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Installing a metal chimney for a wood stove through a spray-foam-insulated cathedral ceiling?

Scott Smith | Posted in Green Building Techniques on

Planning on installing wood stove with pipe exiting near peak of cathedral ceiling with rafter cavity filled with open cell insulation and vent baffle. I understand the needs for space near combustibles but trying to figure how to install pipe to maintain insulation barrier and keep safety in mind.

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Replies

  1. User avatar
    Rob Myers | | #1

    Scott,
    I installed an air tight insulated Excel chimney support available from ICC (www.icc-rsf.com/). Speak with the manufacturer to work out details, but my understanding is that you still cannot have foam in direct contact with the support even though it is spaced out from the insulated chimney. I built a 3/4 inch plywood box for the support (foam can contact the outside of the box) and then used Roxul inside the box to insulate up to the support.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Scott,
    Rob gave you good advice.

    All manufacturers of metal chimneys sell kits for penetrating an insulated ceiling. These penetration kits include some type of air space between the chimney and the insulation -- there is no avoiding the air space and its accompanying thermal penalty.

    What you can control is air leakage -- so made sure to include metal flashing at the air barrier penetration. The metal flashing can be installed with attention to airtightness (seams can be sealed with high-temperature silicone caulk).

    Any chimney increases the chance of ice damming in regions of the country with high snowfall. If you want to avoid all chance of ice dams, don't install a wood stove. Making sure that your chimney penetrates the roof at the ridge (as you are doing) is a good way of minimizing (but not eliminating) the chance of ice dams.

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Rob,
    I stand corrected. Good to know!

  4. User avatar
    Rob Myers | | #4

    Martin,
    However...I'm really not sure how much difference insulation at the collar makes. Theoretically when you are needing the value of the insulation you are also running the wood stove - and the chimney area is always warm. I do think that the air sealing is of value. But the whole assembly is tricky to install - especially with a high slope cathedral ceiling and external foam insulation.

  5. User avatar
    Rob Myers | | #5

    Scott,
    The Excel chimney insulated support is (as far as I know) unique in that it is air sealed and has an insulating sleeve that fits between the insulated chimney section and the radiation shield/support (which is basically a metal pipe with a welded end). Regular insulation (fiberglass or mineral wool) can be in direct contact with the support so it is actually possible to (in effect) have a continuous layer of insulation. Foam insulation in the cathedral ceiling must have a physical barrier between the foam and the support. The technical people at ICC are very helpful and I just sent them drawings which they approved - my WETT inspector then accepted the manufacturers installation approval. As an added note - I also like ICC chimneys because they are not as bulky - there is only 1" of insulation in the stove pipe rather than 2" so the chimney diameter for a 7" flue is only 9"). The chimney has the same (or better) performance characteristics compared to others on the market.
    Here is a link to the US installation manual, the insulated collar is a lot easier to envision using the drawings. The relevant section is part number ERDSI - Insulated Round Support Pge 19
    http://icc-chimney.com/c/icc/file_db/docs_document.file_en/XLUSA-II_2012-01.pdf
    Rob

  6. Scott Smith | | #6

    Thanks to both of you gentlemen for your help!
    Will take a look at that specific model and begin planning.

  7. Rachel Kinney | | #7

    Thanks for sharing this information.

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