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Preventing Frost on Plumbing Vent

ken_o | Posted in General Questions on

Insulating plumbing vents has been discussed many times here, but always from the “home heat loss” point of view.  I am trying to find ways to prevent frost clogging, which is a real issue:

We will be building in West Yellowstone, which has both very cold weather and a lot of snow on the roof (except in July).  So far it sounds like using a 4″ vent pipe, insulating the pipe in the unheated attic space (not inside the heated areas of the house) and also insulating the exposed pipe above the roof penetration are all possible solutions.  Maybe even painting the pipe black.  Does anyone have some experience with this (other than theories…)?


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  1. Expert Member
    BILL WICHERS | | #1

    I've done this with some vertical rain conductors (fancy way to say "downspout" in the commercial world), and I did it by putting a self regulating heat cable down inside the pipe. Worked great.


  2. ken_o | | #2

    Inside the pipe. Interesting. Then no problems with leaking at the boot. How did you secure the heat tape at the top of the pipe?

    1. Expert Member
      BILL WICHERS | | #9

      A loop and a large black zip tie. By "large", I mean one of the ones that is around 3/8"-1/2" or so wide. You can get these at commerical supply houses, and they are *much* more durable than the smaller ones you normally see.

      I used the self regulating heat cable made by Raychem, which is expensive, but very robust.


  3. Expert Member
    Michael Maines | | #3

    The IRC requires a minimum 3" vent pipe in very cold climates: I don't know if that's enough for your situation but I bet it would be.

  4. ken_o | | #4

    I think I'll just bump up to 4" PVC near the roof. I'll implement the other suggestions also. Climbing on the roof at -35F isn't my idea of fun.

    1. Expert Member
      MALCOLM TAYLOR | | #5


      If you have an unconditioned attic and lot of snow, you may want to consider terminating the vent on a side wall.

      Another alternative might be to twin the vent stack with one that had an AAV in the attic that could provide back up when the vent froze over.

  5. canada_deck | | #6

    Do pipes get blocked with frost inside the attic space or only at the top of the vent? I wonder if it would help or hurt to put a long horizontal (with an appropriate low slope for drainage) leg in the attic (without insulation). Assuming the attic is cool but above freezing, you might be able to condense a lot of the moisture out of the sewer air before the vertical leg through the roof. That's probably a poor assumption if the ceiling is well insulated though.

    If you decide to upsize the pipe near where it goes through the roof, I think you will need to approach that carefully. Easy to imagine that becoming an issue with melt-freeze. E.g. You could have a 2" vent that gets upsized to 4" for the top 12". If you do develop an ice plug at the top and then it melts loose when the sun hits, that ice plug will drop 12" and sit there and then melt very very slowly. I'd want any ice plug to be able to drop all the way down into a heated space where it would melt more quickly.

  6. ken_o | | #7

    Lots of great ideas here. I think I can put together a design. Thanks a bunch.

  7. ken_o | | #8

    I just noticed that section P3103.2 in the code says that any increase in vent pipe size must take place at least 1 foot inside the heated envelope. That would address the falling ice plug issue. Again, thanks for all of the thoughts.

  8. plumb_bob | | #10

    BC code required a 3" main plumbing vent that is increased to 4" where it goes through the roof to prevent frost closure. We do that here with no problems and we get spells of very cold weather.

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