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Community and Q&A

Thermal break and insulation for gas fireplace flue pipe

John Culbertson | Posted in Mechanicals on

We are building a tight and well insulated home in WY. It gets very cold here. We are paying special attention to creating a thermal break and insulating all ductwork to the exterior. They installed a “one pipe” gas fireplace and the metal flue pipe comes straight from the outside with no thermal break. I am told that it must have one inch of air around it so we are not able to insulate the pipe. I had suggested stove pipe insulation which is fire proof but contractor says code does not allow anything. So, we then have this super cold metal that will conduct cold into the framed wall area and down to the gas fireplace unit and then out into the living space. In addition to the cold transfer I am also concerned about condensation in winter time when we maintain 30% humidity and 70 degrees. Any suggestions?

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  1. GBA Editor
    Martin Holladay | | #1

    You have to follow the manufacturer's installation instructions, which are almost always available online. What is the manufacturer's name and what is the model number of the gas fireplace?

    A "one-pipe" installation can mean a lot of different things -- you maybe talking about a pipe-within-a-pipe, with the outer large-diameter pipe providing combustion air from the outside, and the inner small-diameter pipe being the flue. But it's hard to know exactly what you're talking about without more information.

  2. John Culbertson | | #2

    I mean a pipe with in a pipe and it is a fully sealed system. The unit is a "fireplace extrordinaire" and their manual does require a one inch air gap. I have spoken with them and they have confirmed that.
    So, I basically have to accept the cold air transfer and condensation risk?

  3. GBA Editor
    Martin Holladay | | #3

    Fireplace Xtrordinair makes many different models of fireplaces. If the installation manual requires 1-inch of clearance between the pipe and combustible materials, that clearance must be maintained.

    Just because you can't insulate the pipe itself, doesn't mean that you can't install an air barrier to limit infiltration or exfiltration. The usual way to seal any air leaks at the point where a flue pipe leaves the conditioned space is either: (a) to use a collar supplied by the pipe manufacturer, or (b) to install a sheet metal collar of your own devising, and to use high-temperature silicone caulk to seal the crack between the pipe and the collar.

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