Installing fireboard, need help!

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MacMac

New Member
Jan 6, 2022
9
95922
Hi!
So I’m building a cabin from scratch and it’s time to build out the area for my wood stove! It’s going in a corner that I’ve already insulated and put up vapor barrier. I’m ready to install the fireboard/cement backer, but I’ve run into a question that Google cannot answer. Do I remove the plastic vapor barrier and the paper backing on the insulation before putting up the cement boards? And does there need to be an air space between the cement backer and the studs? The insulation is fiberglass batts with the paper backing. The vapor barrier is 5mil plastic sheeting
I’ll attach a photo of the walls.

image.jpg
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
30,158
central pa
Hi!
So I’m building a cabin from scratch and it’s time to build out the area for my wood stove! It’s going in a corner that I’ve already insulated and put up vapor barrier. I’m ready to install the fireboard/cement backer, but I’ve run into a question that Google cannot answer. Do I remove the plastic vapor barrier and the paper backing on the insulation before putting up the cement boards? And does there need to be an air space between the cement backer and the studs? The insulation is fiberglass batts with the paper backing. The vapor barrier is 5mil plastic sheeting
I’ll attach a photo of the walls.

View attachment 296114
It really doesn't matter what you do with the vapor barrier or paper face with wood studs and fiberglass it will always be a combustible wall. So clearances will need to be honored to those studs.
 
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MacMac

New Member
Jan 6, 2022
9
95922
It really doesn't matter what you do with the vapor barrier or paper face with wood studs and fiberglass it will always be a combustible wall. So clearances will need to be honored to those studs.
Would it be a good idea to remove as much combustible material, or does it still not matter??
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
30,158
central pa
Would it be a good idea to remove as much combustible material, or does it still not matter??
Unless you are removing the wood studs it doesn't matter
 
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bholler

Chimney sweep
Staff member
Jan 14, 2014
30,158
central pa
I plan on covering the hardibacker board with mortar and riverstones
That won't change the clearance requirements to those wood studs. The clearances are all spelled out in the manual. Page 10 has the double wall pipe clearances that you will go by
 
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begreen

Mooderator
Staff member
Nov 18, 2005
96,312
South Puget Sound, WA
Would it be a good idea to remove as much combustible material, or does it still not matter??
What matters the most is the stove that will be going in. Many modern stoves have rear heat shielding built-in and good corner clearances. Those clearances are to combustibles, vapor barrier included. Our corner install has just drywall behind it. The paper on the drywall is combustible, but there are no issues because the clearance requirements are honored and even exceeded a bit in our case.

What stove will be installed?
 

MacMac

New Member
Jan 6, 2022
9
95922
What matters the most is the stove that will be going in. Many modern stoves have rear heat shielding built-in and good corner clearances. Those clearances are to combustibles, vapor barrier included. Our corner install has just drywall behind it. The paper on the drywall is combustible, but there are no issues because the clearance requirements are honored and even exceeded a bit in our case.

What stove will be installed?
It’s the morso standard 2b
 

begreen

Mooderator
Staff member
Nov 18, 2005
96,312
South Puget Sound, WA
It’s the morso standard 2b
That is a radiant stove with higher clearances than many. It will need 16" from the back corners to the wall if it is installed diagonally in the corner. This is double the clearance requirement of some shielded convective stoves and triple of a jacketed stove like the PE Alderlea T4.
 

MacMac

New Member
Jan 6, 2022
9
95922
That is a radiant stove with higher clearances than many. It will need 16" from the back corners to the wall if it is installed diagonally in the corner. This is double the clearance requirement of some shielded convective stoves and triple of a jacketed stove like the PE Alderlea T4.
Yea that’s what I was reading. I just didn’t know if the fireboard and stone would decrease that distance or not. Somehow I couldn’t get an answer anywhere!! But this forum has been so helpful! I now feel confident I can proceed with a safe install!
 

begreen

Mooderator
Staff member
Nov 18, 2005
96,312
South Puget Sound, WA
The board and stone alone would not change clearances to the studs in the wall. If you feel more comfortable with wall shielding in place and want to reduce the clearance, the most effective way to do this is by putting up an NFPA 211 spec wall shield. This will need to be on 1" stand-offs, and open at the top and the bottom of the wall shield. That would reduce the clearance down to 12" in the corner, as long as double-wall stove pipe is used or the stovepipe is shielded all the way up if single-wall.

If the wall shield is to get a stone veneer then it needs to be well anchored. One way to do this is to make long, 3" wide strips of the 1/2" cement board, double them up to create 1", then screw them vertically to the studs to create firring strip spacers. Alternatively, the wall shield could be made out of sheetmetal or antique ceiling tin for a thinner and easier-to-support shield.