Clearance to non-combustible advice.

[heykman]

Hello forum members!

Just joined and this is my first post. After doing a ton of searches without finding any info thought I would try going direct to the community.

I am building a hearth and surround for a new wood stove rated at a 5" minimum clearance from stove corner to a combustible. Specifically, for my corner surround build I have removed the drywall and replaced with cement board over the studs. My stove corner is placed 6" from the cement board (6-1/2" to the wood stud) and I will be putting a total of 3" of mortar and stone on to the 1/2" cement board. My question is: Would a non-combustible material (river rock/mortar/cement board) make a need to increase the minimum distance to the combustible (wood stud)?

I guess what I'm asking is, do I need to worry about heat transfer through the rock taking away 3-1/2" of clearance, giving me effectively only a 3" clearance to the combustible?

Any clarification on this would be greatly appreciated!

 

[bholler]

Hello forum members!

Just joined and this is my first post. After doing a ton of searches without finding any info thought I would try going direct to the community.

I am building a hearth and surround for a new wood stove rated at a 5" minimum clearance from stove corner to a combustible. Specifically, for my corner surround build I have removed the drywall and replaced with cement board over the studs. My stove corner is placed 6" from the cement board (6-1/2" to the wood stud) and I will be putting a total of 3" of mortar and stone on to the 1/2" cement board. My question is: Would a non-combustible material (river rock/mortar/cement board) make a need to increase the minimum distance to the combustible (wood stud)?

I guess what I'm asking is, do I need to worry about heat transfer through the rock taking away 3-1/2" of clearance, giving me effectively only a 3" clearance to the combustible?

Any clarification on this would be greatly appreciated!

No you still just go to the nearest combustible. But I wouldn't want my stove that close personally. It will make it a pain to clean or work on

 

[Molsonc1]

The 5inch needs to be from the stone you put on wall.

 

[bholler]

The 5inch needs to be from the stone you put on wall.

Not true at all

 

[kuma.jason]

Following is a direct quote from an accredited testing laboratory:

"The short answer is this is really up to the building inspector/AHJ, but in practice, we as a test lab would say the clearance would still be to the exterior surface. The combustibles may be at the proper clearance, but you are effectively making a composite wall, which is combustible, that is closer than the specified clearance".

It must be appreciated that one lab or one AHJ may interpret the standard differently than another. We have seen it both ways. One install passes inspection and another is failed for not having the proper clearance (when clearance is interpreted as visible airspace).

Then, the best practice probably is measuring to the finished surface of the wall, in the OP's case, the stone.

@bholler, If you can point us to something in NFPA-211 or UL1482 or otherwise that is more clear on the subject I know I would appreciate it. It's actually no fun at all receiving a call from a homeowner that just had an inspection fail due to the lack of clarity on the issue.

 

[bholler]

Following is a direct quote from an accredited testing laboratory:

"The short answer is this is really up to the building inspector/AHJ, but in practice, we as a test lab would say the clearance would still be to the exterior surface. The combustibles may be at the proper clearance, but you are effectively making a composite wall, which is combustible, that is closer than the specified clearance".

It must be appreciated that one lab or one AHJ may interpret the standard differently than another. We have seen it both ways. One install passes inspection and another is failed for not having the proper clearance (when clearance is interpreted as visible airspace).

Then, the best practice probably is measuring to the finished surface of the wall, in the OP's case, the stone.

@bholler, If you can point us to something in NFPA-211 or UL1482 or otherwise that is more clear on the subject I know I would appreciate it. It's actually no fun at all receiving a call from a homeowner that just had an inspection fail due to the lack of clarity on the issue.

I will try to find it later busy right now

 

[Ludlow]

I would go to the finished surface myself. I dont believe in cutting it to the minimum.

 

[heykman]

Well crap on a cracker. I set everything based on measurement to the combustible. For my situation the question was posed for safety concerns and not for any inspection requirements. I live in the mountains and my dwelling location/situation has zero code concerns. It wouldn't be impossible for me to frame out the roof again for the chimney pipe placement but I would rather not.

Any thoughts to my situation would be greatly appreciated!

 

[bholler]

Well crap on a cracker. I set everything based on measurement to the combustible. For my situation the question was posed for safety concerns and not for any inspection requirements. I live in the mountains and my dwelling location/situation has zero code concerns. It wouldn't be impossible for me to frame out the roof again for the chimney pipe placement but I would rather not.

Any thoughts to my situation would be greatly appreciated!

What makes you think codes are of no concern because you are in the mountains?

 

[Mech e]

Following is a direct quote from an accredited testing laboratory:

"The short answer is this is really up to the building inspector/AHJ, but in practice, we as a test lab would say the clearance would still be to the exterior surface. The combustibles may be at the proper clearance, but you are effectively making a composite wall, which is combustible, that is closer than the specified clearance".

It must be appreciated that one lab or one AHJ may interpret the standard differently than another. We have seen it both ways. One install passes inspection and another is failed for not having the proper clearance (when clearance is interpreted as visible airspace).

Then, the best practice probably is measuring to the finished surface of the wall, in the OP's case, the stone.

