Help with 1 inch air gap hearth wall

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P

PJP

Member
Mar 22, 2020
26
Maine
Hi All,

I tore down the old hearth and building a new one. I have a question about building a hearth with a 1 inch air gap. All of the research I have been doing and I cannot find a diagram or picture of how it should be built. All I can find is, "3 1/2 inch masonry wall with 1 inch air gap" to reduce my clearances by 66%. There are no specifics even from a formal document written by the State of Maine. But then when you do some research people are saying to leave a gap on the top and bottom. What exactly does this mean?

Does this mean a literal air gap at the bottom of the wall in conjunction with the air gap behind the wall, kind of like the wall would not touch anything anywhere - a floating wall so to speak? Not even touching the hearth floor?

Or does it mean just an air gap from top to bottom behind the wall but with no gap between the hearth floor and wall?

Peter
 
Yes, the wall shield needs to be ventilated. This means open at least 1" at the bottom from the hearth pad and open at the top so that air can freely convect behind it.

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The hearth floor is ok to touch the wall.

What stove is this for and what is the clearance reduction goal in inches?
 
Open at the bottom, behind & at the top.
You need the air to be able to circulate thru the lower gap at the floor level,
up behind the wall & out the top of the wall opening (or from top to bottom).
1" is the minimum air gap.
Steel 2x4s laid with the 3.5" side flat on the wall make nice wall spacers &
will give you a 1.5" gap & a good surface for mounting your backer board for
masonry, stone or tile attachment.
Note: You will have to extend the shield 36" above & to either side of the back of the stove.
Your owner's manual will tell you the minimum requirements
Here's one example of a rear heat shield construction.

[Hearth.com] Help with 1 inch air gap hearth wall
 
Thanks guys. So before I tore it all out, it was a brick hearth wall (3.5") with a 1" air gap. There was no gap at the bottom but had the gap behind the wall and at the top. It had a 3.5" masonry floor and still does as I left the floor intact. I am planning on just building off of the masonry floor that is already there. Attached is a picture of what I am assuming needs to be done with the wall?
[Hearth.com] Help with 1 inch air gap hearth wall
 
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At least 1" gap at the bottom too.
 
So I do need the 1" gap at the bottom that I drew in to the picture? Wouldn't this be totally contradicting the whole point of the wall as a heat shield? I now do not have the clearance I need in this 1" of space. Wouldn't it make more sense to not have the open bottom so that heat is not penetrating the combustible behind it?
 
PJP said:
So I do need the 1" gap at the bottom that I drew in to the picture? Wouldn't this be totally contradicting the whole point of the wall as a heat shield? I now do not have the clearance I need in this 1" of space. Wouldn't it make more sense to not have the open bottom so that heat is not penetrating the combustible behind it?
Click to expand...
No that 1" won't be exposed to radiant heat because the firebox is raised on legs. The air flow allowed by the gap top and bottom is needed to dissipate the heat from the heat shield
 
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PJP said:
So I do need the 1" gap at the bottom that I drew in to the picture? Wouldn't this be totally contradicting the whole point of the wall as a heat shield? I now do not have the clearance I need in this 1" of space. Wouldn't it make more sense to not have the open bottom so that heat is not penetrating the combustible behind it?
Click to expand...
Also, be sure that you do the wall pass-through correctly.
 
Each illustration and the article posted earlier in the thread all show this bottom gap. It is necessary. Done correctly, a proper wall shield is an excellent heat barrier. Maybe post a picture of what is there now and we can help more with future reconstruction ideas.
 
Does your specific stove allow for a reduction using a wall shield?
 
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Followping this as I will be doing in a few months. Also can you leave a one inch air gap between 6” double wall chimney to get through a tight space. Heat shield of some sort and 10” total?
 
Twigster said:
Followping this as I will be doing in a few months. Also can you leave a one inch air gap between 6” double wall chimney to get through a tight space. Heat shield of some sort and 10” total?
Click to expand...
No absolutely not you need the clearance required by the chimney manufacturer. There is no reduction allowed. For the record most modern stoves don't allow any reduction either
 
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I always worry when somebody is trying to build a “legit” nfpa wall shield that they are doing so under the false impression that it will reduce the required clearance to combustibles for a modern stove. Now, for a couple of stove models that is allowed specifically in the owners manual but usually it’s not.

Sure, it’s never a bad idea to build a more effective heat shield for extra safety but it is almost never required.
 
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Highbeam said:
Does your specific stove allow for a reduction using a wall shield?
Click to expand...
@Highbeam, I'm only using your statement as an example, and I don't understand the reduced clearance logic, anyone with sound information, please help me understand this, so you have a stove that has a rear clearance of 16" you want to built a ventilated wall shield to reduce the clearance further to 12", some one here asks "Does your specific stove allow for a reduction using a wall shield?" OP then says "I don't know" then the whole conversation goes with, please call the manufacturer, the stove is tested to certain tolerances..ect…
Next thread from someone else..... "I have this older stove I got from my uncles house, it has a couple of those tube things inside and a baffle, the stove was repainted and I cant find any labels on it, how close can I install this stove?"
The answer, since your stove has no labels you have to install it as a unlisted stove, the nfpa says 36" from combustibles, but if you build a ventilated wall shield you can reduce the clearance by 2/3 or 12". See what I mean here?
 
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kennyp2339 said:
@Highbeam, I'm only using your statement as an example, and I don't understand the reduced clearance logic, anyone with sound information, please help me understand this, so you have a stove that has a rear clearance of 16" you want to built a ventilated wall shield to reduce the clearance further to 12", some one here asks "Does your specific stove allow for a reduction using a wall shield?" OP then says "I don't know" then the whole conversation goes with, please call the manufacturer, the stove is tested to certain tolerances..ect…
Next thread from someone else..... "I have this older stove I got from my uncles house, it has a couple of those tube things inside and a baffle, the stove was repainted and I cant find any labels on it, how close can I install this stove?"
The answer, since your stove has no labels you have to install it as a unlisted stove, the nfpa says 36" from combustibles, but if you build a ventilated wall shield you can reduce the clearance by 2/3 or 12". See what I mean here?
Click to expand...
The fact is pretty much any stove with a proper ventilated sheild will be safe at 12". But that doesn't mean you are allowed to by code. On listed stoves the manufacturers clearances over ride everything else
 
bholler said:
The fact is pretty much any stove with a proper ventilated sheild will be safe at 12". But that doesn't mean you are allowed to by code. On listed stoves the manufacturers clearances over ride everything else
Click to expand...
What’s to stop someone from popping the tag off then, they would then be under the nfpa rule and be good to go.
 
kennyp2339 said:
What’s to stop someone from popping the tag off then, they would then be under the nfpa rule and be good to go.
Click to expand...
That is against federal law for one. It would also increase hearth requirements in most cases and make the setup harder to insure.
 
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