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Possible To Safely Insulate Space Btwn Chimney And Shield?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by turbocruiser, Oct 12, 2011.

  1. turbocruiser

    turbocruiser Feeling the Heat

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    Let me preface this thread's question by saying I searched all around and added posts to slightly/somewhat/sorta related threads but still can't say I understand if its possible to safely seal and also safely insulate the space that's between the chimney pipe and the insulation shields?

    In my case I've managed to almost airtight seal the space (interior chimney chase) between the attic area and the living area by wrapping everything within the chase with cement board and then fiberglass taping and thinset sealing all joints between cement boards. But when I say I've sealed everything its really "everything except" the gap that goes between the chimney pipe and the insulation shields.

    Additionally I've managed to thoroughly insulate the transitions from living area to attic area where an average of R60 is everywhere except again the area the insulation shield sits.

    Now, I realize that the primary purpose of the insulation shield is to prevent the possibility of combustibles contacting the chimney pipe so I'm not trying to defeat that purpose in any way whatsoever but what I'm wondering is can I safely put something like high temp silicone sealant on that gap that goes around the chimney and can I safely put something like Rock Wool, or Roxul or something else specific in that space around the chimney to further insulate things?

    If this is one of those things where you just forbid yourself from altering the original state of things, that's totally fine for me; I just don't know so I'm asking the experts. I'm hoping that at the least it is okay to high temp silicone seal the gap that goes around the chimney though because that's basic air sealing? Anyways, thanks in advance for advice.

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  2. mhrischuk

    mhrischuk Guest

    Sometimes over sealing can create more problems than it's worth. I wouldn't try to make things too airtight.
  3. jimbom

    jimbom Combustion Analyzer

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    I had the same urge to surround our UL103 HT chimney with noncombustible insulation.

    Our chimney is in a heavy gauge metal chase with a 2.5" void between the chimney and chase. I called the manufacturer and asked if I could place pearlite insulation in the void. They said no.

    There is an exterior vent at the top of the metal chase. I asked if I could seal that up and just have a sealed chase with no insulation. They said no.

    I was talking to someone in technical support so I asked why. He equivocated, but my sense of his answer was it would be different than the test conditions for certifications obtained on their product.

    The reason I called originally was I had done a little analysis of my own. I realized a chimney fire in UL103HT would get the chimney hotter if the air space was insulated. Thus lead to more damage or failure of the chimney.

    After all this, I decided to go with the manufacturer's recommendation or as I like to term it - Follow the prudent man rule.

    However, I noticed the different coefficients of expansion between the HT and the steel chase. So I take advantage of that and have sort of a reverse fire damper situation. Or for those in the fire service, a smoke relief damper. If there is a chimney fire, ventilation of my 7 ft ³ chase will increase by 23 inches ² . The UL listing is preserved and I maintain a little warmer chimney surround.

    My recommendation would be to call the manufacturer of your chimney and ask about what you want to do.

    I understand some standards in Europe require insulation in the circumstance we are discussing. Go figure. Perhaps it has to do with the testing protocol.
  4. turbocruiser

    turbocruiser Feeling the Heat

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    Okay, thanks for the advice there. I do definitely understand that over sealing something can have consequences and I also understand that placing the insulation around the chimney then probably makes that portion of the chimney that much hotter. I guess at the least the silicone sealant that goes around the gap would make for more air sealing but I'm wondering whether that would also alter the rating the chimney has.

    I will call ICC and ask about all this but I basically anticipate that they will simply say that anything that is even incrementally different than the test conditions won't work or it isn't endorsed. If they say that something will work however I will share that so that folks here can consider it.

    Thanks again for all the advice I really appreciate it all.
  5. turbocruiser

    turbocruiser Feeling the Heat

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    As an update, I called the technical assistance dept at ICC and asked them this question. As always I was impressed with their service and support. Essentially they explained the specific test conditions the chimneys undergo to get their ratings and very openly offered that they cannot really recommend anything outside of those test conditions.

    They also then explained that although they don't test with the gap that goes around the chimney sealed with silicone, they have heard of "lots and lots" of people who have done that without any worry as long as it is indeed high temp silicone sealant. I really want to emphasize though that they were not in any way whatsoever recommending this; they simply were sharing what they have heard over the years and also admitting that they haven't heard about any worries with this practice.

    They then explained pretty energetically that putting any insulation around the chimney itself is definitely dangerous and really not recommended. The technicians explained that can change the internal temperatures of the chimney there in that isolated area and can cause all sorts of problems both with regular operation as well as with extreme operation (from chimney fires). Additionally depending on the type of insulation one uses there can be all sorts of serious problems there too.

    So, my take home message here was while it is probably "okay" to seal that gap that goes around the chimney with a thin smooth strip of high temp silicone sealant, it is not at all okay and actually dangerous to try to insulate the space between the chimney pipe and the insulation shield. I haven't made up my own mind on whether to seal with silicone yet as I'm still trying to get a good feel for what others would do here. On the one hand I'd prefer not to exchange attic air but on the other hand as some have suggested here I might be making too much out of a 1/8 inch gap that goes around an 8" OD chimney. Lets see that's about 3 square inches of leakage if my math is right? So what would y'all recommend? Thanks as always.
  6. WoodNStuff

    WoodNStuff Combustion Analyzer

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    I definitely would go along with the recommendations/test conditions of the pipe you have. That said, I got Selkirk's Supervent chimney. Supervent has their own insulation material specifically designed and marketed for the air gap between the outside wall of their class A chimney pipe and their attic air shield. After reading other posts similar to this in the forums, I called Selkirk and spoke with one of their technical people who told me that they have tested their pipe with this insulation and it is safe. They sell it for about $25. That said, it's only for their pipe. While on the phone I ordered the insulation directly from Selkirk. It should arrive in a few days. I'll then be able to install my chimney. Until then patience is the word of the day.
  7. turbocruiser

    turbocruiser Feeling the Heat

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    Wow, that's awesome to know about ... would you mind posting a picture as well as material description when it arrives ... I'm thinking it is probably a lot like stove gasket which also made me wonder why using a thin (like 3/8ths) graphited stove gasket wouldn't work and still be safe? Thanks again.
  8. greythorn3

    greythorn3 Minister of Fire

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    I Used that clear high temp silicone sealant where the class a goes into my attic to the firestop. been a few years like that now without any problems.
  9. turbocruiser

    turbocruiser Feeling the Heat

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    That's great to know thanks. Do you happen to remember the rated temp of that silicone sealant? I was going to use the Rutland 600 stuff; I can't imagine the exterior of the chimney getting that hot? Thanks Again!

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