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Using attic joists as ceiling service cavity

Christopher Emr | Posted in Energy Efficiency and Durability on

Once again, my Googling has come up short. Has anyone discussed adding a deck (osb or plywood) to a truss built attic on top of the joists, then adding all the insulation on top of that?

In this way the space between the ceiling and the deck could be used as a service cavity without lowering the ceiling. Given that it would be part of the conditioned space, duct work could be run in there for connecting HRV venting in the bathrooms. 

Would the air sealing around all the truss connections be too tricky? Or risky? 

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Christopher,

    It might work with an stick-framed attic with ceiling joists. Air sealing around the chords ion a trussed roof would be next to impossible.

    Either way, you end up with service bays, as opposed to a service plenum, so you would need to figure out how to, or where to, run the ducts in the other direction.

    if this is new construction, either scissor or plenum trusses are a better idea, perhaps coupled with some strategically placed bulkheads in the ceilings below.

  2. Christopher Emr | | #2

    Thanks for the quick reply Malcom,
    This is for renovation. The trusses are a "double fan" type truss so there are two places where the web connects to the bottom chord.

    I'm curious if this simply comes down to a practical matter of just sealing around the web connections or if there are other thermal/code/performance issues.

    We have a lot of wiring that runs around the attic which means lots of penetrations to seal. In addition we have all the ceiling boxes for fans/lights. Plus a pair of drops in every room for data and coax. Then there is a big patch panel in the hall closet that distributes the coax and data.

    Right now we have direct vent bath fans, but in the renovation I would like to add an HRV. The ducts could have a register high on the wall, but I would rather put them over the shower area to be most effective.

    So, lots of tricky air sealing in either case, not sure which is the lesser of the two evils.

    1. Expert Member
      Zephyr7 | | #3

      Wire penetrations are the easiest to seal -- grab some canned foam, squirt some in on one side of the wire in the hole, then push the wire over a bit and squirt some foam on the other side. When the foam expands, you get a near perfect air seal, and little issues with thermal expansion/contraction to worry about that could cause that seal to fail over time.

      Trusses have surfaces in shear that you need to seal, basically sliding-type gaps on the sides of vertical (or vertical-ish) truss members. These areas are more difficult to properly seal, and much more likely to fail (leak) over time. I'd take 100 wire penetrations to air seal over 10 truss connections any day -- the wires are both easier/faster to seal, and the seal will be more reliable too.

      If you do decide to go with a "deck", and seal the truss connections, I'd get a two part spray foam kit to seal those connections. The two part kits make a closed cell foam which is much more durable than the typical canned foam, and the two part kits make it easy to put on a heavy coating which will make a more reliable air seal in these spots.

      Bill

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