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Community and Q&A

Installing Polyiso Around Drain Pipes when Sealing Undersides of Floor Joists in Vented Crawlspace

etting | Posted in General Questions on

I’m in the warm desert, where a ventilated crawlspace can work.  I will be sealing and insulating the undersides of the floor joists with 2″ polyiso and the rim joist with polyiso and foam.  What is the best way to get the 4′ x 8′ sheets of polyiso around the five 2″ and one 3″ drain pipes that drop through the floor joists?  I could cut holes in the polyiso and slip the pipes through before they’re attached to anything, but I’d rather put the big sheets up after everything else is done so that they won’t be in the way.   The other option that comes to mind is to cut a sheet at the nearest joist, cut a slot into it to fit over the pipe, then glue or foam most of the slot back into place and nail the sheet’s cut edge into the joist.  It would be easier if the pipes dropped down near where the edges of the full sheets would be, but I can’t make that happen, in part because the pipes drop down at different distances from the stem wall.   This seems as if it must be a fairly common problem; how is it typically handled?

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  1. Expert Member
    BILL WICHERS | | #1

    If all the pipes are in the same general area, I'd leave a square/rectangular cut-out in the full panel in the general area of the pipes. Run the pipes, then fill the square/rectangular hole.

    If you keep all the pipes in a straight line, you can do what I like to do: Cut oversize clearance holes in the polyiso panel that will fill that square/rectangular hole for each pipe, then cut the panel in a straight line through the center of all of those holes. This leaves two pieces with a common edge that has "half holes" for each pipe. You can now easily install these two pieces around the pipes and fill the gaps in the oversized holes with canned foam. Seal the edge of the square/rectangular panel to the main sheet of polyiso using either canned foam or tape to complete your air seal.


  2. etting | | #2

    Thank you, Bill. What you suggest sounds good, but my drain pipes zig-zag along 24'.

  3. Expert Member
    BILL WICHERS | | #3

    Then you're going to have a lot of cutting to do :-)

    A hole saw will do a decent job of cutting the holes you need if run by hand. I just hold the hole saw in my hand and twist it back and forth to cut the hole. I do the same in drop ceiling tiles by the way. Using the hole saw by hand limits the chances of tear out or otherwise ripping into the polyiso and damaging it. You could also use a small serrated knife to follow a line. Either way, I'd oversize the holes so that you can fill the gap with canned foam after installation. If you try for a precision fit with the cut holes you're likely to get frustrated pretty quickly because these things never seem to want to line up the way you want them to.


  4. etting | | #4

    Thank you, Bill. It makes sense to cut holes a little big, within the expansion limit of foam. If others chime in, it will be interesting to see whether they cut holes, slots, squares, or something else. I was surprised I couldn't find this problem discussed anywhere.

    1. Expert Member
      MALCOLM TAYLOR | | #5


      I'd vote for getting the plumbing drops roughed-in to just below the bottom of the joists, then installing the foam. If you do the whole rough-in first there will be metal support straps running from the lateral runs up to the joists, making it very difficult to fit the sheets.

      Buy a variable sized hole saw. There isn't much point in worrying about sealing the holes at the foam if your air-barrier is the subfloor above.

      1. Expert Member
        NICK KEENAN | | #10

        As Chuck Berry said to Keith Richards, if you're going to do it you may as well do it right.

        I would want this all to be rodent sealed. Depending on what your local rodents are the foam may be enough, or you may need OSB or plywood covering it. In either case, a mouse can get through a hole the size of a dime. Mice will gnaw at smaller cracks to get in. I have a theory that they feel drafts and use that to decide where to chew, because sealing with can foam seems to discourage them.

        1. etting | | #11

          Thank you. Yes, rodents would ruin the air sealing before causing all kinds of further problems, but my concrete stem wall is well sealed. It has a tight steel door, the vents are covered with heavy steel mesh, and if any rodents were to dig down along the wall, they'd run into the protruding footing. Apparently, it doesn't occur to them to move away from a wall to get around a horizontal barrier.

  5. etting | | #6

    Thank you, Malcolm. I dreaded the idea of trying to install polyiso around straps, but I was thinking I could avoid them because the building drain will rest upon the crawlspace floor, built up and dug down as needed underneath, but now that you mention it, I should check code on which of the short horizontal legs between the bottoms of the joists and the building drain require straps. I assume from your suggestion that if I need straps, I could secure them with nails or screws that go through the polyiso into the joists; that would be MUCH easier. I have to seal the polyiso and around any penetrations to keep cold air from reaching the floor joists and subfloor.

    1. Expert Member
      MALCOLM TAYLOR | | #7


      Unfortunately it was a lesson I had to learn the hard way. It takes an exorbitant amount of time, and yields disappointing results, trying to do a decent job with all the obstructions.

      I use deck screws through the foam to secure the strapping. It allows you to make adjustments.

      1. Expert Member
        BILL WICHERS | | #8

        If doing this with the usual plastic tape-like strap, I like to fold the ends over where the fastener goes through, with the folded piece under the main piece so that it acts like a washer. This is an easy way to reinforce the ends of the strap a bit. If you're using the metal strapping it doesn't really matter.

        BTW, I would install the straps after the foam too. I'd mark the foam with a sharpie marker as you put it up though, marking all the joist locations. This makes it really easy to know where to put in the screws for any supports afterwards, and you can do it quickly by eye while you're putting the foam up.


  6. etting | | #9

    Very helpful, Malcolm and Bill; thank you!

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