebar and concrete layer are the support, once the hearth cures at least a week. The lumber frame support for durock or plywood do the job until at least a week. The durock is flimsy and provides no real structural support.

[james]

I think of the concrete board as a permanent form. It's sole purpose is to hold the concrete in place while it cures, but unlike plywood, that you cut to fit and then drop out of the hearth after the concrete dries, you can lay the concrete board on top of the block stand and leave it there.

Both are good methods.

James

[dalucca2003]

I think of the concrete board as a permanent form. It's sole purpose is to hold the concrete in place while it cures, but unlike plywood, that you cut to fit and then drop out of the hearth after the concrete dries, you can lay the concrete board on top of the block stand and leave it there.

Both are good methods.

James

So do you think it would be okay once finished framing for the hearth to place the cement board inside the frame on top of the blocks as well. Or would it be better to have the cement board only inside the stand opening as one would do with plywood.

[mrpbjnance]

I had the framing sitting on the block and the durolock inside that also sitting on the block. But do not that leaves an awkward ledge around your block when you remove your forms for the hearth, that I have not figured out how to finish yet. Personnaly I build a pier using blocks in the center that I left in place to support my hearth. It does not take a lot of space but is not needed and the 2x4s alone work well. You can also use the blocks to support the durolock during the pour then remove them when the hearth is dried.

[maver]

Either may be ok, I think

Just be sure to use flashing as a slip plane (probably under the board). I'm unsure of any reason to cover the concrete blocks with the cement board though. I just placed mine inside the block stand over the frame. Is there any reason you would want to extend the board over the block?

[james]

Skip the slip plane

The aluminum flashing was an experiement that we tried early on, and with more experience, we have removed that step from the instructions. It does not get hot enough where the hearth meets the concrete blocks to worry about needing a slip plane. You can save the time and effort.
James

[dalucca2003]

So if no slip plane is used, do we just fill the remaining cinder block holes with cement from the hearth pour?

[mrpbjnance]

My Durolock coverd up the holes. Or you can cover them with something else. No need to use all the extra concrete to fill up the holes.

[maver]

So if no slip plane needed then you might consider extending the concrete board over the block to block the empty cells - I don't think you need every core filled with concrete structurally, and if you are mixing your own concrete it will save you a bit of labor.

However, if you are doing super isol (so no need to mix your own perlite or vermiculite concrete) I might suggest you look into getting your concrete premixed. For a job this small you probably would not rent a truck and a pump, but getting a BIG truck -1/4 pound or more such as Ford F250 - with a heavy duty hitch (I borrowed from a neighbor) you may be able to find a rental company like I did who will rent you a big (1 yard capacity) mixer on a trailer, fill the mixer with the concrete and water for you, then you just tow to your house and pour into buckets and dump it into your form. A yard should be sufficient to pour the hearth to six inches and fill every other cell (calculate this for your particular hearth size to be sure).

[dalucca2003]

So if no slip plane needed then you might consider extending the concrete board over the block to block the empty cells - I don't think you need every core filled with concrete structurally, and if you are mixing your own concrete it will save you a bit of labor.

A yard should be sufficient to pour the hearth to six inches and fill every other cell (calculate this for your particular hearth size to be sure).

That was my plan to go get a yard of cement which should be plenty. Question Maver.....first paragraph you state no need to fill empty cells but last sentence you mention filling other cell. Are you talking about the ones which require rebar? I have already filled most of mine and plan to finish every other one prior to pouring the hearth.

Additional question on concrete board....I went to Home Depot and all they have is 3' x 5'. Do they make it bigger and where to go to obtain. Otherwise I see a lot of work reinforcing the board from underneath.

[maver]

I don't think the cells need to be filled, but if you do get a yard of concrete you might as well because I think otherwise you will likely have extra concrete - you need to put it somewhere . I don't think I'd bother filling them prior to pouring the hearth, the every other rebar/concrete filled cells will keep things from moving with the pour.

I used 3x5, which on the inside dimension of the block stand was just under 2 sheets. I duct taped the one seam from above to stop concrete from pushing through. You do need a bit of reinforcement, but no more than you would need for a larger sheet, just good vertical reinforcement for the slab weight, I braced 2x6 laid horizontally on the edges and several through the middle with about 10 vertical 2x6.

[dalucca2003]

Maver,

Sorry for the confusion.....for now I am only filling the cells with the rebar. I thought filling the other ones with the hearth pour would be overkill. But now that I think about it, if I go get a yard of cement from a rental location, I will need to dispose of the extra concrete one way or another. Good advice.

Question for anyone....why did the plans change from utilizing the slip plane?

[maver]

James addressed this above, there just is not enough thermal expansion (with insulation above the concrete structural layer of the hearth) for it to be necessary. I built mine with a slip plane. If it really is unneeded, consider extending rebar from the block stand up into the hearth to lock them together - I believe that idea has been discussed earlier on this forum.