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Insulation around a through wall thimble

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Jclout, Oct 5, 2012.

  1. Jclout

    Jclout Member

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    I installed an ASHT+ double insulated outside chimney from my basement in 2000. This is a product of Security Chimneys. I just replaced the insulation blanket that goes around the chimney pipe inside of the thimble that goes through the wall. I used a mineral wool type called Thermafiber Safing Insulation and have checked this out with the chimney manufacturer today to see if this was suitable. The original was mineral wool and was 2" thick which is what the Thermafiber is as well. In the installation manual it also calls for putting R-20 insulation in the wooden box frame that the thimble is attached to inside of the wall. R-20 seems to be rather hard to find as Home Depot and Lowes don't even carry it. I did a search for it online and the Canadian Home Depots do carry it. The chimney is a Canadian product so I believe this has something to do with it. I also asked the manufacturer if it was ok to use an insulation with at least a higher R value and she said it was, just not lower. This is what I have done in the past but I believe I had the insulation fibers facing the inside and outside of the house instead of facing the thimble. The R-30 I just put in gave me lots of trouble trying to squash it small enough to fit in the thinest spaces of the box between the thimble and the wooden frame. Does anyone know why that insulation needs to be there, is it just to insulate the cold air from coming into the house or is it another layer of protection from the thimble heat for the wooden frame in the wall? I should have asked that question of the manufacturer today while I had them on the line. I think it's there to protect the wooden frame but doesn't the resin in regular fiberglass insulation melt around 250 degrees or so??? I did ask her if it was ok to use more of the mineral wool in the frame cavity and she said no it should be regular insulation. Any ideas? John

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  2. Jclout

    Jclout Member

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    I recently asked a question about insulation in the box around the through wall thimble but no one replied, maybe my question was too long? Can anyone tell me what the purpose for the R-20 insulation to fill the box frame around a through wall thimble to a double wall chimney???
  3. pen

    pen There are some who call me...mod. Staff Member

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    to protect the surrounding framework from heat and to keep heat from entering/exiting that location if the stove is not in operation?

    I'd also guess that it would provide an extra layer of protection to combustibles in an "ohh poo" moment.


    pen
  4. pen

    pen There are some who call me...mod. Staff Member

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    I'm curious, why did this insulation need to be replaced?

    BTW, insulation is not meant to be squished. By shoving r-30 in a space where r-20 is called for, you may have less than r-20 for actual insulation now. An insulation works because of the trapped air that it contains. By squishing it, you remove it's insulation qualities.

    If a wall is 2x4 construction, r-13 to r-15 made for 2x4 (3.5 inch gap) is what should be used. 2x6 construction is meant for r-19, etc. If you shove r-30 in either space, you'll actually do more harm than good.

    The r-30 could be used here, if you were to peel back a few layers of fiber so that it didn't need to be squished in.

    I merged your two threads together in hopes of you getting the best answer possible

    pen
  5. begreen

    begreen Mooderator Staff Member

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    I would use Roxul if the other insulation is hard to get. It's temp rating is 2150F I think and R=22 for 6" wall.
  6. Jclout

    Jclout Member

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    I think when I first started becoming concerned with the insulation was because I had some water leakage and the wood in the area looked as if it may have gotten too hot or could have been damaged from water as well as a sticker that was on the outside of the thimble looking a bit charred. The wood was not charred but just looked suspicious (could be the OCD);) I have had that insulation out of the thimble on several occasions and it has also gotten kind of raggedy, wonder why??? Is the Roxul that was mentioned a regular fiberaglass insulation for walls that is rated for 2150F or is it a mineral wool? I think you are right about squashing that R-30 in there, it's defeating the purpose. If it is in there for heat prevention toward the wood in the frame then the insulation would have to be facing the pipe and not the outside of the house toward the cold air. I can cut and fit the R-30 nicely in the box but the fiber layers are not facing the thimble this way. What should the aprox safe temp be on the outside surface of the thimble inside the wall cavity anyway? Pen thanks for merging the two threads, when I first saw that my second thread was moved I thought I did something wrong.
  7. begreen

    begreen Mooderator Staff Member

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    Roxul is mineral wool with a good R value.
  8. Jclout

    Jclout Member

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    I've just recently read something about not putting insulation near a chimney even though it is very difficult to ignite but my ASHT+ insulated chimney instalation manual calls for R-20 type insulation in the box surrounding the blanket insulated through wall thimble. Also read that the resin in insulation is affected by temps under 300 deg. F Just worried - has anyone else heard this stuff. I just replaced the insulation in that thimble box with new R-30 but the old insulation actually looked fine. Also any ideas on safe temps in wall area around the outside of the thimble?
  9. wingsfan

    wingsfan Feeling the Heat

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    I am doing a chimney install also, and the instructions I got with Supervent pipe from Menards Says you sould use Universal shielding insulation. But that is optional. I googled it and it looks like it is more fire retardant than regular insulation. But anything you put in that opening would still have 2 inch clearances with the way the thimble is made.I ask if some insulated or if some did not.
  10. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    Never use anything other than Roxul or other mineral based insulation. Period. Pink stuff goes in the wall. Mineral wool goes next to something that gets hot. Fiberglass insulation just never was and never will be designed to protect from excessive heat. It keeps your house cooler and warmer. It has no place in any flue installation.
  11. Jclout

    Jclout Member

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    It's the box surrounding the insulated thimble that has the R30 insulation. It touches the thimble but there is 2" of mineral wool type Thermafiber Safing Insulation in the thimble against the double wall pipe going through the wall. I don't know why but that's what the installation manual says. It actually calls for R20 but the woman I talked to from the chimney manufacturer said over 20 is ok just not under. I filled the box with the mineral wool type that is in the thimble but she said it should be regular insulation so I pulled it out and put in the R30. I'm going to post some pictures as soon as I can downsize them.
  12. wingsfan

    wingsfan Feeling the Heat

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    I called a local fireplace store and ask them if they carried the hi heat insulation, and he said he carries it, but when they install a system, them self they don't use it.They caulk well to keep out drafts , but he said they feel that air is the best insulation in that area.Since my manual says it is optional, I am taking the advice of the expert and leaving it out.
  13. Jclout

    Jclout Member

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    I have a long temp gauge and spot to put it in where it will hit the thimble so I am going to monitor the temp where the R30 touches the outside of the thimble wall but I'm not sure what is a safe temp at that point, anyone have any thoughts on that. I have been burning since the installation in 2000 with it in there but this new R30 I just put in is squeezed into the thin areas where the box frame comes close to the outside thimble wall.

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