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Community and Q&A

Concerned about Water Vapor in new wall assembly

shereef | Posted in GBA Pro Help on

Hello folks, really appreciate this forum. 

We’re building a modular home at zomes.com. This is a CAD of our latest model: https://autode.sk/3VeAzdT

Our wall assembly is composed of the following layers from the EXTERIOR to the interior:

1. (outermost)  1/2 inch fiber cement board
2. 1/2 inch zip board structural pan
3. 1.5 inch continuous rigid insulation
4. 3 inch douglas fir frame with 3 inch rockwool comfort board in the insulation
5. 1/4 inch interior decorative panel

We are wanting to replace the 1/2 inch zip board with a 1/2 inch MgO structural panel. The concern being raised is that the MgO panel may not be breathable. And will therefore trap moisture from inside the building, which will condense from the inside of the board, and get trapped there. Creating mold on the framing. 

My limited understanding of vapor is that it won’t get trapped on the inside and will diffuse back into the building. 

Can anyone shed some light on this? 

Much appreciated!

Here are the relevant spec sheets:

Magnesium oxide panel: 
https://www.rockmaxboard.com/structural-mgo-sheathing/

Rigid insulation: https://drive.google.com/file/d/1ybYdAP32vcI5uoPPDu4bHvxh7lTuJFq4/view

Rockwool Comfortboard:
https://www.rockwool.com/syssiteassets/o2-rockwool/documentation/technical-data-sheets/residential/comfortboard-80-non-structural-sheathing-continuous-insulation-techdata.pdf

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    Moisture can get trapped anywhere, so inside vs outside doesn't make much difference. The main reason there is often more concern with inside or outside is that in typical cold climates, where there is a long heating season, mositure tends to get into the wall from the inside, and condense towards the outside where the temperature of the surfaces is colder. Condensation only happens when a surface is below the dew point of the moisture laden air, which tends to be inside of the exterior sheathing in many cases.

    In your case, you have that 1.5" layer of "rigid foam". That is going to be between R6 and R9, depending on what material you are using. 3" of mineral wool is going to be around R12-13 or so (I didn't look up the exact R value for comfortboard). That's a pretty good ratio of outside to inside R value, so the framing probably won't spend a lot of time, if any, below the dew point -- which means little risk of condensation. Note that this depends on your climate zone though, which I don't know, and the ".sk" implies Slovakia, which is outside of North America where most of us on GBA are, so I'm less familiar with your local climate. The same general rules of physics still apply though.

    1.5" of any type of rigid foam is not going to be very vapor open, either, so little if any moisture will make it through the MgO board. The rigid foam also won't really be "breathable", which means the difference between Zip and MgO here is probably not going to make any difference. If you have doubts, use thicker rigid foam, but note that with the structural panel on the exterior of the rigid foam and not against the framing, you will have shear issues with the fasteners. It looks like you're building a modified geodesic dome type of structure, which I have no experience with, but if this was a normal stud wall, I would generally advise to put the structural panel directly against the framing and the rigid foam on the exterior of the structural panel, which eliminates the shear issue with the fasteners.

    Bill

    1. shereef | | #4

      Thank you so much for your generous reply Bill.

      From your response, and others, it's clear structural needs to be right against the frame. Assuming we do that, we would add another thinner MGO panel on the outside. Essentially creating an mgo sandwich on both sides of the rigid board (although it's not glued). We would tape the seams of the exterior and then clad.

      Agreed, this means no moisture is leaving. However I am concerned about it getting trapped. But if I understand you correctly, if we did it this way, the sheathing layer would not get cold enough to get below due point?

  2. walta100 | | #2

    What climate zone???

    I think putting insulation between the sheeting and the structure is foolish at best and silly if there is any chance of hurricane, earth quake or tornado.

    Yes Zip+R is a code approved and they did a lot of tests to prove a building will not collapse if that is the goal. Your copy is not an approved assembly so your plans would require an engineers stamp to get approved. It seems unlikely you would be asking your question here if you had an engineer design this wall.

    I say move the foam to the outside build a stronger house or spend the money to buy Zip+R.

    Why rockwool when damp sprayed cellulose goes in better, faster, cheaper and is greener.

    Walta

    1. shereef | | #5

      Hi Walta, thanks for calling out my foolishness :) Couldn't agree with you more. There's a lot I don't know, which is why I'm asking. I appreciate you taking the time to respond.

      From your response, and others', it's clear structural needs to be right against the frame. Assuming we do that, we would add another thinner MGO panel on the outside. Essentially creating an mgo sandwich on both sides of the rigid board (although it's not glued). We would tape the seams of the exterior and then clad.

      We do have structural engineering, and MEP engineering support. I'm lost as to what kind of engineer would help with the questions I'm asking here regarding vapor. Can you point me to the right direction?

      If we more the structural as you suggested, do you see an issue with vapor?

  3. Expert Member
    MALCOLM TAYLOR | | #3

    shereef,

    You are asking about wall assemblies, but with a structure like that there are no parts of the skin which can be built as walls, they all need to be designed and built as roof assemblies. That affects not only the choice of materials for your control layers, but the cladding too.

    I'm really reluctant to give this sort of reply to someone coming here for advice, so please take it in the spirit it is intended. I think the chances of you being able to build a functioning dwelling from the model you have come up with is virtually nil. The extreme demands of both the structure and building envelope mean even a pre-fab company with expensive engineering resources would struggle with it. My advice would be to re-think whether it's worth pursuing a design which may be impossible to successfully bring to fruition.

    1. shereef | | #6

      Thank you for taking the time Malcolm.

      And I appreciate your honesty saying "I don't think this will work".

      It makes sense as you say to think of almost the whole assembly as a roof assembly. How would you layer it?

      We are confident in structure integrity. We've run finite element analysis, and structural engineers are giving us the green light.

      From your response, and others', it's clear structural needs to be right against the frame. Assuming we do that, we would add another thinner MGO panel on the outside. Essentially creating an mgo sandwich on both sides of the rigid board (although it's not glued). We would tape the seams of the exterior and then clad.

      If we move the structural as you suggested, do you see an issue with vapor?

      1. Expert Member
        MALCOLM TAYLOR | | #7

        Shereef,

        I simply don't have any useful advice on how to proceed. Part of what motivated by caution was that I couldn't think of what assembly would work, or how each facet would interact with those around it. Hopefully someone with more relevant experience will chime in. Good luck with your project.

  4. walta100 | | #8

    “We would add another thinner MGO panel on the outside. Essentially creating an mgo sandwich on both sides of the rigid board (although it's not glued). We would tape the seams of the exterior and then clad.”

    Sure, you could do 2 layers of sheeting but why?

    Seem to me you are trying to reinvent the wheel. All the needless sheeting costs money that is wasted buried in the walls serves almost no function adds zero value.

    If you need something to attach the siding to on top of the exterior insulation, find your local sawmill and buy ruff sawn 1x4 and install one every 16 inches that adds value as a rain screen allowing the back of the siding to dry for 10% of the cost.

    You may find this article interesting.

    https://www.greenbuildingadvisor.com/article/how-to-design-a-wall

    Walta

    1. shereef | | #9

      Thanks Walta.

      We need to tape for intrusion. That's why the last layer.

  5. [email protected] | | #10

    I think you need to hire a professional engineer. This project requires more than silicon valley bravado and fireside chat.

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