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Unique Roof Venting Issue

adam_nw | Posted in General Questions on

Hi All,

I’m a residential remodeler in Seattle in the process of re-building the roof on my 1950’s garage that’s shared with my neighbor.  2×6 framing w/ a 10′ span to center bearing wall, 3/12 pitch.  The finished roof will be standing seam, vented through the framing cavity with eave and ridge vents (my half will be finished and used as a small workshop).

Here’s the difficulty: my Structural Engineer came up with a diaphragm blocking system in the final two bays of the roof–tied into a full height wall and pressure block below–intended to resist the hydrostatic pressure pushing on the concrete foundation wall at the rear of the garage (see photo).  I did not catch that this full-height blocking would interfere with my venting solution through the cavity until it was already built. 

My proposed solution is to fur out the joists below the blocking by about 2″ for the bays in question, effectively creating a ventilation channel below the blocking (see sketch).  My question is whether or not this will work; will I still achieve the stack effect if the venting is below the framing as opposed to up tight against the roof deck? 

Any feedback would be greatly appreciated.  And if my proposal is no good, any alternative proposed solutions would be welcome.  Thanks in advance for your time.

Regards,

-Adam

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Replies

  1. Malcolm_Taylor | | #1

    Adam,

    I don't see how a vent channel on the underside of the structure does much, and I can't think of how you would connect it to vent openings at the top and bottom. See if your engineer will accept this:

    - Cover the rafters with a permeable WRB like Tyvek.
    - Strap all the rafters with 2"x2"s running from the eaves to the ridge to form topside vent channels.
    - Then sheathing, underlay and roof panels.

    1. adam_nw | | #3

      Thanks to the two of you for your replies; has definitely helped me think through the problem. I think what I will do is create a jig for my router with a large straight-shank bit. If I set the depth to 1", I can create a channel at the top of the blocking that is almost the width of the bay. I can then fill that with Cor-A-Vent S400, creating a continuous channel under the roof deck from eave to ridge.

      This topic has me thinking about my entire roof assembly. This space will be used as a workshop, with space-heating provided by an electric heater only when I'm working in the shop in winter. Because the rafter depth is only 2x6, I was planning on insulating the concrete walls with rigid foam, but foregoing cavity insulation in the roof. I will install plywood on the underside of the rafters, and tape and airseal it well. I know this is not the most energy efficient approach, but I'm trying to find a practical solution given the constraints of the project. If I don't insulate the roof, am I risking ice-damming when I occasionally heat the space? Hope my question makes sense.

      Best regards,

      -Adam

      1. Expert Member
        BILL WICHERS | | #4

        I don't see any need for coravent except possibly at the extreme lower edge. All the blocking up through the bay just needs an open channel, using coravent gains you nothing in those areas.

        You'll absolutely want to insulate the roof. The roof in this structure is probably the biggest area for heat loss, since the walls are at least partially earth bermed. If it's just a shed/workshop, you don't really need to worry about code minimum R values. I would probably use 3.5" deep mineral wool batts in those rafter bays, which would leave about a 2" vent channel. That's more vent channel than you need, but it would be easy to build this way using standard materials. I'd probably put 1" polyiso under the rafters to bring the assembly up to about R21, but with some continuous insulation too to get a little better performance than batts alone. I'd tape the polyiso as an air barrier, then put the plywood over the polyiso layer. If you also put 2" polyiso on the walls, you'll have a reasonably well insulated shed/workshop that will perform much better than if you were to leave any of it completely uninsulated.

        Bill

  2. Expert Member
    BILL WICHERS | | #2

    Usually you want the vent channel on the top, since both warm and humid air (not always the same thing) rise. I have in the past used a hole saw to cut half-circles in the edges of blocking before, usually for cables to pass through. You can make a jig for this that will let you cut half circles, you would use a 2x4 flat against a 2x8 and aligned with one edge of the 2x8. Drill through the 2x8 with the hole saw until you get to the 2x4, then go through that too. You can now hold the 2x4 edge against whatever you want to put a half circle in, drill through the 2x8, and the 2x8 and then 2x4 fill keep the hole saw aligned as you go through the edge of the uncut framing with only one side of the hole saw. No pilot bit is needed with this jig. It will help to use some bar clamps to hold the jig in place while running the hole saw.

    I would check with your structural engineer if this will be acceptable though, since that blocking is intended to carry load. My guess is it will be OK as long as you don't notch out too much of the blocking with the hole saw rig. Several smaller half circles might be better structurally than one big one, with my guess use a 2" hole saw for 1" deep half circles, then use maybe 2-3 of those spaced out along the blocking to form an air passage for a vent channel. This will work a little better for venting compared to just cutting holes in the middle of each piece of blocking.

    Bill

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