R-value of cement block question

[Boiler74]

Hi all,

Thanks to this site, I found a website that lists the R-value of various building materials. It is:

http://www.coloradoenergy.org/procorner/stuff/r-values.htm

It lists concrete block R-value for 4", 8", and 12" block. My question is, is this for solid block or block with air spaces? I assume it's with the air space but want to make sure. I'm lowering, extending, and rebuilding my masonry hearth and want to make sure my R-value math is correct.

Thanks
Josh

 

[begreen]

I believe the R value of solid concrete is about .095/in.

 

[Beau]

Which is why I was looking to line my fireplace with Roxul.
My fireplace is below grade, sitting in dirt : /

 

[begreen]

Personally given that scenario I would work to encourage good air circulation around the case of the insert. And maybe run a dehumidifier in the summer. Given the concern about overfiring the insert and the dampness of the environment I would hold off. It can always be retrofitted if necessary. Just my pov given the description of the situation.

 

[Boiler74]

I'm asking about concrete block, not solid, poured concrete. According to the site linked in my first post, the R value of an 8" concrete block is 1.11. The r value of poured concrete is .08 per inch. That is .64 for the equivalent 8" thickness. So the concrete block has a higher r-value. Is that due to an air space or the simple make-up of the concrete in the block? That is what I'm trying to determine.

Thanks
Josh

 

[fossil]

Air space.

 

[Lumber-Jack]

I'm sure Boiler is referring to these kind of blocks with the hollow core.

And I think they do have a slightly higher R value if left hollow rather than being filled with concrete. It's because the heat transfers easier through solid concrete than air gaps. Sometimes they'll fill them with vermiculite which is even better, but still pretty low.
The best thing you could do would be top dig down along the wall outside and install rigid foam insulation. But that might be a big job.

 

[Boiler74]

I think there is some confusion on what I am doing. I'm building a hearth for under my stove. In his post above, Beau talked of being below grade. I am not dealing with below grade. I'm just trying to make sure I get my R-value right and will be using cement blocks to form the interior structure of my hearth.

Thanks
Josh