Air space in hearth, unvented.

[blel]

I've looked around but can't seem to find the answer.
What R value do you get per inch of unvented air space in a hearth.
Thanks

 

[Hogwildz]

there is an R & K value chart on site here somehwere, do a search. I think air is one of the higher numbers.

 

[begreen]

1" of air = ~ R 0.7 dead or alive.

 

[blel]

Thanks. So 2" air, plus 1' wonderboard,cement board etc, plus some sort of tile/stone, should give me R2?

 

[begreen]

Yup, double up on the durock or wonderboard both for strength and additional R value and you are at R 2.

 

[webbie]

Ideal air=space hearth might be something like this - one sheet of sheet metal or durock on the floor, then some drywall hat channels (for airspace), then 3 or 4 layers of durock or wonderboard on top - finish with tile or slate.

That should do up to about R=2

I would be possible to build something like this so it could sit on carpet (thinner) - in that case, you want to have a number of smaller points - maybe 1/2" square, where the assembly hits the carpet.

 

[blel]

Couldn't I just put hat on floor and then 2 layers of Durock, then tile? I suppose I could put strips of Durock under the hat instead of full sheet. What are dimensions of hat?
2" air =about 1.4R, plus0.5 for Durock, plus tile should be approx 2.0R from what I have read.

 

[Highbeam]

I used the "dead air space" for my high R value hearth. I used the steel studs to make a platform and then attached the platform directly to the subfloor. The platform was topped (2 layers ) and sided (1 layer) with wonderboard cement board and then tiled. Dead air space is an extremely high R-value item.

The Hearthstone manual for my Heritage lists a chart for R-values. I just now checked the online manual for the Heritage 8021 (my stove) and....

Horizontal Still Air gets 0.92 per 1/8" which is totally different than above quoted.

Funny that to be safe, my hearth would need to be built out of 6 layers of wonderboard OR a 3/16" air gap.

 

[blel]

"1” of air = ~ R 0.7 dead or alive."

"Horizontal Still Air gets 0.92 per 1/8” which is totally different than above quoted."

This is why I am confused. These two values aren't even close. Don't know what to believe. Any help?

 

[Highbeam]

http://www.hearthstonestoves.com/assets/files/document_library/Heritage8021Manual.pdf

Here's my source for my stove, page 8. When the inspector shows up and wants to see the manual it is best when the installation agrees with the manual vs. me saying that I read it on the net. It's been awhile but I am fairly certain that I emailed Hearthstone to verify this with a tech and he assured me of the accuracy in writing. In my case, I wanted to be sure that I did not need to vent the air space and he wrote no. Inspector loved it.

 

[begreen]

Wow, my uninsulated walls just jumped up to R25 according to Hearthstone!

(just kidding) You bring up an interesting point. These figures seem to be all over the map. I suspect someone at Hearthstone has erred. Here's another chart that lists 1/2" to 4" trapped air at R 1.0.

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

 

[blel]

RankRank
CT
Total Posts: 25
Joined 2008-06-06


“1” of air = ~ R 0.7 dead or alive.”

"Horizontal Still Air gets 0.92 per 1/8”

So 1" =7.36R ?

Is there any definite source on this or are we all just guessing?