Protected Surface - Sheet Metal w/ Brick Veneer

[karl1989]

Hello!

I am building a new home and have ordered a Jotul F500 V3.

I have a class A chimney installed and plan on using double wall pipe with a rear heat shield on the F500. With this configuration I can have a 6" rear clearance. See attached photo of my stove area. There is no drywall behind the durock, I installed it directly on the 2x6 wood studs.

my issue is that the guy who installed my chimney left me almost no room to work with as far as finishing my wall with tile. I will have 6.5" inches from pipe to 1/2" durock on studs. This only gives me 1/2" of room and I'm not comfortable

I wanted to have veneer bricks but I don't think that is possible unless I create a protected surface.

My thinking is that I will have 1" spacers off the Durock. I will use sheet metal and put 1/2" brick directly on the sheet metal. Will this idea work? I'm thinking this will be more work but will provide more protection to my wall than is needed (which is what I am after).

thanks

 

[begreen]

The clearance measurement is to the nearest combustible, which in this case is the studs.

 

[karl1989]

So do I need the air gap? I'm so confused by the code. What you wrote is exactly what the chimney installer told me but it seems to conflict what I read in the code. If I have durock on studs do I measure from stove pipe to studs?

ideal scenario for me is studs > Durock > thinset > 1/2" brick veneer. But I imagine this will end up being slightly less than the 6" clearance.

 

[coaly]

Cement board transmits heat very well. When in contact with the wall, it becomes part of the wall, considered combustible, so measurement is to the finished surface. There is no approved heat shield. The stove would need to be 6 inches from finished surface.

When you add a 1 inch airspace behind a heat shield, you measure from stove to wall, not heat shield. The shield also has to be open at bottom and top for air circulation behind it. The cooling is due to air flow. Only then does the cement board become a heat shield.

For the least amount of space, and an air space to wall, cover studs with sheet metal like you did with the rock board. This becomes the finished combustible wall. Then use 1 inch spacer behind rock board sitting on spacers (normally a few bricks) at bottom to support weight. Finish with brick veneer, Your measurement now is to sheet metal surface with good airflow up and out the open top. No heat shield spacers in centerline of stove. This gives you the brick look you want behind stove with overkill with heat shield.

 

[karl1989]

thanks @coaly

On the sheet metal, is there anything that should be done to seal the gaps between the sheets?

 

[coaly]

thanks @coaly

On the sheet metal, is there anything that should be done to seal the gaps between the sheets?

No. Overlap if possible. If there are horizontal seams I would install the top first and overlap from the bottom so air flow moves up over the seam. It's not there for fire protection, you could mount the heat shield directly to studs, but the open stud space would not vent as well as a narrow chimney type chamber behind shield.

 

[karl1989]

@coaly @begreen

Aside from the extra 1/4" of space gained, do you see any other reason I should remove the durock on my wall, replace it with sheet metal?

My thinking is that the sheet metal will be pretty expensive. Not to mention I have to spend the time to remove the durock I already installed.

I think I have 2 options as shown below. Both should be perfectly safe and up to code right???

stove to studs is 7"
Stove will have rear heat shield and double wall pipe
clearance minimum is 6" to protected or non protected surfaces

Option A (no sheet metal)
studs > 1/2" durock > 1" air gap > 1/2" durock > 1/2" veneer brick > ~4.5" of air > stove/pipe

Option B (sheet metal)
studs > sheet metal (~3/16") > 1" air gap > 1/2" durock > 1/2" veneer brick > ~4.75" of air > stove/pipe

 

[begreen]

So do I need the air gap? I'm so confused by the code. What you wrote is exactly what the chimney installer told me but it seems to conflict what I read in the code. If I have durock on studs do I measure from stove pipe to studs?

ideal scenario for me is studs > Durock > thinset > 1/2" brick veneer. But I imagine this will end up being slightly less than the 6" clearance.

The code you are reading is for clearance reduction with a non-compliant (no UL test) stove. Your stove has been UL tested and has documented clearances.

stove to studs is 7"
Stove will have rear heat shield and double wall pipe
clearance minimum is 6" to protected or non protected surfaces

You need nothing, the clearance is to the studs, not to the tile or brick facing. The manual does not say protected vs unprotected. It talks about clearances to combustibles when it says:

The following clearances have been tested to UL and ULC standards and are the minimum clearances specifically established for the F 500 V3.
The following diagrams give the required clearances you must maintain when installing the F 500 V3 near combustible surfaces. See pages 22-23.
A combustible surface is anything that can burn (i.e. sheet rock, wall paper, wood, fabrics etc.). These surfaces are not limited to those that are visible and also include materials that are behind non-combustible materials. << aka the studs in the wall>

Use Option C, leave the durock in place and cover with the desired non-combustible surface veneer. Install stove.

 

[karl1989]

@begreen

Are you sure? I have read a lot online that durock, bricks will transfer heat to the combustable wood behind it. What you are saying is what the chimney installer told me, but there is conflicting information online.

I obviously want to err on the side of extreme caution since this is a new house.

 

[begreen]

Yes, I am sure. I updated the prior post with the documented info.

 

[begreen]

Also note that the current Durock or Wonderboard Lite have perlite pellets in them. That's what makes the board lighter. This increases the insulation value of the cement board to R=.39.

