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Community and Q&A

Air barrier with a vented attic

Beardoh | Posted in Energy Efficiency and Durability on
New subscriber here…Looks like lots of great conversations and posts.

With vented roof assemblies, it seems that primarily soffit vents leading to a roof or gable vent is common practice.

To a leyman, like myself, it seems odd that we introduce a soffit vent into a structure that we have worked so hard to air seal at the foundation/rim joist area, walls and wall penetration etc. I believe it is fairly common, but the two assemblies (wall and roof) seem very different. 
For reference, here is an assembly show by Alexander Baczek at this link for a vented roof:
Is my intuition wrong on this?  
I appreciate the details in the above link, but how good of an air seal would this be? 
I am building in central New Hamshire, climate zone 6 and hoping to achieve passive home levels for a blower door test.  Building this home myself, and will take the time needed to button up as best as I know how.
Has anybody done a roof/ceiling assembly where they were relying on a vapor barrier such as Majrex in their ceiling for an air barrier and can share their blower door test results?  Was it hard to achieve a low ACH?

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  1. Expert Member


    In both vented attics and vented cathedral ceilings, the primary air-barrier (which as you say is continuous from foundation to roof) continues along the ceiling plane. The air introduced to vent the roof system is above the insulation, so they never mix.

    We do the same thing with rain-screen gaps on walls. The primary air-barrier (which may be in a variety of places in the wall) stops interior air from moving though it, so it never gets to the vent cavity outside.

    1. Beardoh | | #9

      Thank you Malcolm

  2. jadziedzic | | #2

    We used Intello Plus as an interior air barrier (walls, ceiling) in our new build. Our framer laid wide strips of Intello across the top plates of interior partitions and between interior partitions where they joined the exterior wall. Those strips were connected to the Intello installed later on the interior walls and ceiling with Tescon Vana tape.

    Our pre-drywall ACH was in the 1.5 - 2.0 range (we had some duct boots that weren't sealed very well which probably contributed to that result). While that measurement would probably have dropped somewhat once drywall was installed, I opted to have the New England Air Barrier folks do their AeroSeal process after drywall was installed. They achieved approximately 0.8 ACH50 by the end of the test, and the price was not at all unreasonable. (Contact me directly if you'd like those details.)

    1. Beardoh | | #7

      Thanks for your response and explanation. I came across a recent blog article from GBA about laying wide strips of the air barrier on top of the top plates. Good to hear it was successful in practice. I'll send a private message about the AeroSeal.

    2. Beardoh | | #8

      Looks like there is no facility to write directly via - at least I am not finding it.

      Can you mention the price and square footage of your place?

      Also - do you know much about the longevity of the material that is being sprayed?

  3. sommerbros | | #3

    We regularly use a membrane like Intello or Majrex as an interior AB at the ceiling with excellent results. A typical AB approach for us is to use our WRB as our primary AB at the exterior wall. We fold our WRB over our top plate and towards the interior to provide a connection point for our ceiling membrane. The key to simplifying this detail is to install a service cavity (we use 2x3) at the ceiling after the membrane has been installed and detailed. This allows us keep all of our electrical wires and boxes inside of our membrane and almost completely eliminates penetrations.

    1. Beardoh | | #6

      Thanks for that explanation. That makes a lot of sense, and your photos are quite helpful.

      2x3 usage on the ceiling solves another problem that I had been pondering. I wanted to use toungue and groove pine on some of the ceilings in this build, but with a membrane on the underside, it would not be possible to use a 'motivator' (decking plank straightener.)

      Couple questions:
      1-What is your WRB typically (I do not recognize the blue wrap on your top plate photo)
      2-If using T&G on a ceiling like you have shown, would you put anything between the T&G and the 2x3s?
      3-Are your 2x3s attached to the under side of the trusses on the 2 or 3" side?

      Thanks in advance

  4. walta100 | | #4

    Seems like you need to look at your plans and draw a red line that separates what is inside your thermal envelope and what is outside. Any location where you must lift the red pen off the paper is a break in your thermal envelope and a weak point in your design that needs work. So long as the soffits vents and ridge vents are both outside of the red line, they are not a problem.

    The fact that you are inclined to seal the attic make me think you are looking for the free lunch of an unvented unconditioned attic. That is a very risky high stakes game with mold and rot. You are betting that the dew point of the air in that attic will always be lower than the temp of every surface in the attic. Yes, some do win the free lunch but picking the winners will be is impossible.

    Hint if you avoid half story designs and cathedral ceiling it is a lot easier to draw the red line.


    1. Beardoh | | #5

      Thanks for your comments Walta. Actually, I do want to do a vented attic, but wanted to hear some feedback on the interior air barriers that are being used today.

      Your hint is well taken. While I do hope to have a cathedral ceiling, I'd like to build with trusses (raised heel), that will allow for a ceiling height above 8 feet, but not as high as the underside of a rafter if we were using traditional rafters. This will allow for more insulation, and allow for venting via soffit/ridge/gable.

    2. freyr_design | | #10

      Why is cathedral harder to draw redline vs vented attic?

  5. walta100 | | #11

    Everything about a cathedral ceiling is harder, risker and more expensive.
    Simply put you are trying to fit ten pounds of stuffing in a five pound bag.

    If the sloped ceiling is a must have please read this article and pick one of the options.


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