OK to pour concrete directly over Micore?

[WendyMuster]

I need to extend my existing brick hearth for a wood stove insert that requires 1.24 R-value, 20" total hearth extension in front of the stove. I would like to pour appx 4" deep concrete right over micore (set on top of the subfloor). Is that possible? I've read a lot on this forum about putting cement board on top of micore because it's too soft for tile installations. But I'm not putting tile or brick, just the concrete over the micore.

I need an R-value of 1.24 and my existing hearth is about 4" above the wood flooring. I will cut out the wood flooring (which is 3/4") and put the micore directly on the subfloor, then build a frame and pour concrete up to level with the existing hearth.

Can you please let me know if this is possible? Do I need to attach the micore to the subfloor? Do I need cement board between the micore and the concrete? Can I just use a lath (?) layer over the micore, under the concrete?

Thank you for any guidance you can offer!

 

[sesmith]

Can't answer your question directly, but as an alternative you could do 2" of concrete over 2" (4 x 1/2" sheets) of USG Durock cement backer board to get your r-value requirement. 1/2" Durock has an r value of .39.

 

[WendyMuster]

Thanks for the alternative idea!

However, I already have the micore (from many previous iterations of this plan) so I'd like to use it if i can. Obviously because I have no other use for it and because achieves 1.03 of my r-value in only 1/2". The thickness of the micore, on the subfloor, would be completely below the height of the existing wood floor. I'd also rather not have lines/layers of materials showing above the existing flooring.

I'm having a hard time finding out what the R-value of quikcrete is as well. Maybe I only need concrete and no micore or cement board?

 

[kennyp2339]

4" of concrete is pretty heavy, I'd double or triple up the boards underneath then pour a 2" layer if it were me, you'll be over your K factor, but lighter on the rafters.

 

[jmb6420]

Concrete r value is pretty low. Also, double the thickness doesn't double the r value!

 

[Highbeam]

I’ve poured lots of concrete directly over expanded poly styrene foam. Like styrofoam but 20 psi rated. No idea what the psi rating of micor is but if it is too low and compresses you could crack your new slab and/or lose your r-value and/or the whole slab will settle.

I would much rather use the layers or durock and then hold them short of the form so that from the side you don’t see The durock.

 

[WendyMuster]

I’ve poured lots of concrete directly over expanded poly styrene foam. Like styrofoam but 20 psi rated. No idea what the psi rating of micor is but if it is too low and compresses you could crack your new slab and/or lose your r-value and/or the whole slab will settle.

I would much rather use the layers or durock and then hold them short of the form so that from the side you don’t see The durock.

that's a great idea! thank you!

 

[WendyMuster]

Concrete r value is pretty low. Also, double the thickness doesn't double the r value!

wow, this isn't very good news for me. thanks for the info. i do NOT know what i'm going to do about this hearth extension problem