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Any Field Test or Hygrothermal Modeling Results to Support the IRC 2021 Class I Variable Vapor Retarder Requirement?

Gary601 | Posted in General Questions on

As Martin said in his GBA article “Building Codes Update Vapor Retarder Requirements”, the code has gotten a lot more complicated.  Hope I don’t screw this question up!

 For years we had BSD 106 Side Bar 2 permitting a class II VR in CZ5 for walls with exterior sheathing/cladding having a minimum water vapor permeance of 0.1 perm.  To the best of my interpretation the 2021 IRC, not yet in effect here in SE Michigan, requires a class I adaptable (smart) VR for this same CZ and sheathing/cladding permeance based on ABTG recommendations.  But none of the available smart VRs including Membrain (<1 to 10 perm), Intello (0.17 to 13 perm) or DB+ (0.8 to 13 perm), strictly meet the Class I requirement of less than 0.1 perm.  However, assuming ABTG is referring to these available membranes, here’s my question.

 Are there any field test or hygrothermal modeling results showing the advantages (say, a reduction in sheathing moisture content or RH within the wall) of these newer smart retarders over the older kraft-faced batt (0.3 to 3 perms) for this CZ and sheathing/cladding minimum vapor permeance?  Not looking for just a comparison graph of permeance vs RH for these VRs, but actual instrumented test or modeling results.

 I’ve searched the ABTG website but can’t find these data.  Perhaps they are considered proprietary.  The two ABTG reports referenced below have a couple of wall calculators but are devoid of any comparative test or modeling results.

 My project has 2×6 CZ5 walls sheathed with OSB/Tyvek, exterior and interior air barriers, drying to a ventilated brick gap.  They are completed on the exterior (unfortunately, no opportunity for continuous exterior insulation) and ready to insulate.  Although old school vapor variable kraft faced fiberglass batts have been used for many years in this region, if there are test or modeling results to show a significant improvement in wall performance with these newer smarter VRs, I would consider installing one on this build.  

 What are your thoughts GBA members? Thank you.


BSD 106:

ABTG Report:

ABTG Report:

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  1. Expert Member
    Akos | | #1

    Might not directly answer you question but could be interesting to set it up here:

    Once you get the assembly in there, it is simple to adjust the warm side VR and see what happens in the moisture content of the assembly.

    Overall, I think a class II variable perm assembly is your best option. Also moisture issues in walls are usually caused by air leak or flashing details, I would focus on these and not worry too much about your VR perm rating. Get the wall air tight and your flashing correct and the wall will last no matter what your VR is.

  2. maine_tyler | | #2

    >"To the best of my interpretation the 2021 IRC, not yet in effect here in SE Michigan, requires a class I adaptable (smart) VR for this same CZ and sheathing/cladding permeance based on ABTG recommendations."

    I think you need to provide more specifics. It depends on exterior insulation thickness. Are you asking about a situation where the exterior cladding is a class 1 VB and is also not in accordance with table 702.7(3) OR table 702.7(4)?

    If referring to table 702.7.(4), note that it calls for an interior VR to have a perm rating "greater than 1 perm when measured by ASTM E96 Water Method (Procedure B)."

    I don't see where it states the VR must have a perm rating below 0.1 perm.

  3. Gary601 | | #3

    Akos, thank you for your reply.

    Definitely will pay attention to getting the double air barrier correct. Stud cavities are picture frame sealed to the OSB and use DeNarco drywall gaskets under the drywall. Personally flashed all windows, doors and OSB penetrations. Thanks for the link to the calculator. Will give it a go after brushing up on my Sd to perm conversions. What about VR drywall primers for this application? Do they have variable permeance in your experience? I thought the primers had a fixed permeance between 0.5 and 0.9 perms. Martin mentions vapor retarder paints as having variable vapor permeance in his article, Smart Vapor Retarders for Walls and Roofs. But he may be making a distinction between the paints and primers.

  4. Gary601 | | #4

    Tyler, thank you for your reply.

    I approximated the water vapor permeance of the subject OSB/Tyvek/ventilated brick cladding walls with no exterior continuous insulation as 1/(1/2+1/50+1/40) = 1.83 perms. Per ABTG Report 1701-01, Figure 3(a) I selected Table 2(A), for walls with vapor permeable cavity insulation only. Using Table 2(A) and selecting CZ5 with water vapor permeance of 1.83, these walls seemed to fit in the Class I column.

    I also used the ABTG wall calculator (link below) which seemed to confirm the Class I VR requirement and with the following qualifying note 2:

    Note 2, Class I: 0.1 perm or less. Use of a Class I "smart" vapor retarder is also recommended for use on walls relying on compliance OPTION 2 (net permeance of exterior material layers) where permitted and used.

    ABTG wall calculator:

    1. maine_tyler | | #5

      I'll have to dig into the ABTG stuff a little bit because I'm not that familiar with it. That said, I'm confused: you are asking about the 2021 IRC are you not? Why not reference the 2021 IRC?

      I don't see where your interpretation of the 2021 IRC is coming from...

  5. Gary601 | | #6

    Yes, I may have assumed that ABTG was the source of the 2021 IRC VR recommended changes and that all the ABTG recommendations were adopted by the 2021 IRC, which as you suggest is probably not the case. Thanks for the awakening. I was really only interested in any modeling or test data that might be available for the use of a Class I smart vapor retarder for my CZ5 application which I wrongly assumed had become an 2021 IRC requirement. No need to spend more time on it for my sake. Thanks for straightening me out.

    1. maine_tyler | | #7

      I did peak at the ABTG report, and it is interesting that they appear to be suggesting a class 1 with osb in CZ5 like you say. I didn't see reference to 'smart' in that particular report, but it is funny that the calculator would specify 'smart' class 1 as I am also unaware of any.

      I would follow the IRC or BSI instead. Class 1 in climate zone 5 seems a little heavy handed to me, especially if there is no 'smart' class 1 available.

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