what is still air? R value 2.5 needed

[boboiatl]

I have a hearthstone homestead with a 2.5 R value for floor protection and I have 5 inches to work with to achieve that.The owners manual says horizontal still air =.92. I have read of using strips of 1 1/2 inch wide by 1/2 inch thick hardiebacker as spacers with full sheets separating each layer. My plan would be to use 1/2 inch thick strip on subfloor -------followed by 1/4 inch full sheet-----------then
1/2 strip as spacer
1/4 full sheet
1/2 strip as spacer
then full sheet with tile to finish. I think that would get me to around 2.96 and keep me around 3 inches in height.
Is this right and if not any suggestions would be appreciated.

 

[willoweep]

I am reinstalling my Homestead in our new home, and have seen that table in the instructions for years. My question is, what will keep the 1/4" hardibacker from deflecting when you set a 375 lb. stove on top? If you have spacers under the feet, it's not R-2.5. Just layer up 3 sheets of Micore with Durrock on top, add tile and be done. That's what I'll be doing this weekend.

 

[Grisu]

Agreed. For that r-value your best bet will be Micore or similar plus some Durock for stiffness. This table should help: (broken link removed)

 

[begreen]

I have seen all sorts of ratings for dead air space. This site gives 1-4" dead air space an R=1.0 rating.
http://www.coloradoenergy.org/procorner/stuff/r-values.htm

That said I would skip the airspace and I would not use Hardibacker in this application. It has a much lower R value than Durock NexGen. (R=.20 vs R=.39). Instead I'd put down 1" of Micore 300, then 2 sheets of 1/2" Durock NexGen for a final value of R=2.84. You'll want the double sheets of cement board on top for added stiffness.

 

[edyit]

I have seen all sorts of ratings for dead air space. This site gives 1-4" dead air space an R=1.0 rating.
http://www.coloradoenergy.org/procorner/stuff/r-values.htm

wait am i reading that right? dead air space say from 2x4 framing then covered gives an R of 1?