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Comparison of four wall systems

ClevelandOhio | Posted in General Questions on

Over the last few days i have been trying to understand the vapor dynamic in my proposed wall system. I do not want to accidentally use a vapor barrier where it is not necessary. Is using any XPS a good idea in my wall system if I go with rockwool as my main interior insulator? XPS perm is from 1.0-0.6 depending on the thickness (0.5-3″)

My #1 goal is to have a wall that can dry fast and efficiently to make sure that there is never mold or rot. This is why I left the original balloon cavity uninsulated — way back when I only read 3 books on this subject. Now I realize that I read the equivalent of 10 books, but my confidence level on making a good decision are lessened drastically.

I feel like SYSTEM D makes the most sense to me from the information that I cobbled together on here and elsewhere online.

Would you guys please help me analyze the dynamics and pros and cons of these 4 wall systems:

***SYSTEM A***
1) Vinyl siding
2) EPS 1/2″
3) Asbestos shingle
4) Wood lap siding
5) 2×4 uninsulated cavity (old baloon frame)
6) OSB
7)1/2″ XPS over entire OSB
8) 2×6 stud cavity filled with 6.5″ of rockwool
9) 3/4 gypsum board
10) Latex paint

VS.

***SYSTEM B***
1) Vinyl siding
2) EPS 1/2″
3) Asbestos shingle
4) Wood lap siding
5) 2×4 uninsulated cavity (old baloon frame)
6) OSB
7)1/2″ XPS over entire OSB
8) 2×6 stud cavity filled with
a. 1.5″ of XPS (on exterior side of the cavity; closer to OSB layer)
b. 4″ of rockwool
9) 3/4 gypsum board
10) Latex paint

VS.

***SYSTEM C***
1) Vinyl siding
2) EPS 1/2″
3) Asbestos shingle
4) Wood lap siding
5) 2×4 uninsulated cavity (old baloon frame)
6) OSB
7)1/2″ XPS over entire OSB
8) 2×6 stud cavity filled with
a. 4″ of rockwool (on exterior side of the cavity closer to OSB layer)
b. 1.5″ of XPS
9) 3/4 gypsum board
10) Latex paint

VS.

***SYSTEM D***
1) Vinyl siding
2) EPS 1/2″
3) Asbestos shingle
4) Wood lap siding
5) 2×4 uninsulated cavity (old baloon frame)
6) OSB
8) 2×6 stud cavity filled with 5.5″ of rockwool
9) 3/4 gypsum board
10) Latex paint

*1-8 are already built; i will be able to modify 1-5 in the future.

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Replies

  1. GBA Editor
    Brian Pontolilo | | #1

    Hi ClevelandOhio,

    I know that you gave a description of your wall assembly in another post, but I am not sure of a couple of details. Are you installing the OSB to the interior of the original 2x4 wall? Are you then planning to push the 2x6 wall up against the OSB (or the continuous rigid foam if you choose to go that route)?

    The way I see your situation is that you will ultimately have a double stud wall, when down the road, you take off the layers of siding and insulate the balloon framed 2x4 wall. Most of the high performance builders I've talked to about double stud wall prefer cellulose insulation because it has the capacity to buffer moisture. So, consider cellulose. Also, if you don't install continuous rigid foam, consider installing a smart vapor retarder on the interior.

    If you do install the continuous rigid foam, treat your 2x6 assembly like an exterior wall. Make sure you adequate R-value of rigid foam to keep its interior surface above the dew point, which is a ratio of rigid foam R-value to air permeable cavity insulation R-value. You can read about that here: Calculating the Minimum Thickness of Rigid Foam Sheathing. In this assembly, use a class III interior vapor retarder.

    1. ClevelandOhio | | #2

      Brian,

      Thank you for your reply. Here are the answers to your questions:

      >I know that you gave a description of your wall assembly in another post, but I am not sure of a couple of details. Are you installing the OSB to the interior of the original 2x4 wall?

      Yes, to the interior.

      >Are you then planning to push the 2x6 wall up against the OSB (or the continuous rigid foam if you choose to go that route)?

      The 2x6 wall is already built. It is not touching the osb -- there is about 3/4-1" clearance between the OSB and the 2x6 wall.

  2. ClevelandOhio | | #3

    I am looking for answers that help me analyze the performance of each of the wall systems.
    I have to pull the trigger buying on the insulation next week.

    If I do not understand these assemblies then I will default to System D because I understand it the best: the vapor from outside travels throught the OSB under certain humid conditions (above 80)goes through the rockwool goes through the drywall and paint. Less of an issue, but vapor from the interior, goes through the paint, drywall, and rockwool, and under certain conditions dryes to the outside.

