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Community and Q&A

Detailing Insulation Around Fireplace Chase

Plasko2 | Posted in Energy Efficiency and Durability on

I’m replacing a manufactured wood burning fireplace with a gas fireplace. This means I have access from inside the house to a framed chase on an exterior wall.

I’d like to define the thermal/vapor boundary of the chase at the firestop and want to get the detail right. Is it advisable to insulate with multiple layers of mineral wool between two sections of drywall ? The flue pipe would pass through metal firestops installed on each section of drywall to maintain clearance to combustibles. Because the mineral wool isn’t combustible, it could be set in direct contact with the flue. I’d use high temp caulk on both firestops to stop air leakage. Should I apply a layer of latex primer to the interior section of drywall as a vapor barrier? I’m in St. Louis, MO (zone 4a).


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  1. MattJF | | #1

    I don't believe any insulation is allowed with 2" inches of the flue at a penetration like you describe. Rockwool is not rated for such an application. You would need a product specifically rated for such an application.

    Remember that rockwool is basalt fiber+binder, so the binder can burn or at least outgass at high temps.

    I am confused about the vapor barrier comment. Are you thinking the wall behind the sheetmetal won't be painted and therefor won't have a vapor barrier? The sheet metal is pretty much a complete vapor barrier itself.

  2. Plasko2 | | #2


    The flue pipe requires 1" clearance to combustibles and isn't any more specific than that. Are you aware of insulation suitable for high temps?

    The chase is about 16 feet tall top to bottom. The ceiling for the adjoining room is 8' up and that's where I'm defining the thermal/vapor barrier. At this point I'd install a piece of drywall with the center cut out to allow a metal firestop to be inserted. The firestop is basically a square piece of sheet metal with a hole cut out for the flue pipe to pass through. I'm proposing that I'd paint the drywall that the firestop attaches to to reduce vapor migration because a plastic or polyiso vapor barrier seem like a terrible idea near anything hot. I'm further proposing another piece of drywall with a second firestop 15" or so above the first, effectively sandwiching the insulation between the two sections of drywall. My thought is that I need to minimize air movement through the insulation to get the best R value and to keep the insulation from getting full of dirt over the long term, thinking that might also impact performance. Does this makes sense or is there a better approach?

    1. nhbean | | #3


      Rather than paint, you might consider cutting sheet metal to fit around the firestop over the drywall, and then caulk the whole assembly with high-temperature caulk.

      For insulation, take a look at flue pipe "chimney blankets". These are made of ceramic fibers or similar materials specifically tested for high temperatures, and can even reduce the required clearance around your chimney liner. There are a number of suppliers that sell online, or you can purchase through a local firepace installer.

      1. MattJF | | #5

        Note that chimney liner is a different product than B-vent. Most b-vent manufacture installation guides say no insulation. Doesn’t mean it wont’t work, just that it is not approved.

        There are a lot of different chimney products and we need to be careful not to mix and match.

        Maybe there is an approved product out there, I am just not aware of one.

  3. MattJF | | #4

    I missed the gas part and was assuming Class A pipe, which needs two inches. If your appliance is rated for B vent, and you are replacing the flue with B vent, then 1" is acceptable. None of the pipes I am aware of are rated for direct insulation contact.

    I am having trouble picturing where in the chase you want to put the firestop. We may need a sketch or photo. Multiple firestops are acceptable. They would actually be required each time the pipe passes through a floor or ceiling in a chase. Does the chase have drywall or plywood on the outside wall side or does it incorporate the stud bay?

  4. Plasko2 | | #6

    I'm installing a direct vent fireplace, so it uses direct vent pipe, not b-vent. I understand these to be different things. It's an 8" pipe with a 5" pipe inside. The outer pipe is cool intake air. The fireplace store simply told me 1" to combustibles. You could be right that I should keep 1" clearance even to non-combustible insulation. That's still much better than what I had prior which had the old fireplace sitting outside the thermal barrier.
    The chase is 2x4 framed and from the floor to the firestop I've added mineral wool to the bays, applied polyiso over the studs and then drywall over the polyiso. Above the firestop it's just empty 2x4 bays.

    Hopefully the sketch helps shed some light.

    Many Thanks,

  5. MattJF | | #7

    Here is one manufacture of that style of pipe. They state clearances are defined by the appliance manufacture and air spaces are not to be filled with insulation.

    Check the manuals for you appliance and the brand of pipe you will be using.

    Your double fire stop seems good. I would line the upper one with the roof sheathing.

    I assume the interior wall of the chase is insulated in addition to the outside. You basically have two insulation layers, which is good as these units sink a lot of heat out if the exterior of the chase is not insulated.

    The interior painted wall should be a sufficient vapor barrier. There won’t be a lot of out to in vapor drive here.

  6. Plasko2 | | #8

    I'll check up on the requirements from the venting manufacturer and the appliance manufacturer. If I need to keep 1" clearance that isn't a big deal.

    Why use roof sheathing for the upper layer instead of drywall? Is that to better tolerate incidental moisture?

    The front of the fireplace chase isn't insulated. If I insulate that, where am I drawing the thermal boundary? Wouldn't that encourage the fireplace unit to get cold and radiate cold into the room? I thought the idea was to keep the conditioned portion of the chase warm in the winter by insulating/air sealing the firestop, floor (it's cantilevered), and all exterior walls.

  7. MattJF | | #9

    For the fire block location, put the first one lined up with the drywall, second one at the sheathing to line up with the cathedral ceiling insulation. In between install a piece of duct 2” larger than you pipe to enforce clearance and stuff the rest of the cavity with rockwool.

    From a comfort standpoint I think your idea of not insulating the inside wall makes sense. I don’t think there are any moisture vapor concerns with this assembly not addressed by the interior paint unless there is a lot of air leakage around the insert.

    I have an insert in a bump out on my house that I loath. I made a sandwich of 1.5” polyiso foam and hardboard and then upholstered it. It fits right into the surround. I haven’t pulled it out since I put it in as it is super cold. I haven’t figured out what, if any insulation is in the bumpout. The wall above it appears cold on an ir camera.

    1. Plasko2 | | #10

      Using duct to enforce clearance is a good idea. What's the rationale behind using plywood for the second layer? I'm planning to use several layers of mineral wool to achieve R50-R60 on the top. Shouldn't it be as well insulated as any other ceiling?

      I'm planning to spend time on the air sealing aspects to minimize air leakage as much as I can. Comfort is a significant driver behind this project. My chase only had insulation on the front/inside wall and nothing on the sides/back wall/floor of the cantilevered chase. This put the old fireplace outside the thermal envelope making it miserable/ridiculous from a comfort perspective.

  8. ThomasMiller | | #11

    Here is one assembling of that style of line. They state clearances are characterized by the apparatus production and air spaces are not to be loaded up with protection.

    As opposed to paint, you should seriously think about slicing sheet metal to fit around the fire stop over the drywall, and afterward caulk the entire gathering with high-temperature caulk. For the fire block area, put the first agreed with the drywall, second one at the sheathing to agree with the church roof protection.

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