@bholler, If you can point us to something in NFPA-211 or UL1482 or otherwise that is more clear on the subject I know I would appreciate it. It's actually no fun at all receiving a call from a homeowner that just had an inspection fail due to the lack of clarity on the issue.

As an OEM, you are responsible for instructing your users on the safe installation and operation of your product. If your product causes a fire, someone is injured, or God forbid killed, you will end up in a lawsuit and have to prove that not only is your product safe, but that your instructions for the installation and operation of the product give users clear instructions for its safe operation. UL is just a private company with great brand recognition. That UL mark on your product will show a jury an outside company verified that you built a safe product. If there is perceived ambiguity in any of your instructions for safe operation of the stove however, a good attorney will leverage that to prove negligence on your part.

If I were in your place as an OEM, I would get this question sorted out and then update my manuals accordingly.

 

[billb3]

Well crap on a cracker. I set everything based on measurement to the combustible. For my situation the question was posed for safety concerns and not for any inspection requirements. I live in the mountains and my dwelling location/situation has zero code concerns. It wouldn't be impossible for me to frame out the roof again for the chimney pipe placement but I would rather not.

Any thoughts to my situation would be greatly appreciated!

No code enfoercement was how it used to be many years ago north of, say, Conway NH where you could truly live free. We'd go skiing in Stowe and there was always someone who had a friend of a friend that had a place we could stay at for free. You wouldn't always necessarily want to or be terribly comfortable when there.
As codes are often subject to interpretation, This is one advantage to hiring local help as they know first hand how inspectors are interpreting or amending codes.
With no local enforcement I don't know why you're sweating the small stuff.
Rocks beat flammable wallpaper any day.

 

[bholler]

No code enfoercement was how it used to be many years ago north of, say, Conway NH where you could truly live free. We'd go skiing in Stowe and there was always someone who had a friend of a friend that had a place we could stay at for free. You wouldn't always necessarily want to or be terribly comfortable when there.
As codes are often subject to interpretation, This is one advantage to hiring local help as they know first hand how inspectors are interpreting or amending codes.
With no local enforcement I don't know why you're sweating the small stuff.
Rocks beat flammable wallpaper any day.

No enforcement doesn't mean no codes. If anything ever happened your insurance company will care if it was installed to code. As someone who has done insurance evaluations for insurance claims I can tell you that it doesn't matter in the least it it was permitted and inspected or not. If you ignored code when you installed it you are screwed.

 

[heykman]

What makes you think codes are of no concern because you are in the mountains?

Hey thanks for all your input on this situation. What I mean is by living in the mountains...I live in a goat barn that was built in the late 1800's which has been converted into a dwelling at some point over the years. As well built and awesome as it is, nothing is built to code.

I'm a stickler to doing things the right way and following codes, that's why I made extra space to the combustible. However, given the light of differing opinions on the distance with added non-combustible, I need to weigh out whether 1/2"-1" is going to be a safety concern. I know code and safety usually go hand in hand but in my case, I really only need to be concerned about safety.

 

[EbS-P]

The question is one of how hot the combustible surface will get. The manufacturer has determined that the heat transfer from the stove is at a level that allowed combustibles 5” from stove. It is my guess that this is based on temperature. So does adding some material between the heat source and the combustible material result in a higher temp? By moving closer you are decreasing the air gap more on this later but radiation is the dominant mode of heat transfer. If we assume this , the radiates power per unit area or intensity, decreases with distance away from the source and assuming the non combustible. If assume the material conducts 100 percent of the energy that it absorbs. The energy per unit area incident on the combustible material is the same. Now the reality may be different a lot of assumptions were made. Biggest one is the combustible material now no longer can lose energy to the air. Would I want my combustible materials touching a solid piece of metal thats 5” thick and was 1 mm away from the stove? No. This tells me the air gap is important. The Requirement of 5” takes into account the material is in contact with air that has a temp less than the temp of the material and can loose energy to the air.

If you are concerned adding a metal heatshield using Non combustible standoffs would be an option but it’s not a pretty one. I have a PhD in physics and can’t even begin to answer your questions about the safety of 1” difference. do the manufactures include a safety factor in their minimums maybe but now you have to guess what it is. Guessing with personal safety is not something I am comfortable with.
Evan.

 

[Woody Stover]

Would I want my combustible materials touching a solid piece of metal thats 5” thick and was 1 mm away from the stove? No. This tells me the air gap is important....Non combustible standoffs would be an option but it’s not a pretty one.

I was thinking along these lines. But it seems that a massive piece of metal would intercept the radiation (which air space doesn't prevent) and the mass of the metal would dilute the heat being transferred through it, as might cement board, mortar and stone to a lesser degree.
Instead of all cement board, mortar and stone, could you insert an R-value insulator such as Micore into the thickness and provide the needed protection? I don't know.

 

[Mech e]

Personally, I am with bholler on this one. The stove manuals (including my own) I have seen spec "distance to combustible materials". In the case of the OP's question, I would interpret that the distance to the wood studs. Attaching cement board, mortar, and rock to wood studs and then calling that a composite that is combustible is adding unwritten definitions to the owners manual that are not intuitive or obvious.

