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Video

How to Ventilate Rafter Bays When Adding Insulation

This video is only available to GBA prime members

See two ways to keep air flowing from your soffit vents to your vented attic if you’re planning to add blown-in insulation between your ceiling joists.

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Video Transcript:

We’re going to add insulation to his attic eventually so we want to put insulation blockers and air channels in the rafter bays. We want the most amount of insulation as far down as possible over the top plate of the wall. The insulation blockers should still allow the air to flow up under the roof sheathing and to the ridge so it cools the attic. One way to do this is by using ventilation channels. They’re a piece of plastic or piece of foam which is designed to be installed between the rafters. It leaves a space that the air can flow up and through. To do it right we must install a blocking panel at the very end underneath the ventilation channel and down to the top plate. This will stop any of the air from short circuiting the ventilation channel and going through the insulation where it can degrade the value of the insulation.

The blocking panel has already been cut to size to go between the top plate at the bottom and the underside of the roof sheathing at the top, and to fit between the rafters on either size. I need to carve out the shape of the ventilation channel. The cuts don’t need to be neat; as long as I’m within a half an inch the spray foam will fill the rest. The next thing to do is trim all the way around the perimeter a 45 degree angle off the face about a half an inch back. That’s going to leave a channel for the spray…

10 Comments

  1. COPat | | #1

    Does it matter which way the foil side of the rigid foam faces? My assumption is that there would be more beneficial raidant heat reduction results from facing upwards not inwards.

    1. GBA Editor
      Martin Holladay | | #2

      Patrick,
      Many brands of foil-faced rigid foam have foil on both sides. But if your brand has foil on only one side, you're right -- the foil faces the air space.

  2. iainb | | #3

    I’ve got basically no soffit or overhang. I’ve read some of the articles which say that what venting the roof deck can do is sometimes exaggerated and that a good air sealing job os the most critical part. Should I go to the trouble of installing some kind of edge vent or just make sure there is an air gap as shown?

    1. GBA Editor
      Martin Holladay | | #4

      I.B.,
      Climate matters. The colder the climate, the more important it is to have good soffit-to-ridge airflow under your roof sheathing (if we're talking about vented roof assemblies).

      For information on your options, check out this article: "All About Roof Venting." For you, the most relevant section of the article can be found by scrolling down to the section with this heading: "Roofs with no overhang."

      1. iainb | | #5

        Thanks. Looks like I need to pay someone to come out and either install venting or spray foam. Which I think I knew, I was just hopeful there was another option. Right now there’s a few inches of cellulose in the ceiling joists, a couple of windows, and somehow I don’t have ice dams. Makes me not want to tinker too much.

        1. COPat | | #6

          I'm essentially in the exact same position.

          5" overhang on the eaves, with a 3:12 pitch. I'm planning on using 1.5" foil faced rigid foam as baffles and then blown in cellulose.

          Currently I have no ice damming or signs of mold, but that has to be because my conditioned space leaks into the attic like a sieve with only 4" fiberglass batts and minimal ventilation (2 mushroom caps).

          I'm also very nervous that tightening my envelope and getting to an average ~R50 may cause ice damming.

          1. GBA Editor
            Martin Holladay | | #7

            I.B. and Patrick Ovens,
            Both of you seem to imply that adding insulation increases the chance of ice dams. In general, it does not. A lack of adequate insulation at the eaves is more likely to cause ice dams than a generous amount of insulation at the eaves.

            Again, climate matters, and I don't know where you two live. If your climate rarely gets snow, ice dams aren't a concern.

  3. iainb | | #8

    Martin,

    I’m not concerned about added insulation causing ice dams exactly. It’s that right now the existing assembly has worked for 80 years to keep the roof intact and I’d prefer to mess with success only if I’m sure I’ll get what I need. I’m honestly shocked I don’t have ice dams (zone 5, central MA), and I keep wondering if it’s because I lose enough heat that I don’t get a freeze thaw.but just thawing;

    IB

    1. MartinHolladay | | #9

      IB,
      If your improvements are in the right direction, the chance of ice dams will be lower, not higher. If you have ice dam worries, the first line of attack is always air sealing. Spray foam kits can be extremely useful to improve the air barrier at your ceiling, especially when addressing air leaks in the tight spaces near the eaves.

      The second line of attack would be R-value improvements.

      The third line of attack would be improvements intended to encourage air flow through your ventilation channel directly under the roof sheathing.

  4. merges | | #10

    Just moved in to a 12 year old house in Ontario, Canada and am having my attic insulation removed and replaced with cellulose R60. While taking this opportunity to air seal all attic penetrations, I stumbled on the situation below and hoping for some expert advise:

    There is a total of 6 baffles for an attic space of about 1000 sq.ft. While investigating how to add more, I realized that there is nothing but a batt of fiberglass insulation material that separates the attic and sofit vent. The way I see it, I have three options before the insulation company shows up::
    * Install foam based block and/or baffle (as shown in the video for this article)
    * Pay for a spray foaming over the fiber glass batt and creating a air barrier (the insulation company I have contracted do not do spray foam)
    * Leave it as is

    I have attached two pictures (with and without legend). What would you recommend?

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