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Community and Q&A

Frost buildup in wall cavity in unfinished house

Dpcvt | Posted in Energy Efficiency and Durability on

Happy New year. My wife and I are totally rebuilding our house, starting from the foundation up. We are living in it while completing interior work. We live at 1600 feet in central

The walls are 2×6 with zip system sheathing with all joints carefully taped. The house feels very airtight. R23 roxul batts. Most of the insulation is currently exposed. I’ve periodically pulled the insulation back to check for moisture and never found any. It’s been unusually cold this week. Today it is -11F outside, 60 inside with an interior humidity of 35%. I pulled some insulation back and found frost behind every place I checked. Not just near possible leaks, but everywhere. It is a consistent 1/8″ covering, and not melting or dripping out anywhere. The roxul fits very tightly, and this surprised me.

I’m assuming having tight sheetrock would help with this, but I’m not sure if this indicates a problem. I do have sheetrock in the kitchen and bathroom, but no way to check those cavities. All of the exterior receptacles are in the draft proof boxes.

I wasn’t planning on a vapor barrier, and I still don’t think I want one. My thinking was to allow any moisture that might build up in the walls a way to escape.

I’m just not sure if I need to rethink my design or Not. I’ve never lived in a house with exposed insulation, so I have nothing to compare to.

Do you guys think this is normal for my situation or indicative of a problem?

Thank you for any thoughts on the matter.

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  1. GBA Editor
  2. Dpcvt | | #2

    Thank you for the links. It sounds like I will have to wait to sheetrock until the weather brakes and the bays can dry out. Not a huge problem since I still have much rough-in to complete.

  3. Dpcvt | | #3

    I have a follow up question:
    I realized after pouring over the articles above (and several more) that I need to rethink my rim joist insulation strategy.

    In the basement I plan to use spray foam since all of my penetrations are through the rim joists and old basement window openings. I think spray foam will nicely seal those, and seal the sill down to the concrete foundation.

    Between the first and second floor, I currently have roxul batts. Those also have condensation behind them (no drywall up in most of the house).

    It sounds like I would be better off pulling those roxul squares down and replacing them with 2" foam board sealed with foam to the joists, plates and floors. Not a difficult task since nearly everything is still accessible.

    I am wondering If I should put those scraps of Roxul back into the joist bays on the interior side of the foam board, or just leave them out all together.

  4. GBA Editor
    Martin Holladay | | #4

    1. Can you tell us your name?

    2. What is the purpose of installing insulation between your first floor and your second floor? Is the insulation there to reduce sound transmission?

  5. Dpcvt | | #5

    My name is Dana.

    Sorry, I didn't explain clearly. I was inquiring about insulating the rim joist bays at the edge of the second floor joists. I do not plan on insulating between the floors themselves.

    In the attached photo (not mine, taken from the internet), spray foam is being used in this cavity. I currently have roxul bats cuts to fit here. I am contemplating pulling the roxul and using 2" foam board here with the roxul (possibly) re-installed on the inside surface of the foam board.

  6. GBA Editor
    Martin Holladay | | #6

    You have condensation there now because of the lack of drywall. Condensation is much less likely after the drywall is up. That said, using either rigid foam or spray foam at rim joists is always the preferred method. Once you have installed either rigid foam or spray foam, you can re-install the mineral wool.

    More information here: Insulating rim joists.

  7. