GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

How to insulate prefab chimney interior wall

Laura Mitrescu | Posted in Energy Efficiency and Durability on

I have had a roof leak around the chimney masonry which was apparently “fixed” by the previous owner while continuing to leak small amount of water into the wall by the side of the fireplace. This wall is now open to allow for replacement of the rotted studs, and I’d like to improve on the original insulation from the inside. On the other side of the wall is the open prefab chimney cavity – apparently just one layer of brick lined on the interior with some plywood and fiberboard.

My first question is how to insulate the wall itself. The original build (30 yr old house) has 2×4 stud cavities lined with unfaced fiberglass in this area, followed by a layer of plain plastic on the interior, then drywall. To the exterior, there is only masonite siding (no house wrap). Directly above the fireplace insert, they omitted the plastic and used felt paper instead (why?)
I am planning to spray all available studs with boracare, then use canned spray foam insulation around the window casing (there was some moldy fiberglass stuffed in here originally). I’m not sure that I should replace the plastic sheet again (and tape every seam). For insulation I could use either new fiberglass (backed or plain), or a couple of 2 inch thick XPS panels, perhaps sealed at the stud junction with caulk or foam. How do I create a higher R value while minimizing moisture problems (in humid NC) and keeping things relatively breathable should future leaks occur (esp with aging masonite siding)?

For the interior of the fireplace, I have removed a small strip of swollen and mildewy fiberboard, only to reveal the brick immediately under. I wonder if I need to replace this at all since it’s certainly not serving as any kind of insulation. I would be tempted to just take out any other section with water damage, do a breathable waterproofing on the outside of the fireplace, and caulk around pipes.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Your description of the current construction is hard to follow, and you are asking lots of different questions.

    I suggest that you ask one question at a time, and that you try to do a better job of describing the building assembly that you are repairing.

    It sounds like you have a wood-framed house insulated with fiberglass batts. It also sounds like you have a metal chimney. Is this a metal fireplace or just a metal chimney? If it is a fireplace, is it a gas-burning fireplace or a wood-burning fireplace?

    It also sounds like there is a masonry surround of some type near or around the metal chimney. It's hard to understand from your description how this masonry was installed.

  2. Laura Mitrescu | | #2

    Sorry Martin, I was planning to post some pics, but didn't see the link.

    The house is 2x4 stud, originally insulated with fiberglass. The outer siding is masonite and fiberboard. To the interior, I have a plastic sheet (regular 6 mil I'm guessing) and drywall. You can see the current construction on the picture. I will be removing the rest of the drywall from around the fireplace and whatever moldy fiberglass insulation is left.

    First question is how to insulate the wall, inside the stud cavity, now that the drywall is off and old fiberglass removed. Do I just replace the fiberglass and close it up, or should I try to use a rigid foam panels instead? If going for foam, I only have access to 2" XPS or 1/2" polyiso (from HD or lowe's). I would like to use foam instead of fiberglass batts, since fiberglass would end up being squashed inside all those small areas. I am not planning to replace the vapor barrier plastic sheeting the builder had originally installed right behind the drywall. Would using 2 sheets of 2"XPS (4 inches total thickness) inside the wall cavity cause any condensation problems? I am also thinking that I can seal rigid foam panels around their edge to the stud to prevent air circulation, as this wall opens directly to the chimney cavity behind.

    A second question concerns the tar paper that the builder installed right above the fireplace insert (gas burning), rather than using plastic. Is there a reason why they did this? Were they worried that heat from the flue would melt the plastic? Would this mean that I cannot use foam in that area for insulation and need fiberglass?

    As for the chimney, it has been built on a stud structure that supports it all the way to the top, and then lined with brick to the exterior. It is not a solid brick construction. The brick is just 1 layer thick on the exterior (a veneer). Behind the brick but still on the outside of the stud frame is also a layer of fiberboard. You can see in the pictures the fiberboard water damage. You can also see the brick show where the previous owner busted the fiberboard by nailing ivy supports on the exterior of the chimney.

    So the third question concerns the interior of the brick chimney. The fiberboard is swollen and mildewed in the areas that had water leakage, so I want to remove the damaged portions. Do I need to replace it with anything, or can I leave the brick exposed? I'm not sure that the fiberboard lining actually served any purpose. It is not insulation or vapor barrier.

  3. GBA Editor
    Martin Holladay | | #3

    If you approach this situation as a patch job, you may miss factors that have led to deterioration, and the problem may occur.

    The construction of your chimney remains unclear to me, but it sounds like you have a metal chimney surrounded by a wood-framed chase. The wood-framed chase is clad with brick veneer. Is that correct?

    You also said that you "have had a roof leak around the chimney masonry which was apparently 'fixed' by the previous owner while continuing to leak small amount of water into the wall by the side of the fireplace." This sentence causes alarm bells to go off.

    How did the water enter? Has the leak really been fixed? Do you understand the water entry mechanism?

  4. Laura Mitrescu | | #4

    No, I don't understand the entry mechanism and neither did 1 chimney guy and 2 roofers. There was an accumulation of leaves and debris several years old behind the chimney (in the cricket area) as well as at least one uncovered nail hole in the roof deck and tar paper missing in an area (perhaps as consequence of previous "repair"). The shingles around the damage area are much more loose and brittle than on other parts of the roof and yet they are almost permanently shaded by an overhanging tree). In addition, the gutter next to the problem area is sloped a bit in the wrong direction (away from its drainage) but since that is lower than the rotted area I'm assuming that's not it.

    The damage is centered at one of the top corners of the chimney chase so I don't think the water flow is really backing up into this spot.

    Both roofers have confirmed that the chimney flashing is good and embedded into the masonry. One repairman has refused to help me any further, while the other has offered to redo the whole roof and reinstall flashing. At the previous rain I could not feel any new moisture, so perhaps it was just lack of maintenance??

    I am however considering redoing the roof (the shingles are old and brittle though they could last a bit longer). I have asked the roofer who offered the replacement about an insulated roof deck (closing up the attic) but he said this is not common in NC and also that shingle manufacturer's warranty would only apply with ventilation (namely putting in a ridge vent).

    I am not sure what to do about the roof and I'll post something asking for advice regarding my options but meanwhile with cold temperatures at night I really need to close up the wall.

    As for the chimney construction, you have it correct.

    Btw, enclosing my crawlspace is also on the menu, but it will be a while before I can address that.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |