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Do manufactured rafter vents provide adequate ventilation?

NateSc | Posted in General Questions on

For a simple gable roof, insulated amply with rock wool on the ceiling plane, in a cold snowy climate (upstate NY).

Are premanufactured baffles adequate ventilation channels at the eave? I have seem product spec sheets saying a 1.4″ channel – I need to measure them myself to confirm.

A lot of what I have read says 2″ minimum. Without special ordering something expensive, to get 2″ it seems I would need to rip down some rigid foam insulation and construct things myself in place.

Thanks for any advice, Nathan

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Nathan,
    Is this an existing house or a new construction house?

    If this is an existing (old) house, and there are no signs of moisture problems in your attic, don't worry. The existing vents are fine.

    If this is a new construction job, it makes sense to get the details right. Buy AccuVents or Smart Baffles -- or make your own site-built baffles. For more information this topic, see Site-Built Ventilation Baffles for Roofs.

  2. NateSc | | #2

    Thanks Martin. This is new construction, I've been insulating wall cavities and will start doing the attic soon.

    No drywall on the ceiling yet, so I have good access.

    Darn article is behind the pro-paywall. I am an owner builder with a subscription to fine homebuilding. It is hard to justify a sub to both.

    Thanks again.

  3. GBA Editor
    Martin Holladay | | #3

    Nathan,
    Concerning the paywall:

    1. The first 10 days at GBA Prime are free. Sign up for your free 10 days here:
    https://subscribe.greenbuildingadvisor.com/membership

    2. If your 10 free days are up, you can get another month for $15. If you are building a house, it's $15 well spent. In many cases, reading a GBA article has saved an owner-builder thousands of dollars.

  4. charlie_sullivan | | #4

    $15/mo is tiny compared to $15k/mo paid to a builder. It's well worth it, including all the good stuff you get for free.

    That said, I would be delighted to see a package deal to subscribe the FHB and GBA, maybe even with Fine Woodworking as well.

  5. Expert Member
    MALCOLM TAYLOR | | #5

    Charlie,
    I am a builder, and would welcome any readers who would rather just send $15 a month to me rather than subscribe to GBA :)

  6. brp_nh | | #6

    Is this a vented unconditioned attic where you have or will have rock wool on the attic floor....or a conditioned attic with rock wool between the rafters and rigid foam above the sheathing?

    Was just curious and confused by the mention of ceiling and foam.

  7. NateSc | | #7

    Vented unconditioned attic. It is getting two layers of R-30 rock wool. 7-12 gable with concealed fastener standing seam (that was fun), only obstacles on the roof are a wood stove pipe, plumbing vent, radon vent. Even at 7-12 i am shedding snow better than just about all the other roofs in the area, and better than the 9-12 shed I built with standard 'exposed fastener' metal.

    The stores around here stock foam and plastic baffles. The foam is really flimsy, the plastic seems nice. The plastic baffle is quite rigid and after a test fit, it actually aligns with my blocking vent gap just about perfectly. So I'm just going to use those and be done with it. The extra rigidity of the rock wool should also allow me to finesse it in there and somewhat keep the channels that receive the staples open as well. We'll see, that's certainly not science.

    I like GBA a lot, but I think it probably goes 'a little further' than I have gone. I have had the 10 day trial before. I am kind of an in-betweener.. most of the builders here take it way further on insulation and air tighteness and various other approaches than we have. I see people caulking every stud bay in addition to taping the sheathing.. wow, special ordering materials from Europe. We EDPM'd every where the zip wall does not touch other zip wall.

    Anyhow thanks again.

  8. brp_nh | | #8

    So, the two layers of R30 rock wool are going in between and/or on top of the bottom chords of the trusses? And you'll use the ceiling drywall below the chords as your air barrier?

    And where is the rigid foam you mentioned in the original post ("I would need to rip down some rigid foam insulation")?

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