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

Open-cell foam contacting a water heater flue

John Sexton | Posted in General Questions on

I have open cell foam being put into my attic. My hot water heater is in the garage below, but where the flue goes through the rafters, is it OK for the open cell foam to contact around the flue? If not, how much of a gap is left around it? I guess they would have to just cut that gap after foaming since the foam isnt going to just cooperate….

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. Richard McGrath | | #1

    What type water heater is this and you do not mention flue material or construction either ?

  2. John Sexton | | #2

    Gas tank water heater. Flue is metal...not sure if there are other pertinent details...

  3. GBA Editor
    Martin Holladay | | #3

    This may be a flue (single-wall pipe) or this may be a metal chimney (double-wall pipe, or so-called B vent).

    If it's a flue (single-wall pipe), the situation is more dangerous that if it is a metal chimney (B vent). But in either case, building codes require you to maintain an air gap between the flue / chimney and wood framing. A similar precaution should be taken for a flammable insulation.

    Note that the use of a flue instead of a chimney in this location may be a code violation.

  4. Steve Knapp CZ 3A Georgia | | #4

    Martin. Could John cut back the open cell and use fireblock foam to seal the opening where the flue penetrates the sheathing, or would that approach also violate code?

  5. GBA Editor
    Martin Holladay | | #5

    For all code questions, I defer to the interpretation of the local building inspector. That person's opinion is the only opinion that matters.

    The usual method for air sealing a chimney penetration (where a chimney penetrates the home's thermal envelope) is explained in this article: Air Sealing an Attic.

    While most codes call for a 1-inch or 2-inch air space around a chimney penetration, some building inspectors have been known to accept mineral wool insulation in contact with a chimney. I can't imagine than any inspector would accept spray foam in that location.

  6. John Sexton | | #6

    Thanks for the answers. Since I had to cut out drywall in a knee wall location just to see the flue and penetration, I am not concerned much with what a local code official may see at some point. I just want to make sure what I do is safe and effective. So it is likely safe for foam to contact the flue if it is double walled and NOT safe if single walled? Can I swap out the material used for the last few feet of the flue to "fix" that if needed?

  7. Steve Knapp CZ 3A Georgia | | #7

    Thanks! In case anyone else is reading this threat, here are the relevant paragraph's from Martin's "Air Sealing an Attic" article.

    "Gaps around brick chimneys are dealt with differently than holes above soffits. Because chimneys can be hot, these gaps should be covered with sheet metal, not rigid foam. After nailing four pieces of sheet metal in place — one on each side of the chimney — the seams where the metal pieces overlap and the gaps between the metal and the chimney can be sealed with high-temperature silicone caulk.

    Gaps around metal chimneys are sealed with techniques that are similar to those used for brick chimneys. The easiest way to seal around a metal chimney is with two overlapping pieces of sheet metal; of course, you'll need to cut each piece with a half-moon hole that corresponds to the chimney diameter.

    Manufacturers of metal chimneys and most building codes require a 2-inch air space between the chimney and any framing lumber. Respect this air space; avoid the temptation to fill the air space with insulation."

  8. John Sexton | | #8

    If it helps any, here are pictures of the penetration through the roof, the floor where it comes up from below, a joint showing the pipe material up close, and the water heater and flue from below.

  9. John Sexton | | #9

    The last two pics... sorry had to separate them for this site ...

  10. GBA Editor
    Martin Holladay | | #10

    I don't think you want any spray foam to contact the metal pipe, whether it is single-walled or double-walled.

    Remove any spray foam that is touching the metal flue / chimney. And you can air seal the penetration if you want, following the advice in my article.

  11. John Sexton | | #11

    OK thanks very much. What about the new PVC piping that will go with the 96% direct vent furnace that is being installed in my attic? Is that OK to have foam around?

  12. GBA Editor
    Martin Holladay | | #12

    Q. "What about the new PVC piping that will go with the 96% direct vent furnace that is being installed in my attic? Is that OK to have foam around?"

    A. Probably. But it never hurts to call your local building department. (OK -- it almost never hurts.)

  13. John Sexton | | #13

    Great, thanks again Martin!

  14. David Meiland | | #14

    The best thing to do is to use the chimney pipe manufacturer's attic insulation baffle, if they make one and you can get it. They are common for woodstove chimneys, not so sure about B-vent as it's been about 20 years since I installed any. It would look like a short section of pipe that is about 4" larger in diameter than the actual chimney, and it is installed around the chimney during the original installation. You can place insulation against the outside of this fitting. If that isn't an option, I would probably make something up out of sheet metal that performs the same function, and place it around the chimney before the spray foam is installed. Here's a picture from the web that shows the general idea.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |