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Rigid insulation on exterior walls with high interior humidity

user-1136771 | Posted in Green Building Techniques on

I built an addition on my house that includes a solar thermal array on the roof that heats an endless pool in the addition. The existing house is brick, so when i poured the foundation (foam blocks), i included a 4″ brickledge. i have not finished the exterior yet, and am rethinking my options. I am in zone 6b. The construction is 1/2″ sheetrock over conventional (not staggered) 2×6 studs with faced fiberglass bats in the stud cavities. exterior sheathing is 7/16″ osb. finally, tyvek house wrap. i am considering using 4″ xps with some stucco type material (not researched this yet) over the foam. I am concerned about the high humidity from the pool. it is usually covered, but when it is open, the humidity in the room is much higher. the room is virtually a greenhouse with plants so the plants love the humidity. I am considering putting 4″ of xps foam over the tyvek house wrap but am concerned about condensation on the sheathing. From what i have read, the 4″ of xps foam should always keep the sheathing above the dewpoint, even in the coldest (-30f) of nights. can you give me some advice on this idea? I also am not sure how i am going to attach the two 2″ xps sheets to the sheathing. I am considering notching the sheet to “inlay” a 1×2, then screw that to the wall. I am concerned about the weight of any covering on the exterior of the foam. thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    Rooms with indoor swimming pools fall into a special category. Lawyers who handle building defect litigation and building science consultants know that the walls and roofs of these rooms often rot and the designers and builders of this type of room often ends up in court. So your details need to be perfect.

    I would never include OSB sheathing in the wall of such a room. Nor would I depend on fiberglass batts for insulation.

    Ideally, you need a near-perfect air barrier. The best wall for such rooms (according to building scientist Joe Lstiburek) is a PERSIST-type wall with all of the insulation on the outside of the building structure. For more on PERSIST methods, see Getting Insulation Out of Your Walls and Ceilings. I would insist on the use of peel-and-stick membrane on the exterior of the plywood wall sheathing if I were building such a room, and I would make sure that all of the insulation (several layers of rigid foam) were on the exterior side of the peel-and-stick.

    Don't forget to have an airtight detail to handle the intersection of your walls and your ceiling. Good luck.

  2. wjrobinson | | #2

    Remove the OSB and any paper faced drywall. Install paperless drywall, closed cell sprayfoam. Or get rebuild details from an experienced engineer.

    Also, but too late for you, I would build the frame entirely out of pressure treated materials.

  3. user-1136771 | | #3

    well, thats not the answer i was hoping for. i guess i still have option to fix some of the problem because i only have the tyvek up on the exterior. The links you provided, and the links in those articles are invaluable but i wish i had known about this method before i put the roof on. however, they do go into REMOTE retrofit on remodels, which I will have to do the best i can. One thing i have a question on is your comment on osb for sheathing. Is that a personal preference for you or are there other reasons? the REMOTE link says either is ok. I am also wondering about the use of tyvek drain wrap vs a peel and stick membrane. the REMOTE manual shows the use of both. regarding the ceiling, which is already finished, do you think it would be advisable to cover the finished drywall with a membrane, and then attach another layer of sheetrock on the finished side of the room? Right now, there is no membrane, only faced fiberglass batts. although i would not have an airtight seal at the intersection of the walls, it seems this would be better than none at all. finally, it looks like another approach to the moisture issue is a dehumidifier or forced ventilation during the times the pool is open and the humidity high. any info you can direct me towards for this concept is appreciated. thanks for all your help.

  4. GBA Editor
    Martin Holladay | | #4

    If you have already installed drywall on the ceiling, and if you know that the connection between your walls and ceiling is not airtight, you are in trouble. I suggest that you hire a consultant from the Building Science Corporation in Massachusetts. The money you spend on consulting will be less than the cost of rebuilding your house if you have problems with moisture and rot.

  5. user-1136771 | | #5

    Here is a followup to this post, 2 and a half years later. I did open up all the exterior walls (from the outside) and pulled out all the fiberglass batt insulation. Just a note, there was no evidence of damage from humidity in the wall cavities at all. I then used drainwrap as recommended in the REMOTE manual to sheath the outside of the OSB. (Removed Tyvek first) I attached 4" of foam insulation and covered it with a standard EIS finish. Finally, i bought a small commercial dehumidifier (HI E DRY 100). I have no humidity showing as frost in the attic. The windows in this addition show no more condensation than any other windows in the house.,

  6. GBA Editor
    Martin Holladay | | #6

    Thanks for the followup report. I'm glad to hear that things are going well. It sounds like you took the right approach.

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