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Insulation Around Gas Water Heater Flue-Updated Question with Photo

Lindaloowho | Posted in General Questions on

**Update: I discovered that I do have a fire stop on the underside (main floor ceiling) of the water heater flue. Am I good to go with the Roxul butted up against the B vent without any further modifications? I have used some high temp caulking around the circumference of the vent on the attic side. 

A Canadian Government publication indicates the fire stop can be on the underside of the attic floor. But they do not discuss the use of mineral wool specifically,  just that no “combustible material should touch vent”. I remember reading that Roxul may not be approved for direct contact with B vents, but then read that approval may be different in Canada.

The Ontario Building Code is like a different language to me. Can anyone clarify? 

Note: view of fire stop is upside down…that’s a view looking up.

This is for a vented, unfinished attic.



I just want to be sure of something: I asked a question last week about insulation around the gas water heater vent penetrating ceiling and roof deck in the area in which the baffle would sit.

I sealed off the venting in this bay, but now I must surround the vent with Roxul (fire-rated material) because foam can’t be used within several inches of vent.  The Roxul will have contact with the roof deck because, as I understand it, the foam baffle to protect the roof deck can’t be used. I just wanted to be sure this was okay, as it seems that Roxul is not recommended to have contact with concrete or wood in crawl space applications.

in Southern Ontario
in a Vented Unfinished Attic

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  1. Expert Member
    BILL WICHERS | | #1

    This can get more complicated. I'm assuming you have a gas water heater using B vent (double walled metal vent pipe). If you are using single wall vent pipe, you need much more clearance to combustible materials. If you have a high efficiency unit vented with PVC pipe, then you don't have these clearance requirements.

    B vent installation manuals will specify the clearances required. The general requirement is you must maintain a 1 inch clearance between the B vent vent pipe and combustible materials, which includes wood and any type of rigid foam insulation. Mineral wool is not a combustible materail and can be in contact with the vent pipe.

    Note that the usual way to do this is to make a sleeve out of aluminum flashing that will keep things 1" away from the vent pipe. There is some info about doing this in the "Step by step instructions" section here:

    I would NOT put mineral wool in contact with the roof deck around the vent pipe. I'd just leave that 1" gap open to avoid potential condensation issues with a vapor-permeable insulating material like mineral wool here. I would just use the protective sleeve described in the above link to keep the other insulation away from the vent pipe.


    1. Lindaloowho | | #2

      Thanks Bill,
      If I’ve already put the Roxul with contact to the B vent, then I should just cut down the height of the insulation, to a minimum of 1 inch away from the roof deck in that area? Is that what you are saying?

      1. Expert Member
        BILL WICHERS | | #3

        You want air to be able to circulate around that roof sheathing unless you were using vapor-impermeable insulation, or had insulation ABOVE the sheathing (neaither of which are particularly common in residential buildings).

        It would be safest to NOT have the roxul in contact with the sheathing, and to leave the 1" gap clear along the vent pipe so that you aren't greating an area near the sheathing that is insulated and seperated from the ventilated interior attic space.

        I'm entirely sure on your particular situation though -- if you have a vented attic (which is my assumption), then you shouldn't be insulating against the roof deck unless you're using vent baffles under the sheathing, in which case you'd want to seal off the vent baffle where the vent goes through, keeping the 1" gap from the pipe, then keep the insulation clear along the pipe from that gap down.

        If you're insulating the FLOOR of a vented attic, then the roxul can contact the vent pipe, but you still want the air seal described at that link, you just don't need to make the cylindrical barrier part that would be needed to maintain the 1" space from the vent pipe to the insulation.


  2. Lindaloowho | | #4

    I am insulating the floor only. It will remain an unfinished vented attic.

    1. Expert Member
      BILL WICHERS | | #5

      That makes it easy then!

      Put in the metal flashing PLATE shown in that link, caulk it to the attic floor and caulk it to the vent pipe using high temperature caulk. That is going to be your air barrier / air seal to prevent air leaks around where the vent pipe penetrates the floor. Note that it's often easiest to make two "half plates", and fit them on either side of the pipe than it is to try to make one plate with a hole and a slit and trying to wrap it around the pipe.

      Once that plate is in, install the mineral wool around it as you normally would. Mineral wool isn't a "combustible material" (it's actually commonly used in commerical buildings as a fire stop material), so it can be in contact with the vent pipe. Since you have a vented attic, you don't have to worry about any further issues here.


  3. Lindaloowho | | #6

    Okay great! I’ll do that and make sure the Roxul does not touch the roof deck too. Thanks for your help!

  4. Expert Member
    BILL WICHERS | | #7

    I recommend keeping those wires away from the vent too. Those look like small phone/network/thermostat wires. If they get hot often, the insulation tends to get brittle, crack, and fall off of the wire. Low voltage wiring like phone and network poses little fire risk, but it is frustrating when your phone or network start to be unreliable for no obvious reason...


  5. torontoboyjim | | #8

    Thanks for this posting. I have a similar situation (or worse!) where the B vent goes from the basement (conditioned space) through the garage (un-conditioned space) in a drywalled but uninsulated wall cavity, then up through the second floor walk-in closet (conditioned space), then into the attic and out the roof. As it passes through the garage this drywalled chase is of course cooling down things as they got nervous (or ignorant) in 1991 and did not insulate it well - except for an exposed portion through the garage that was wrapped when the previous owners did spray foam on the garage ceiling. I am just trying my best to stop the conditioned air hemorrhaging in this fun split-level house!

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