    But what if it condenses on the interior of the OSB? Or exterior of dywall (It won't, right because it is warm during the rinter...but wait, due to the AC, in the summer the drywall will be cold -- so yes?) I do not understand if it is likely in my zone? Where will the liquid water go? Does it turn to vapor and go back trough OSB? After how long? Will my OSB become a soggy mess in time? Or will it be my drywall? It is also my understanding that my insulated cavity is being heated underneath by my radiant heat floor during the winter via conduction through the baseplate -- what does this change? And these are questions for the wall system (System D) that I understand the best... My head is spinning! Where do I go to find information like this? What else do I need to read?

    The problem is that osb acts as some type of smart vapor barrier in the middle of my assembly. And that causes me a lot of confusion.

    https://www.ecohome.net/sites/www.ecohabitation.com/files/imagecache/G12-Image-640/guide/page/vapour_permeability_of_sheathing_1.jpg

    I can't tear down the 2x6 wall and/or the OSB.

    I am as confused as ever, very frustrated, and have only few days to act on the non-celulose insulation purchase. I think everyone has their favorite wall system that they know very well and so they give such a system as a solution. I am stuck in a very rigid framework of what is already built and I hope a building scientist would read my cries for help and help me fix my wall assembly so it does not mold or rot in the future. This house stood here for over a hundred years and it would be shame that at this juncture, I make a wrong choice that causes it to rot or mold.

    1. Expert Member
      Michael Maines | | #5

      Cleveland, yes, with any of the systems you propose, in late winter you will likely get moisture accumulation at the osb sheathing. How long the osb remains damp depends on how fast it can dry, to either the interior or the exterior. If your existing 2x4 stud bays were open to air movement, the osb could dry quickly when exterior dewpoints are lower outside than inside. Alternatively, you could insulate the existing framing bays. That would likely keep the osb warm enough that condensation would not occur there; it could still occur closer to the exterior, but those materials are more forgiving of repeated wetting.

      When you are running the A/C the wall wants to dry to the interior, so the situation is reversed. If interior continuous insulation has enough R-value compared to the rest of the assembly, the outer face will be warm enough that condensation won't occur. But it's probably safer at the interior to use materials vapor-open enough to let the wall dry to the interior. I recommend a variable permeance interior membrane, such as Pro Clima Intello from foursevenfive.com--it slows moisture from getting into the wall from the interior, but opens up to allow drying to the interior if the wall gets damp.

      1. ClevelandOhio | | #8

        Thank you so much for a detailed reply!

        > in late winter you will likely get moisture accumulation at the osb sheathing.
        In the winter, things might be helped along a bit by the fact that my radiant heated floor will transfer heat to the 2x6 insulated cavity and thus the OSB. So if I understood you correctly, and you speak of the condensation on the interior of the OSB (Where would it have come from -- interior or exterior? ) It will be then (maybe?) turned back into vapor. Now things get confusing for me: if drywall perm is 50, but I use some sort of primer (1.2 perm) does it mean that the wall will not dry to the inside and is most likely just stay in the drywall and eventually leave out of the OSB?

        1. Expert Member
          Michael Maines | | #14

          >In the winter, things might be helped along a bit by the fact that my radiant heated floor will transfer heat to the 2x6 insulated cavity and thus the OSB.

          During winter, yes. But in late winter/early spring the walls may still be damp but you aren't using your heating system, so the drying force of heat slows down.

          >So if I understood you correctly, and you speak of the condensation on the interior of the OSB (Where would it have come from -- interior or exterior? )

          Water vapor always moves from warm to cold and moist to dry. So in the winter, the warm moist indoor air is trying to get outside. In summer, the warm, moist outdoor air is trying to get inside. In your climate, heating dominates, but I know that northern Ohio can get pretty steamy.

          >It will be then (maybe?) turned back into vapor.

          Yes, if it can dry to the exterior, the accumulated moisture will evaporate into the outdoor air.

          >Now things get confusing for me: if drywall perm is 50, but I use some sort of primer (1.2 perm) does it mean that the wall will not dry to the inside and is most likely just stay in the drywall and eventually leave out of the OSB?

          Regular latex paint qualifies as a class 3 vapor retarder, semi vapor permeable, so it slows water vapor movement but does not stop it. Intello, on the other hand, goes from a class 2 (semi vapor closed) to vapor-open, depending on moisture levels.

      2. ClevelandOhio | | #9

        > If your existing 2x4 stud bays were open to air movement, the osb could dry quickly when exterior dewpoints are lower outside than inside.

        I believe this to be the case because the exterior assembly is not airtight at all even withe the EPS. It is very much the opposite of airtight actually.

      3. ClevelandOhio | | #10

        > But it's probably safer at the interior to use materials vapor-open enough to let the wall dry to the interior. I recommend a variable permeance interior membrane, such as Pro Clima Intello from foursevenfive.com--it slows moisture from getting into the wall from the interior, but opens up to allow drying to the interior if the wall gets damp.

        Again I would like to ask the primer-on-drywall question.
        Also, what would happen if I do not use any vapor barrier?
        What will happen if there is no vapor barrier and there is primer on the drywall that is 1.2 perm?

