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HRV duct penetration

Hobbit _ | Posted in Green Building Techniques on

I’ve got a bit of a condensation problem with a cold-air duct. It’s
the fresh-air intake for the HRV, made from a big PVC pipe [for less
thermal bridging than metal] run through a wall and air-sealed
around where it passes through the inner and outer wall layers.
It connects to a piece of insulated flex-duct that runs to the HRV,
with the fiberglass blanket and outer cladding of the flex pushed
right up against the inner surface of the wall to try and expose
none of the cold parts to the inside.

The fiberglass is at the heart of the problem: interior air appears
to have filtered its way in and started *condensing* around the cold
pipe, into the fiberglass blankie, and capillarying its way into the
wood of the wall as well. While I’ve caught this before any damage
occurred, obviously a different configuration is needed.

As an interim fix I’ve backed the flex-duct insulation away from the
wall surface and cinched it down around the pipe leaving about a half
inch of PVC exposed, and the fiberglass completely hidden inside the
duct’s outer cladding. This is mostly to let the wood of the wall
dry, assisted at the moment by a warm light bulb [remember those?]
aimed in its general direction. That sliver of pipe is still going
to be cold, though, and attract moisture from inside.

I’m interested in ideas on the longer-term fix, and discussion of
best ways to do cold-air duct penetrations in general. Everybody
with an HRV in zone 4 and up must have to deal with this, right?
I’m thinking that the pipe/wall junction should have a surround of
impermeable sprayfoam and the fiberglass should be as contained as
possible, but haven’t quite worked out how to expose the least cold
surface to the inside of the house. Ideally the insulation should
be continuous from the wall right down into the inside of the HRV,
but it’s not like there’s a product designed to provide that. It
also has to remain able to be disassembled when needed, so blasting
the entire thing with sprayfoam isn’t in the cards.

All ears … I also floated this question over at hvac-talk, if
any of you read that.

_H*

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Hobbit,
    First of all, for any GBA readers considering following your approach, it's worth mentioning that ventilation contractors usually avoid the use of PVC for fresh-air ductwork because of concerns about possible offgassing.

    If you wanted to build a rectangular sleeve out of rigid foam, the sleeve could be installed in your wall, and the duct (whether metal or PVC) could be inserted into the sleeve. The sleeve could terminate beyond the wall at the home's interior, and you could use canned spray foam to seal the gap between the rectangular sleeve and the round duct.

    At that point I would recommend transitioning to insulated flex duct. You would have to cobble some type of connection between the flex duct cover and the rigid foam sleeve.

  2. Hobbit _ | | #2

    Hmm, PVC intake pipes and PT lumber in my air handler ... if I
    suddenly stop posting someday, I guess you'll know what happened.

    Perhaps I should simplify the question ... what are some typical,
    real-life, *already fielded* methods of bringing a cold-air supply
    duct through a wall in an insulated and moisture-safe fashion?

    _H*

  3. Hobbit _ | | #3

    Well, a few days later, the gap between the better-sealed flex
    duct cladding and the pipe/wood interface seems to be keeping
    things fairly dry. The pipe temp has been running about 40F,
    theoretically under the dewpoint of my indoor air but it's
    been staying dry so far.

    Here's what I'm probably going to do once warmer weather arrives:
    urethane the wood around the pipe penetration, to make it a bit more
    water-resistant; run an additional bead of caulk around the junction;
    run a light layer of low-e foam around over top of that; and reattach
    the duct [probably just put in a new piece] with its outer cladding
    clamped down to the pipe up next to the sprayfoam edge but making
    sure that no fiberglass is meeting indoor air. That should let the
    entire passage stay cold but remain isolated away from any sources
    of moisture. If I shape the cured sprayfoam right, any condensation
    that manages to form on the pipe should drip off before it reaches
    the wood wall panel.

    Caveats? War stories? How have any of you solved this problem,
    if it's come up?

    _H*

  4. Jenny Belman | | #4

    It is a piece of PVC pipe that comes in through a sealed wall penetration. It's PVC instead of metal to avoid thermal bridging to outside.

  5. Hobbit _ | | #5

    Let's try a picture:

    http://techno-fandom.org/~hobbit/hse/myduct.jpg

    The wall is, from the cold side: 3/8" sheet PVC, two layers of
    1" XPS, 3/8" ply. The thru-pipe is PVC, and the flex duct
    connects on the inside end. The outside end continues into a
    riser and weatherhood. The "downhill" slope of the pipe is a
    little exaggerated here but definitely present as engineered in.
    There are little red bits of sealant and PVC cement sort of shown
    where they are. It all sits down on the remains of what used to
    be the basement-window frame, parged into the CMU wall.

    You'll immediately see that the PVC pipe is not really preventing
    a thermal bridge, except perhaps through its own wall thickness,
    as the "cold outdoors" is effectively being brought all the way in.
    The challenge is to prevent or at least safely contain condensation
    on the outer diameter of the pipe on the *indoor* side, next to
    the wall penetration.

    _H*

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