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Vent Channels for I-Joist Rafters

Timothy Godshall | Posted in General Questions on

I am building a 6/12 pitch cathedral ceiling with 14″ I-joists. I plan to create a vent channel between the top flanges of the I-joists and filling the rest of the joist cavity with open cell spray foam.

In the past, I’ve created these vent channels by nailing 7/16″ OSB to the underside of the upper joist flanges, but this approach is much more costly now with the price of OSB at $27/sheet. I’m wondering – is there any reason I couldn’t use housewrap to create the vent channel, pulling it taught and stapling it well to the underside of the upper I-joist flanges?

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Replies

  1. Expert Member
    Zephyr7 | | #2

    I think the labor to put in housewrap vent “baffles” would be higher than you might expect. That material is going to flop all over as you try to get it straight, and generally fight you the entire time you’re working with it. You’ll also have the challenge of cutting long, straight-edged strips.

    Since the baffle material doesn’t have to be super strong, maybe just try 1/4” or even 1/8” hardboard, which is usually pretty cheap. A quick check shows 1/4” at $13/sheet and 1/8” at $8/sheet. The 1/4” would probably be the better option — less chance of breakage during installation. You could also use luan.

    Unfaced or rigid foam with vapor permeable facers would be another option.

    Bill

  2. Expert Member
    Akos | | #3

    If you have wide flange I-Joists, it is pretty simple to staple up house wrap. Get one of the smaller rolls and cut the whole roll on a miter saw to the width between the joist webbs. You then staple as you unroll along the web.

    For narrower flange, as Bill suggested, 1/4" OSB/plywood is much easier to work with. You can also try cardboard or even ramboard.

    1. Expert Member
      Zephyr7 | | #5

      >"Get one of the smaller rolls and cut the whole roll on a miter saw to the width between the joist webbs."

      I like that idea! Have you had any issues with tear out though? Since housewrap is a random weave type of "fabric", I'm curious if the edges have a tendency to shread when cut with a saw.

      Cardboard might be a fire issue here. I'm not certain it would be a problem, but I know regular cardboard has no fire ratings and is more easily ignited than wood, so that may be a concern.

      Bill

      1. Expert Member
        Malcolm Taylor | | #7

        Bill,

        I've even used a reciprocating saw on rolls of house-wrap without problems. A circular saw works on building paper too, but your blade gets all gummy.

  3. Jon R | | #4

    Keep in mind that the intent is for moisture that makes it past the interior surface to harmlessly flow-through into the vent channel. So the vapor permeability ratio between the interior side (drywall, barrier, insulation) and the vent channel matters. How much depends on climate. But for less demanding walls, there is good support for 3x, 5x, and 15x in zones 5, 6 and 7. So something more for a ceiling.

    Say the interior side is 1 perm. I wouldn't use 3 perm foam to construct the channel (in Z5-Z7) - it would be an exterior side vapor retarder likely to cause some condensation and moisture retention.

    Highly permeable house wrap like Tyvek (56 perms) certainly wouldn't retain any Winter moisture. But with AC use, it might allow too much moisture in in the Summer, causing some condensation/sorption on the drywall. Better to use Typar (11.7 perms) in Z5 or Z6.

    Summary: in flow-through designs, perms matter - don't just use whatever is cheap and easy to construct vent channels.

  4. Patrick OSullivan | | #6

    How about 1/2" drywall? Perhaps use mold resistant board which, at time of this comment, is $14.67/sheet from Home Depot in quantity 1. Cheaper in quantity or at some other building supply.

    Given that mold resistant panels are rated for exterior soffits, I suspect it would be just fine as a vent baffle.

  5. Timothy Godshall | | #8

    Thanks for all the responses! I like the idea of cutting the roll of house wrap with a miter saw for a nice, clean cut. I think I'll give that a try and see how it goes - I'll post a follow-up in a few weeks to let you all know if it was miserable to install!

    1. igrigos | | #9

      Any feedback on this process Tim? I'm considering this for our upcoming build and was curious as to how it went. Thanks!

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