 

[begreen]

Cement board transmits heat very well. When in contact with the wall, it becomes part of the wall, considered combustible, so measurement is to the finished surface. There is no approved heat shield. The stove would need to be 6 inches from finished surface.

When you add a 1 inch airspace behind a heat shield, you measure from stove to wall, not heat shield. The shield also has to be open at bottom and top for air circulation behind it. The cooling is due to air flow. Only then does the cement board become a heat shield.

For the least amount of space, and an air space to wall, cover studs with sheet metal like you did with the rock board. This becomes the finished combustible wall. Then use 1 inch spacer behind rock board sitting on spacers (normally a few bricks) at bottom to support weight. Finish with brick veneer, Your measurement now is to sheet metal surface with good airflow up and out the open top. No heat shield spacers in centerline of stove. This gives you the brick look you want behind stove with overkill with heat shield.

This is not correct. Old style cement board has some insulation value, R=.22. It is not a conductor like metal. Modern cement board is lighter and can have an R value of .39 to .52 depending on the manufacturer. The cement board is not combustible. The clearance is measured to the nearest combustible, which is the studs. Also note that by the NFPA tables, full brick is an insulator and qualifies for a 50% clearance reduction.

 

[karl1989]

@begreen

I understand what you're saying, but the manual is confusing as the attached diagram from the manual clearly states "Unprotected surface" To me this means to the surface of a non protected wall per NFPA 211

 

[begreen]

These are UL and Jotul tested clearances. It means that the manufacturer has documented proof of a safe clearance reduction when an NFPA protected surface is used on the wall. Note that there is no rear clearance gained by using the protected surface with the rear heat shield and double wall stove pipe. The main gain is with no rear heat shield on the stove.

 

[karl1989]

@begreen

I think I will add the air gap just to be safe. Plus this will let the brick stick out from the surrounding drywall. I think this may allow me to finish the edge with some corner pieces. Otherwise flat brick can look goofy transitioning flat next to drywall

((broken link removed))

 

[begreen]

What you have is totally safe, but there is no harm in that plan if you choose it. There is also no clearance gained. Just be sure the drywall side by the window is at least 14" unless you are trying to reduce the clearance on the window side too?

BTW, why is the stove being installed parallel to the wall and not at a 45 facing into the room?

 

[karl1989]

@begreen I considered that, but I didn't want my pad to stick out into the room anymore than it is. The F500 requires 18" in front of door and I just didn't like how the room flowed with that layout. I wanted to have a large stove but keep the footprint square and as small as possible

that window wall is ~23 feet long so I think it will look OK.

 

[begreen]

Cool. It requires 18" hearth on the left side too to facilitate side loading, but still will need 14" from the window wall unless that is turned into a protected surface. It's more radiant from the sides than the back with the heatshield.

 

[karl1989]

Yeah I am locking the side door to reduce pad to 14". My concern with corner install was also keeping the stove from blocking the window. The parallel placement doesn't block the window much at all.

 

[begreen]

I think you will find that this stove is more pleasant to load with side-loading. If so, you could do a hearth extension at floor level.

 

[karl1989]

@begreen that is a good idea. I still could extend my pad 4 more inches. i will think about it some more.

 

[begreen]

I think you will appreciate the difference. The front door on the F500 tends to spill ash. It works best as a side-loader.

 

[coaly]

This is not correct. Old style cement board has some insulation value, R=.22. It is not a conductor like metal. Modern cement board is lighter and can have an R value of .39 to .52 depending on the manufacturer. The cement board is not combustible. The clearance is measured to the nearest combustible, which is the studs. Also note that by the NFPA tables, full brick is an insulator and qualifies for a 50% clearance reduction.

I thought anything in direct contact with a wall or studs became a assembly, so cement board with brick veneer would measure from the finished surface to stove. Isn’t that considered measuring from the combustible wall? I never subtracted for the thickness of cement board, but I never pushed it to the nearest half inch.

I’ve leaned cement board close to stoves and the side facing away from the stove got extremely hot. Possibly as hot as the side facing the stove. This was the old heavy stuff years ago. Doesn’t it conduct heat very well?

Cement board was installed in a commercial kitchen I maintain on the studs with stainless sheathing over it. There is a line of high BTU woks that divert flames towards that back wall and a inspector wants the wall covered with metal first, then rock board covered with stainless.

 

[begreen]

The clearance is to the nearest combustible. Cement board is not combustible. The measurement is to the studs. Some stove manuals are nice enough to make this clear. For a non-compliant stove, cement board + a full brick veneer = a 50% reduction, even though it is a solid assembly. I see a lot of old Fishers installed depending on this clearance reduction.

I had a cement board wall shield for the F602 when it was in the house. That little stove could radiate heat pretty intensely. Enough so that you would only place your hand on the close by cement board quickly. The backside of the cement board was definitely cooler, but I can't say by how much. We have many hearths installed here that have taken advantage of durock nexgen's higher R value to build up to meet the spec. 4 layers - R=1.56.

 

[Pertzbro]

How do modern double/triple pain windows handle being this close to high heat? I imagine the window will fail much sooner than normal? What type of window is it? Wood, Vinyl, fiberglass, etc?