Seal between flue and conditioned space

[iron]

I am building a new home soon and it will be pretty airtight (should be passive house standards easily). We're looking at small wood stoves or small ZC fireplaces for our place for the ambiance (no natural gas, no pellets, etc).

Is there a product that will go around the outside of the flue and allow a sealed connection to the house air barrier? I assume a double wall flue will get hot, but what about a triple wall? Would there be a ring that slides over it and then you can tape/seal to your air barrier?

 

[Highbeam]

Are you using a ceiling support box hung from the ceiling framing? I sealed the VB to that framing on all four sides and caulked around the metal box. Not sure how much more air tight you can get. The actual class A pipe needs 2" of clearance to combustibles so it won't be up against sheetrock or VB.

 

[iron]

I'm not sure of the ceiling support box. Is there a detail for this or is it a standard product?

DuraVent DuraPlus 6 in. x 14.25 in Square Ceiling Support Box and Trim Collar 24 in. Tall 6DP-CS24 - The Home Depot

DuraPlus is designed to stay cool on the outside, to provide a hot draft on the inside, to boost stove efficiency, and to provide for a fire-safe design that protects both the chimney and the building.

www.homedepot.com

If doing a ZC fireplace instead of a stove, can this box be used?

 

[Highbeam]

I'm not sure of the ceiling support box. Is there a detail for this or is it a standard product?

DuraVent DuraPlus 6 in. x 14.25 in Square Ceiling Support Box and Trim Collar 24 in. Tall 6DP-CS24 - The Home Depot

DuraPlus is designed to stay cool on the outside, to provide a hot draft on the inside, to boost stove efficiency, and to provide for a fire-safe design that protects both the chimney and the building.

www.homedepot.com

If doing a ZC fireplace instead of a stove, can this box be used?

I have two of these boxes in my home right now above stoves. You said you are considering stoves or ZC. Each of those ceiling support boxed are sealed tight to the VB around the outside which gives the required clearance to combustibles. Then the height of the box offers a defense against insulation contact farther up into the attic.

 

[bholler]

I'm not sure of the ceiling support box. Is there a detail for this or is it a standard product?

DuraVent DuraPlus 6 in. x 14.25 in Square Ceiling Support Box and Trim Collar 24 in. Tall 6DP-CS24 - The Home Depot

DuraPlus is designed to stay cool on the outside, to provide a hot draft on the inside, to boost stove efficiency, and to provide for a fire-safe design that protects both the chimney and the building.

www.homedepot.com

If doing a ZC fireplace instead of a stove, can this box be used?

A support box will not be used for a zero clearance fireplace. But firestops will be and those can be sealed. Btw if you want air tight don't use triple wall. It is by design an air leak

 

[iron]

A support box will not be used for a zero clearance fireplace. But firestops will be and those can be sealed. Btw if you want air tight don't use triple wall. It is by design an air leak

Do you have an image of what the firestop looks like in a modern ZC (EPA) fireplace?

When the double wall pipe enters the attic, can insulation (cellulose) be packed up against it?

When the pipe exits the roof, it can be just a pipe, and not an enclosure (like you'd see on a more traditional fireplace), right?

 

[iron]

Or, should I be using one of these? Note: I'm in Canada

 

[bholler]

Do you have an image of what the firestop looks like in a modern ZC (EPA) fireplace?

When the double wall pipe enters the attic, can insulation (cellulose) be packed up against it?

When the pipe exits the roof, it can be just a pipe, and not an enclosure (like you'd see on a more traditional fireplace), right?

No, chimney pipe needs 2" clearance to combustibles it can't have insulation against it.

 

[bholler]

Or, should I be using one of these? Note: I'm in Canada

View attachment 285614
View attachment 285613

It will depend what chimney is specified for the unit you pick but yes it will be something similar to that

 

[iron]

I'm still a little lost here, so my apologies for lack of knowledge.

If I run 6" double wall pipe up into the attic, is there an actual code-legal way to air seal this so that my air barrier isn't destroyed? Do I just take 2 pieces of sheet metal, cut half circles in them, and then slide together to form a tight fit against the 6" pipe and then tape/seal all overlapping joints + to my air barrier?

I get that there needs to be a gap between the flue pipe and combustibles, but sheet metal is obviously not combustible. My understanding is this general technique is normally what you'd do for a firestop anyway.

It seems like if I had a stove, I would use the ceiling support box linked earlier and this product could be air sealed. What is it about a ZC fireplace that would be different?

 

[bholler]

I'm still a little lost here, so my apologies for lack of knowledge.

If I run 6" double wall pipe up into the attic, is there an actual code-legal way to air seal this so that my air barrier isn't destroyed? Do I just take 2 pieces of sheet metal, cut half circles in them, and then slide together to form a tight fit against the 6" pipe and then tape/seal all overlapping joints + to my air barrier?

I get that there needs to be a gap between the flue pipe and combustibles, but sheet metal is obviously not combustible. My understanding is this general technique is normally what you'd do for a firestop anyway.

It seems like if I had a stove, I would use the ceiling support box linked earlier and this product could be air sealed. What is it about a ZC fireplace that would be different?

There are components available from the chimney manufacturers to do this no matter what the situation is. We can talk about specifics when you have more of an idea what you are installing.

