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Thread: Rigid Foam next to PVC exhaust?

  1. #1
    DIY Junior Member DJL's Avatar
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    Default Rigid Foam next to PVC exhaust?

    I'm insulating my basement rim joists with 2" of rigid foam and foil faced polyisocyanurate. Then I'll seal the edges around the rigid foam with spray foam (Great Stuff) to eliminate air leakage. The process I'm following is described at:

    http://www.homeconstructionimproveme...ating-options/

    Is there any problem (fire, or other) in putting rigid foam and polyiso next to the pvc exhaust pipes of my water heater and furnace? These exhaust pipes are over 20 feet away from the heat source and are only slightly warm to the touch, but I didn't know if it was against code to do this, or if it posed a fire risk to do so.

  2. #2
    In the trades Dana's Avatar
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    Polyiso IS rigid foam, foam that has a higher service operating temp than CPVC vent pipe. The same can't be said for EPS bead-board, or XPS (pink, blue, green, whatever.)

    Rigid polyiso is good to about 250F.

    CPVC is good to about 200F.

    Expanded or extruded polystyrene is only good to about 165F.

    Spray polyurethane is good to about 175F.

    So...

    It's fine to put the iso next to the vent- if it's hot enough to degrade the iso the PVC venting is already melted. (And even when it's fully aflame polyiso doesn't melt- it chars.) EPS/XPS, hard to say- it can be degraded by extended exposure to temperatures well below the max operating temp for the CPVC, but there has to be something seriously amiss for the vent to reach the service temp limit of EPS or XPS. But if in doubt, use only polyiso next to the vent pipe, since it can take the heat better than the vent itself.

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    DIY Junior Member DJL's Avatar
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    Thanks Dana for the detailed information. I think I'm going to go ahead and do it. There's well over 20 feet of pipe and like you said, something else would have to be seriously wrong for the vent pipe to get that hot.

    By the way, I'm installing polyiso over blue XPS. I couldn't find thick enough polyiso locally, so I'm padding it out with 2" of blue XPS.

  4. #4
    In the trades Dana's Avatar
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    XPS is pretty crummy stuff from an environmental point of view, due to the high global warming potential (about 1400x CO2 GWP) of the HFC134a used for blowing it, most of which leaks out in the first 50 years, after which point it's R-value is about the same as EPS. EPS & polyiso are blown with pentane (about 7x CO2 GWP), most of which has escaped from EPS by the time it hit's the distributor's yard. (With foil-faced polyiso the facers slow the escape of the pentane.)

    If you used 2" of EPS you'd take about an 18% hit in R value for the polystyrene layer, but it would still have the same insulating characteristics for life. But if you used 2" of rock wool or stacked up polyiso where the vent penetrates the foam there wouldn't be any question about the insulation being able to take the heat, whereas with polystyrene there is, however small.

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