As I mentioned earlier, at the end of the day it is the OEM's responsibility to define safe installation parameters for their product. If creating a multi-layer (composite) surround for the product is not acceptable, then say it plainly and clearly in the owners manual. If UL is being inconsistent in the interpretation of their own standards, then as an OEM I would press them to update their standards and clear up the confusion. It would be interesting to see if this has been addressed by others besides UL (i.e. Intertek, CSA, TUV, etc.).

 

[Woody Stover]

do the manufactures include a safety factor in their minimums maybe but now you have to guess what it is.

I think they try to determine what is still safe in a worst-case scenario...the stove is severely over-fired and glowing red. But yes, it should be specified more clearly by them, if there can be varying interpretations.

 

[kuma.jason]

The simplest interpretation of clearance is visible air space. It's also the only interpretation that is 100% inspector proof.

And, to my knowledge (positively with our own products), all testing is performed with combustible walls/ceilings. In our experience, protecting the wall/ceiling isn't part of testing. Instead, protecting the wall/ceiling comes down to application of other codes/standards, like NFPA-211, for example.

In this case, no application of NFPA-211 can be made due to the 12" minimum clearance requirement in that code. Interestingly, If I recall correctly, when using an NFPA-211 clearance reduction method the new reduced clearance is measured to the original wall surface.

I'll invite a representative from the test lab we partner with to come here and be part of the discussion. I don't know if there are many test labs represented here. They would be a great addition to the community, I'd think.

 

[John Galt]

Everything I have read would require a 1" air space between the stone and any combustable material behind it. This allows air circulation to dissipate the radiant heat absorbed by the stone.

 

[bholler]

Everything I have read would require a 1" air space between the stone and any combustable material behind it. This allows air circulation to dissipate the radiant heat absorbed by the stone.

That is to gain the 2/3 clearance reduction down to a minimum of 12" that is not what is going on here

 

[thunderhead]

So does adding some material between the heat source and the combustible material result in a higher temp?

I think that really depends on the material and IR emissivity of the wall it is covering. A good IR absorber with high thermal conductivity, like unpolished metals, would increase temp slightly I think, by virtue of being a little closer to the source: intercepting slightly more of the stove's emitted radiation, and also probably having a slightly higher IR emissivity than the common wall. An IR reflector with low conductivity would certainly decrease temp. So... what is brick and rock? In between good and bad... where exactly?

material is in contact with air that has a temp less than the temp of the material and can loose energy to the air

Good point, but thinking it over I don't think this matters much. The surface of the rock still be in contact with the air. Perhaps very slightly hotter air with less flow, closer to the stove?

 

[Mech e]

Just for reference, the (approximate) thermal conductivity of some familiar materials in W/(m*K). I would just look at these in relative terms.

Copper 400
Aluminum 236
Cast Iron 52
Carbon Steel 45
304 SS 14
Rock 2-7
Cement, mortar 1.73
Common Brick 0.6-1.0
Fire Brick 0.47
Fiberglass 0.04
Air 0.026

My intuition with respect to the OP's question is that the wall studs are probably at a lower temperature after attaching the cement board, mortar, and stones. These materials are poor thermal conductors, have absorptivity values similar to wood, and they are not combustible.

The real rub here is that the stove manufacturers and companies like UL are only going to approve what they have tested. Anything beyond that would open them up to potential liability problems.

All that being said, I would still interpret the distance to combustibles literally. But even this interpretation has limits. Would I install a piece of copper bar from my stove to a wood wall stud? Nope.

 

[heykman]

Moved my stove to 7-1/2" to the cement board giving me 5" air space to the face of the rock. Reframed the ceiling to accommodate the roof support chimney. No real harm done other than I now have a 13" diameter hole in my roof as opposed to a 12" and my distance from the stove door opening to the edge of my raised hearth is now 18-1/2" rather than my preferred 20".

In hindsight after reading the well thought out information in the replies, I probably should have just stuck to the simple first reply from bholler, "go to the nearest combustible". However, being a single dad to an 8 year old daughter my over-worry default mode is safety.

One thing is certainly clear...very glad I joined this forum. The wealth of information you members have in this arena is amazing! bholler, Mech e, EbS-P...blown away by your expertise! Another thing that is clear or perhaps I should say "unclear" is the information given by the manufacturers, very ambiguous. When I called the stove manufacturer with this question, it was a very uncertain but repeating "5 inches to the combustible."

Thanks for everyones input on this project! Now if I can just figure out how to correctly install the flashing under the shingles dealing with 6" support brackets on either side of the chimney pipe. Will probably post another thread on that one. Ha!

 

[begreen]

Would I want my combustible materials touching a solid piece of metal thats 5” thick and was 1 mm away from the stove? No. This tells me the air gap is important.

If that sheet of metal was 4' tall and wide it would be dispersing the heat over a very large area and at 5" thick might remain quite cool on the backside. All this is theoretical of course. The tested distance is to the nearest combustible.