Q&A Homemade Insulated Chimney - Is it OK?

QandA Posted By QandA, Jun 17, 2007 at 6:15 PM

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  1. QandA

    New Member 2.
    Staff Member

    Nov 27, 2012

    My husband and I recently purchased a used woodburning or coal stove. We live in a very rural area in northern Pa and choose to burn wood due to its easy availability. We contacted a local manufacturer to fabricate a stack with a through the wall thimble. This company has been in business for over 30 years and makes industrial boilers, ovens etc. The system was made of 20 gauge steel, welded and was attached to our 7 inch stove flue. There is an increase close to the back of the stove that changes the pipe to 8 inch and from there the thimble surrounds this stovepipe. This thimble is approximately 14 inches long and about 21/2 inches deep, all sides welded, and is lined with insulation that is used in ovens and race cars. At this present time I cannot tell you the R factor, But I am expecting a reply from the company with regards to this matter. The pipe and thimble to the wall is about 40 inches long, and then goes through to the outside of the house, where it is welded to a tee. The tee has a cleanout at the bottom which can be detached for cleaning, and the stack then goes up to the top about 3 feet past the roof line. There is a cone shaped spark arrester cap on top of the pipe. The stack is made in 4 foot sections and rises approximately 16 to 17 feet. The stack rests on a steel platform that is braced to the house along with brackets about every 4 feet to support the structure. The sections are secured with sheet metal screws . I would like to add that there is layer of insulation that wraps around the circumference of the thimble between it and the wall. Our county has 2 traffic lights. I saw the illustration with regards to using terracotta pipe for a thimble and the specifications that are recommended, and this is what prompted me to write to you. We hoped that you could lead us in the proper direction. Any response would be appreciated. Thank you.


    Dear Friends,

    According to code, you do need to use a class "a" chimney, which means a pre-fab metal one that has been approved or a "site built" masonry chimney. That said, it sounds like the boiler company built a heavy-duty assembly...probably way overkill for your use.

    It is true that some industrial boilers use site-built metal stacks such as these for venting, but these are designed and approved by an engineer or other party that is responsible for the operation and safety.

    You are having the condensation problems due to the fact that the chimney is uninsulated. Many UL listed class A chimneys are insulated with a blanket of ceramic wool (Kaowool is the name of one brand of this type of material) and then covered with another layer of metal.

    I cannot tell you how to try to build or fix a chimney which may not meet code standards. However, some rural areas do not even have building codes, so your chimney may not violate local codes. If the company that installed it knows their stuff, they may be able to upgrade it by insulating it. In any case, you should have the chimney inspected by a knowledgeable fire safety professional, such as the local fire official, a chimney sweep or the construction officer in your township.
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