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Tyvek as an *interior* air barrier

Kyle Bentley | Posted in Energy Efficiency and Durability on

I’ve seen a lot of reference to membrain, intello, and other interior air barriers.

These have dynamic permeability values, but are essentially vapor barriers at low RH values, and not vapor barriers at large RH values.  Membrain says to installation is ok  in climates zones 4C and above, and intello I’m sure is similar.

For climate zone 3, mixed humid, or 3A, an interior air barrier has not traditionally been encouraged.  However, the need for an air tight wall assembly is at the same time desired by everyone.  This has lead to things like “air tight drywall” and other concepts. 

I was thinking about Tyvek, and it’s use as an interior air barrier.  It is more permeable than any other wall assembly component, even more so than 2″ of mineral wool.  It will not stop vapor from moving through an assembly.  This is a feature of the material, and it would make it unsuitable for cold climate interior vapor barriers.

I called Tyvek’s tech support line, and asked if this was a typical use.  The spokesperson confirmed it can be used in this way, and provided a link to a detail showing it being installed on the interior faces of a stud wall (link below). 

For warmer climates, this provides a much cheaper alternative to the smart vapor retarders, and reduces the burden for ensuring a good air seal for a wall assembly, as a backup to the exterior air barrier.  There’s no sealant on drywall, slowing that process,  and it can be done with materials that are often left over from other parts of build.

Question:  Has anyone used tyvek this way in practice, and can you comment on how the job went?  To me it seems like a no-brainier, considering the ease of installation, cost effectiveness, and greatly reduced air exfiltration(?) from the interior to the stud bays.  This is more of a discussion than a question I suppose.  Just wanting some thoughts.

Assume for the sake of the discussion a standard wall assembly of

1/2″ drywall
(tyvek interior air barrier)
2×6 studs
Fiberglass/mineral wool/cellulose (no sprayed foam, but vapor permeable insulation)
7/16 osb sheathing (taped seams)
tyvek,
2″ comfortboard / XPS
3/4″ furring strips
James Hardie / LP Smart Side siding

https://www.dupont.com/content/dam/dupont/amer/us/en/performance-building-solutions/public/documents/en/interior-air-barrier-applications-install-bulletin-43-d101033-enna.pdf

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Replies

  1. Earl Bates | | #1

    Kyle,

    I have the same questions on Tyvek as interior air barrier. Maybe someone (GBA) smarter than me can chime in here.

    As long as the air movement in walls is negated, and if the perm of ½” drywall is around 50, and Tyvek perm is around 58, then why not use TYVEK for an interior air barrier?

    I too called Tyvek 3 times last few years and got no rock solid answer – I saw the bulletin you linked, but it has no guidance on when to use Tyvek on the interior.

    I have a historic project with 3 types of wall assemblies. Getting a continuous air barrier in this project is very difficult. The entire interior cladding is a combo of tongue and groove or shiplap, with NO drywall. So, that in itself is a porous assembly, calling for an air barrier on the inside to prevent air infiltration into the wall cavities.

    In another post on here, they recommend the Membrain, Intello or drywall under the wood clad interior with no mention of Tyvek or others. I am Central Texas (CZ 2/3), so Membrain is not a choice. Intello is real expensive. Agreed, Tyvek (or similar?) is simple and cost effective.

    Proposed Wall Assemblies:

    WALL A: Old growth Pine 2x4 studs with:
    -- ORIGINAL Pine Wood Horiz. Lap Siding directly on Studs
    -- Insert Tyvek to the inside of the outside wall and seal in stud cavity
    -- Rockwool Ins.
    -- Tyvek Interior Air Barrier over studs - Sealed
    -- Horiz. Wood Clad Interior ¾” T&G

    NOTE: The original wood exterior is not removed for preservation purposes – the insulation and Tyvek are retrofitted from the inside of the building. This home previously had drywall over the inside wood for 30 years with no ill effects.

    WALL B: 2X4 Studs with:
    -- Reproduction Horiz. Wood Siding
    -- 1/8” Shimmed Airspace
    -- 30LB Felt ATSM226D
    -- 1/2” Plywood
    -- Rockwool Ins
    -- Tyvek Interior Air Barrier
    -- Wood Clad Interior – 3/4” Pine T&G

    WALL C: 2x6 Studs with:
    -- Wood Board and Batten Siding
    -- 1” Airspace
    -- 30lb felt ATSM 226D
    -- 7/16” OSB
    -- Rockwool
    -- Tyvek Int Air Barrier
    -- Interior Wood Clad ¾” Shiplap

    Would these assemblies not have the same effect as if I had used drywall in place of the Tyvek?

    Earl

  2. Expert Member
    Malcolm Taylor | | #2

    Kyle and Earl,

    I don't see why not. My two reservations would be:

    - The air-barrier function of interior membranes is usually very secondary to their use as variable perm vapour-retarders. If you aren't getting any vapour-retarding benefit, is it worth using the Tyvek just as a secondary air-barrier, when except at the edges, the drywall makes it redundant?

    - Being opaque it's bit harder to drywall. You don't see studs, pipes, and wires while boarding.

    1. Kyle Bentley | | #3

      I haven't used it for that purpose to date,. But I imagine it being more useful in scenarios where drywall isn't being used, ie shiplap accent walls, etc.

      Otherwise, I can also imagine that having a bunch left over from a job, but not enough to worry about taking home. Just staple it to the studs as a back up air barrier.

      I hadn't thought about nailing troubles, I thought you'd be able to feel it, start one nail, and then send it up or down with the screw gun.

      Earl,

      As far as when to use it, I don't think there's a set answer. I think the only thing you can say is that it will not retard vapor, and will allow vapor movement between the other wall components. If you try it and have good success with the install, please report back about it.

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