# 3/8 Millboard or Equivalent



## BuildOne (Sep 5, 2018)

I am building a hearth for a circa 1986 Jotul Alpha stove. I have the manual and my county will be inspecting the installation. The manual says 3/8 millboard or equivalent for the hearth. I called Jotul and they don't like to talk to consumers so I've traded messages with them, but I still don't have the answers. Maybe someone here knows:

1) What is the R-value of 3/8 millboard? 
2) Is Durock an equivalent to millboard? 

I will be building up the hearth on top of a plywood sub-floor to the height 3/4 of an inch to meet a 3/4 inch finished wood floor. I'd like to use a continuous sheet of 16 gauge rusted metal as the finish. 

1) If I start with a sheet of 1/4 Durock on top of the sub-floor, and then add a plus of minus 3/16  layer of metal strips to create a thin layer of air space, then top that with another layer of 1/4 Durock, and finish with the 16 gauge rusted metal (1/16 inch), would that be equivalent to 3/8 millboard?


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## begreen (Sep 5, 2018)

IIRC millboard value is K=.84 or R=1.19. That would require 1.5" of Durock NexGen before topping with the metal sheet. 3/4" is not a lot of space to build up R value. It would be better to use 1/2" Fiberfrax Duraboard LD or 1/2" Micore 160, then 1/4" Durock, and maybe with pads under the stove feet to reduce the chance of dimpling of the steel sheet.


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## BuildOne (Sep 5, 2018)

I just now got a message from Jotul that the K-value of millboard is in fact .84, but they say the K-value is "always" for an inch of material so in order to get the R-value for 3/8 millboard you would need to multiply by 3/8. That would bring down the R-value to .446 for 3/8 millboard (1.19 x .375). 

Still with only 3/4 of an inch, 1/4 Durock or Hardie board is only R .13. It looks like I need to use a 1/2 Micore product either the 160 or 300. 

I did find where 1/2 drywall has an R-value of .450. Do you know if drywall with the paper on it is considered noncombustible?

The Micore will work but I have to order it on-line. My lumber yard says they cannot get it and it's expensive getting it delivered. 

Do you have any suggestions to get R .446 in 11/16 (3/4 of an inch minus 1/16 for the sheet metal)? 

I have the metal and have rusted it and it's heavy enough that I don't see it crimping or denting. It will scratch but that's the rustic look I'm after. 

Thank you,
Steve Smart


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## begreen (Sep 5, 2018)

I think the K value is for 3/8" millboard, not 1", but I'd have to see the actual text. Typically it is something like:
_The specified floor protector should be 3/8” thick material with a k-factor of 0.84._

You could ask Jotul to provide the required R value to verify.

Paper coated drywall is combustible.


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## bholler (Sep 5, 2018)

BuildOne said:


> I am building a hearth for a circa 1986 Jotul Alpha stove. I have the manual and my county will be inspecting the installation. The manual says 3/8 millboard or equivalent for the hearth. I called Jotul and they don't like to talk to consumers so I've traded messages with them, but I still don't have the answers. Maybe someone here knows:
> 
> 1) What is the R-value of 3/8 millboard?
> 2) Is Durock an equivalent to millboard?
> ...


I would check with your local inspector before doing anything.  Is this stove ul listed?  If not they may not care at all what the manual says and go by nfpa211.  Or they may not pass it at all because it is unlisted.


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## begreen (Sep 5, 2018)

bholler said:


> I would check with your local inspector before doing anything.  Is this stove ul listed?  If not they may not care at all what the manual says and go by nfpa211.  Or they may not pass it at all because it is unlisted.


It should be. It was tested and passed EPA phase 1 specs.


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## bholler (Sep 5, 2018)

begreen said:


> It should be. It was tested and passed EPA phase 1 specs.


Ok then disregard my post in that case


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## begreen (Sep 5, 2018)

OK, more info. I knew I had researched this a long time back. Here is a link to the thread:
https://www.hearth.com/talk/threads/r-value.53445/#post-670711
As you see, millboard's insulation value is not a hard spec. When I looked at this I compared it to regular Durock with an R value of .26 for 1/2". Durock NexGen is R=.39. I think you would be ok with 3/4" of Durock NexGen underneath. Hope that helps.


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## begreen (Sep 5, 2018)

bholler said:


> Ok then disregard my post in that case


I had to look it up. It's an early (for Jotul) steel cat stove.


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