 

[iron]

There are components available from the chimney manufacturers to do this no matter what the situation is. We can talk about specifics when you have more of an idea what you are installing.

Thanks BH.

I'm looking at the RSF 3600 Focus. Those product images snipped above are from their catalog, which I've included as attached.

Unfortunately, their technical support dept person I've written seems uninterested in helping me find a solution or presenting a clear detail on how this should look.

 

[iron]

Thanks BH.

I'm looking at the RSF 3600 Focus. Those product images snipped above are from their catalog, which I've included as attached.

Unfortunately, their technical support dept person I've written seems uninterested in helping me find a solution or presenting a clear detail on how this should look.

I just spoke with the local fireplace and stove dealer. He basically told me there is NO stove or fireplace that can have an airtight seal between the flue and attic. Is this true? They keep mentioning heat build up being an issue.

If this is the case, it doesn't seem like a stove or FP would ever be doable in an airtight home. I know there was always the issue of performance (too airtight = hard starts, backpuffing, etc), but that's a separate topic.

It seems like the only product that could be used is a free-standing European style stove like this: https://us.rais.com/duplicate-of-viva-l-120.html, and even with that, I'm not sure you'll get an airtight connection.

Am I missing something?

 

[bholler]

I just spoke with the local fireplace and stove dealer. He basically told me there is NO stove or fireplace that can have an airtight seal between the flue and attic. Is this true? They keep mentioning heat build up being an issue.

If this is the case, it doesn't seem like a stove or FP would ever be doable in an airtight home. I know there was always the issue of performance (too airtight = hard starts, backpuffing, etc), but that's a separate topic.

It seems like the only product that could be used is a free-standing European style stove like this: https://us.rais.com/duplicate-of-viva-l-120.html, and even with that, I'm not sure you'll get an airtight connection.

Am I missing something?

The stove itself will never be air tight. But honestly if installed correctly the amount of air leakage is going to be so minimal I don't see that it will be a big issue.

 

[begreen]

There will be a firestop at the entry to the attic that will act as a seal. Not air tight, but snug.

 

[iron]

The stove itself will never be air tight. But honestly if installed correctly the amount of air leakage is going to be so minimal I don't see that it will be a big issue.

Understood. No stove or ZC fireplace is airtight (door seals, fabrication, etc). However, if you put an 8" pipe through your air sealed attic, and you need a 10" hole for clearances, you just introduced an incredible amount of leakage to an otherwise airtight house.

Is there a way to "seal"/caulk a ZC FP to its enclosure?

 

[iron]

There will be a firestop at the entry to the attic that will act as a seal. Not air tight, but snug.

This is the detail I'm trying to figure out. My understanding is the firestop is typically two pieces of sheet metal with half circles cut out, and slid tight to the pipe. However, if I use the ICC products (linked earlier a few posts ago), I don't see how they are compatible.

 

[bholler]

Understood. No stove or ZC fireplace is airtight (door seals, fabrication, etc). However, if you put an 8" pipe through your air sealed attic, and you need a 10" hole for clearances, you just introduced an incredible amount of leakage to an otherwise airtight house.

Is there a way to "seal"/caulk a ZC FP to its enclosure?

It is an 8" hole filled with pipe. And a 10" hole covered by a firestop that can be sealed outside and caulked to the pipe. Not much of any leakage there. You will probably have more leakage through the air intake of the stove.

 

[iron]

It is an 8" hole filled with pipe. And a 10" hole covered by a firestop that can be sealed outside and caulked to the pipe. Not much of any leakage there. You will probably have more leakage through the air intake of the stove.

did you get a chance to look at the ICC products? they make things that are specifically rated for attic penetrations (including shielding for the insulation). i just don't see how the firestop integrates into this. see post #12

 

[bholler]

did you get a chance to look at the ICC products? they make things that are specifically rated for attic penetrations (including shielding for the insulation). i just don't see how the firestop integrates into this. see post #12

I havnt but the firestop is probably integrated into the attic shield. A firestop is required there so there has to be a way to do it. I don't work with icc products so I don't know their line

 

[iron]

these are the products that seem most appropriate. however, they all just seem like shields (i.e. put a gap between the 8" flue pipe and the framing/insulation material). it seems like there is no firestop in that inbetween space and thus, i conclude, a huge hole for air to pass through.

blue = insulation
yellow = air leakage

 

[bholler]

View attachment 286970 View attachment 286971
View attachment 286972

these are the products that seem most appropriate. however, they all just seem like shields (i.e. put a gap between the 8" flue pipe and the framing/insulation material). it seems like there is no firestop in that inbetween space and thus, i conclude, a huge hole for air to pass through.

blue = insulation
yellow = air leakage
View attachment 286973

Again I don't use icc products so I don't know the specifics. But a firestop is required any time the chimney passes through a level so there has to be a way to do it. I can tell you how ventis or jerimiahs does it.

By code that yellow area in your drawing needs to be covered at every level with a firestop.

 

[begreen]

The ICC insulation shield ERSA says it also acts a firestop. The yellow area in the diagram is metal in this case.

 

[iron]

Thanks begreen. Do you think it's airtight or would I need to tape with foil tape or high temp silicone?

 

[bholler]

Thanks begreen. Do you think it's airtight or would I need to tape with foil tape or high temp silicone?

It will not be absolutely air tight no