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Wall moisture question for Phili area

tXWMcUDnxx | Posted in PassivHaus on

We are building a passive house in the Philadelphia area and had a wall moisture question. Phili is a very humid climate zone 4 for those of you that don’t know the area.

We are building the home with 2X6 studs. We are going to use sheetrock on the inside, 5.5” of dense pack cellulose, low perm gyp board, 7” Larson trusts on the outside, 7” cellulose, plywood on the outside of the trusts, home slicker, and then vinyl siding. Anyone have any concerns regarding moisture in this wall assembly? If you do what would you do to resolve the problem?

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  1. Riversong | | #1

    I have a concern about the excessive materials, when you could get the same R-value much more easily and resource-efficiently with a single double-walled cavity. You're using enough framing material and sheathing to build two houses.

    And I have a concern about the use of vinyl, which is not only one of the least durable of all siding materials but has major health and environmental impacts in all parts of its life cycle.

    Moisture isn't the issue. It's ecological footprint and environmental responsibility.

    And, by the way, that's Larsen trusses (not Larson trusts).

  2. J Chesnut | | #2

    What are you defining as your air barrier?
    To avoid wetting issues with infiltration/exfiltration an continuous contiguous air barrier is required for the entire thermal envelope.
    Your proposed wall doesn't necessarily raise any red flags for trapping moisture without allowing drying potetional. However the most critical details include defining and executing the air barrier and the related window details.
    I'm having difficulty imagining how the Larsen trusses attach to the 2x6 studs without sheathing between the studs and the Larsen truss. Have you run this past a structural engineer?
    Lastly what do you mean by "low perm gyp board"? Do you mean gyp board with a coating of low perm latex paint?

  3. Chad | | #3

    This is similar to the 11" thick double stud walls we are using in Philly filled with dense cellulose. The great thing about cellulose is that it has an amazing ability to take on moisture and then dry out over time.

    Some get nervous with this much cellulose. In that case, you could put R-10/15 of rigid insulation on the outside to handle the due point from entering your wall in this area. Overkill, expensive and high embodied energy if you ask me.

    I'll second the critique of vinyl. It's hard for me to imagine any super-insulated and sustainable home with vinyl on the outside. Also, vinyl is it's own rainscreen, so it would be redundant to use homeslicker under it. Please no vinyl. Fiber cement is a much better option on a budget.

  4. homedesign | | #4

    Of Course if you Invest your Money in A Larsen Trust then you would not have to worry about Due points.

  5. Riversong | | #5


    I think you mean a Larson Trust, which is derived from larcenous.

  6. tXWMcUDnxx | | #6

    The air barrier would be the OSB and I think the perm rating of the OSB is .95 or .86 for the low perm OSB. My goal was to try and limit moisture movement through the wall, but give it the ability to do so if needed. If needed I can use a low perm paint to seal the OSB, but I have some reservations about doing so.

    We are buying the Zehnder HRV. We are also getting the ERV cartridge so we can play with both. My goal would be to try and keep the interior relative humidity down in the winter (40%) and switch to the ERV cartridge in the summer.

    We were planning on going with vinyl to keep the costs down. We had looked at hardy plank, but the material costs are double that of vinyl, it takes much longer to install, and the labor triples over vinyl. Once we have all of our numbers we will see if there is money left. Thank you for the advice! If the money is there we plan on switching.

    Thank all of you for your help. That is the ones that refrained from making insults (Robert). It’s much easier to win people over when you don’t insult them and you back your points up with additional thoughts and materials. If you want to insult someone why don’t you pick one of the conventional builders that don’t care about making the world a better place.

  7. Riversong | | #7


    My comments were factual and accurate. The wall section you're proposing will do nothing to "make the world a better place".

    I suggest you put your injured ego aside and take an objective look at the issues. Materials, like vinyl siding, appear "cheap" only because they don't factor in the human health impacts and environmental destruction that they leave in their wake. They have a low first cost, perhaps, but will cost more in the long run when replacement becomes necessary.

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