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Vapor Barrier

nkhandyman | Posted in General Discussion on June 10, 2014 05:00am

Would someone direct me to the ICC section that discusses when an air (vapor) barrier is required.

Helping to build a home for Habitat for Humanity.  OC foam in the walls; blown in celluose in the attic.  Local BI told us that a plastic air barrier is needed over the foamed walls prior to drywall; and on the ceiling prior to drywall.

Have built last 4 houses the same way and no plastic was needed.  Thank you.

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  1. DanH | Jun 10, 2014 06:44pm | #1

    Where are you located?

  2. User avater
    deadnuts | Jun 11, 2014 12:20pm | #2

    if you mean the IECC

    then this may be what you are looking for:

    Table R402.4.1.1 (2012 IECC).

    Air Barrier and Insulation Installation*

     

    Also, this link may provide you with the detailed info you are looking for:

    http://www.energycodes.gov/sites/default/files/documents/BECP_Buidling%20Energy%20Code%20Resource%20Guide%20Air%20Leakage%20Guide_Sept2011_v00_lores.pdf

  3. User avater
    Mongo | Jun 13, 2014 01:52am | #3

    I don't have a reference for ICC, the best I can offer is IRC. From the chart your requirements are dictated by your climate zone. The spray foam note at the end doesn't apply to open-cell foam due to the density.

    It should be understood that open cell foam is an air barrier. But without adequare drying cycles moisture vapor can pass into it and the open cell foam can accumulate and hold moisture to the point where it can saturate like a sponge.

    Example: I live in Zone 6. I'd need Class I (6-mil poly) or Class II (kraft paper FG batts) vapor retarder because of 702.7. I can't go with a Class III retarder with open cell foam because open cell doesn't meet the density requirements of "Note A" in 702.7.1.

    If I was using an adequate thickness of closed-cell instead of open cell in the framing bays, then I could use a Class III (latex or enamel wall paint) retarder on the interior walls.

    The only way I could use open cell foam in the framing bays with a Class III vapor retarder on the interior wall was if I used  vented siding/cladding OR if I use an adequate thickness of rigid foam sheathing on the outside of the wall.

    Here's IRC:

    R702.7 Vapor retarders.

    Class I or II vapor retarders are required on the interior side of frame walls in Climate Zones 5, 6, 7, 8 and Marine 4.

    Exceptions:

    1. Basement walls.

    2. Below grade portion of any wall.

    3. Construction where moisture or its freezing will not damage the materials.

    R702.7.1 Class III vapor retarders.

    Class III vapor retarders shall be permitted where any one of the conditions in Table R702.7.1 is met.

    TABLE R702.7.1 CLASS III VAPOR RETARDERS

     

    CLIMATE ZONE

    CLASS III VAPOR RETARDERS PERMITTED FOR:a

    Marine

    4

    Vented cladding over wood structural panels.

    Vented cladding over fiberboard.

    Vented cladding over gypsum.

    Insulated sheathing with R-value ≥ 2.5 over 2 × 4 wall.

    Insulated sheathing with R-value ≥ 3.75 over 2 × 6 wall.

    5

    Vented cladding over wood structural panels.

    Vented cladding over fiberboard.

    Vented cladding over gypsum.

    Insulated sheathing with R-value ≥ 5 over 2 × 4 wall.

    Insulated sheathing with R-value ≥ 7.5 over 2 × 6 wall.

    6

    Vented cladding over fiberboard.

    Vented cladding over gypsum.

    Insulated sheathing with R-value ≥ 7.5 over 2 × 4 wall.

    Insulated sheathing with R-value ≥ 11.25 over 2 × 6 wall.

    7 and 8

    Insulated sheathing with R-value ≥ 10 over 2 × 4 wall.

    Insulated sheathing with R-value ≥ 15 over 2 × 6 wall.

    For SI: 1 pound per cubic foot = 16 kg/m3.  

    Note A. Spray foam with a minimum density of 2 lb/ft3 applied to the interior cavity side of wood structural panels, fiberboard, insulating sheathing or gypsum is deemed to meet the insulating sheathing requirement where the spray foam R-value meets or exceeds the specified insulating sheathing R-value.

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