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Can Spray Foam Around Fireplace Chimney

nj_homeowner | Posted in Energy Efficiency and Durability on

Hi Everyone –

We are renovating a house in NJ with a masonry wood-burning fireplace that we plan to use occasionally.

This week, we did some blower-door directed air sealing.  In order to seal the gap between the chimney and the framing, where the chimney penetrates the roof, the contractor used generous amounts of orange Great Stuff Gap + Crack foam.

After the work was done, I checked the manufacturer data for the foam and learned that it’s combustible, and therefore should not have been used.  It appears this is also a code violation.

Is this a serious safety issue?  Should we try to scrape out all of the applied foam and replace it with a metal or fiber cement barrier, sealed with intumescent caulk?  Is that a waste of time?

Thank you in advance,

Brian

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Replies

  1. Expert Member
    Michael Maines | | #1

    Brian, I can't say how much you should worry about having spray foam around the chimney, but it's not code-compliant. The gap should be sealed with sheet metal and the space left clear. The gap around chimneys is usually a huge source of air leaks so it's definitely worth air-sealing it. The small amount of additional heat loss from not having insulation around the chimney is less important than having that space airtight, preferably on both the top and bottom of the assembly, to create a dead air space.

  2. nj_homeowner | | #2

    Thank you Michael. I understand the importance of air sealing the area.

    The contractor put a lot of canned foam in the narrow space, which extends up a foot or two. It will be difficult to remove.

    I wonder if the foam presents a real danger enough to justify the effort of removing it, regardless of what might pass inspection?

    Brian

    1. Expert Member
      Michael Maines | | #3

      Keep in mind that this is just thinking out loud, not providing any advice. The foam sell sheet says to not expose to temperatures above 240°F: https://www.dupont.com/content/dam/dupont/amer/us/en/Greatstuff/public/documents/43-D100013-enNA.pdf. Does your chimney have a lined flue? If not, I wouldn't use it until you get a liner installed. Do you run the fireplace for extended periods? I don't know how hot the brick would get but the longer it runs, the hotter the brick.

      1. nj_homeowner | | #5

        Thank you for these thoughts. The 240 F is actually what made me nervous more than anything.

        Why do you think I need to line the flue?

        We rebuilt the firebox and throat last year, and the mason said the flue looked good to him, did not see a problem. The flue is constructed of stacked orange clay sections with mortar in between the joints.

        I used the new fireplace numerous times last winter even though we were under construction, and it was quite nice.

        Is there something that I should look for that would indicate the need for lining the flue? Or do you think that is always needed?

        The outside of the chimney that goes through the house will be exposed until we put up drywall…. I could get a roaring fire going and check the chimney temp at various places using an infrared thermometer, if that would be useful.

        1. Expert Member
          Michael Maines | | #7

          Lining the flue creates a small, insulating space between the flue and the masonry structure, and helps ensure that flue gasses and sparks can't get through the brick. If you have orange clay flue tiles, those are considered flue liners. Many old chimneys are just brick and nothing else, so when acidic flue gasses erode the mortar, you can see right through to the interior.

          When it comes to fire safety I don't mess around; I would just dig out the foam and be done with it. Use a coarse Sawzall blade and it shouldn't take too long.

          1. nj_homeowner | | #9

            Thank you. The foam itself is not hard to remove, but the framing there is so tight and crazy that it's really hard to reach.

            I hope we can get it!

  3. Walter Ahlgrim | | #4

    Generally the code books are written in blood and someone died and the code changes.

    It is not uncommon for remodelers to find chard lumber around fireplaces showing how close to disaster that home had come.

    The chances are the foam will not be a problem for you but the question is could you live with yourself if the worst did happen.

    Walt

    1. nj_homeowner | | #6

      Walt you are absolutely right! I was not trying to rationalize something risky but to determine whether the risk is real in the first place.

      I think we need to remove it!

  4. Expert Member
    Kohta Ueno | | #8

    If you don't already have a sketch detail for the recommended retrofit (sheet metal air barrier closure), see this guide, under page 37/MASONRY CHIMNEY CHASE.

    GM-1001: Attic Air Sealing Guide and Details
    https://www.buildingscience.com/documents/guides-and-manuals/gm-attic-air-sealing-guide/view

    1. nj_homeowner | | #10

      Thank you Kohta. Your guide as usual is very nice, but the framing around my chimney is much more complicated than your example. One side of the chimney is at the peak of a gable roof, while the other side is the peak of a lower gable roof. There is also 2x4 framing around the whole thing. It's going to take some work. I'm actually not even sure how we will reach some of the foam!

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