  3. Expert Member
    Michael Maines | | #4

    Cleveland, are you in Cleveland, zone 5A? If not, what is your climate zone?

    Your wall assemblies are somewhat hard to analyze because of the exterior 1/2" EPS and not knowing how airtight or drafty the existing 2x4 cavities are. But it's probably safe to ignore everything outside of the osb when it comes to condensation control. 1/2" of XPS on the inside face of the OSB is not worth the effort or carbon footprint. I would use cellulose, which will fill the gaps between the studs and the osb, but if you're committed to mineral wool you can cut strips to tuck into that gap. If the gap was smaller you could ignore it, but it's just big enough that I believe convective looping could occur.

    Are you taping the osb to make it airtight, or using another airtightness method?

    At the interior, I would not use XPS in any case because of its blowing agents are potent global warming gasses, plus its R-value decreases over time to near-parity with EPS. I would consider cross-strapping with 2x2s and blow in additional cellulose for an equivalent R-value. (Search "Mooney Wall.") Or check out the Bonfiglioli approach.

    1. ClevelandOhio | | #11

      >Cleveland, are you in Cleveland, zone 5A? If not, what is your climate zone?

      I am not sure. Some maps say zone 6A -- other maps say differentlty. Zip code is 44119.

      1. Expert Member
        Michael Maines | | #15

        >I am not sure. Some maps say zone 6A -- other maps say differentlty. Zip code is 44119.

        You are showing a plant hardiness map. Cuyahoga County is considered zone 5A by the International Code Council. (https://www.greenbuildingadvisor.com/article/all-about-climate-zones)

      2. Expert Member
        Dana Dorsett | | #17

        DOE climate zones are not the same as and are completely unrelated to USDA plant hardiness zones.

        Only the DOE zones are relevant to the IRC prescriptives & building science matters.

        Cleveland is in DOE climate zone 5A.

    2. ClevelandOhio | | #12

      > Your wall assemblies are somewhat hard to analyze because of the exterior 1/2" EPS and not knowing how airtight or drafty the existing 2x4 cavities are.

      I believe this to be the case because the exterior assembly is not airtight at all even withe the EPS. I think it is very much the opposite of airtight actually.

      1. Expert Member
        Michael Maines | | #16

        >I believe this to be the case because the exterior assembly is not airtight at all even withe the EPS. I think it is very much the opposite of airtight actually.

        That's good, and it means that you can follow standard advice for your climate zone. I'd agree with everything Brian says below in response #7.

    3. ClevelandOhio | | #13

      Are you taping the osb to make it airtight, or using another airtightness method?

      I am taping the seams with aluminum foil tape and sprayfoaming (great stuff)around the perimiter of the wall. From what you said, it sounds like I might have to spray vetween wall stud and OSB as well (due to conductive looping).

  4. Jon R | | #6

    It's best to design your walls to have:

    a) good interior side air sealing and
    b) about 5x higher wet perms to the exterior than dry perms to the interior.

  5. GBA Editor
    Brian Pontolilo | | #7

    Hi again Cleveland Ohio.

    This is the second thread you have started on this wall assembly and you are still uncertain of how to proceed. If you can't determine how to proceed based on the thoughts of other GBA members and GBA articles, it seems like at this point you may want to consult a local professional. but I'll offer a few more thoughts.

    First of all, your concern with vapor drive is backwards and perhaps unnecessary. In a heating dominant climate there is a greater concern with outward vapor drive than inward--warm, wet, heated indoor air driving vapor to the exterior where it can condense on cold sheathing. That's why, in your climate, when a vapor retarder is required by the International Residential Codes (IRC), it is required on the interior of the wall. That's not to say interior vapor drive can't be a concern, particularly if you cool the house with A/C; it's just commonly less of a concern. Also, vapor drive is much less of a concern than air leakage when it comes to walls getting wet. Air seal the OSB sheathing, the 2x6 stud bays, and the drywall diligently, and you have reduced the risk of wetting by condensation greatly.

    In this odd assembly, you also essentially have vented siding, which is one of the IRCs stipulations for using a class III vapor retarder on the interior.

    A 1/2 inch of continuous rigid foam insulation seems like it may not be the best idea. It won't provide high enough R-value for it's interior face to stay warm, creating a possible condensing surface and it will slow outward drying, the same section of the IRC that allows for a class III interior vapor retarder when siding is vented would like the R-value of continuous exterior insulation to be 7.5. Though the IRC allows you to do one or the other, vent the siding or meet the required R-value for continuous insulation to use a class III interior vapor retarder, most high-performance builder are cautious and do both.

    Of your choices, D may be the safest option, but again if it were my project, I'd go with dense pack cellulose, which will fill the gap between the 2x6 wall and the OSB creating a thermal break, will buffer moisture, and is the most environmentally benign insulation option. And I'd use a smart vapor retarder on the inside. to slow outward vapor drive in the winter and allow the walls to dry in both directions in the summer.

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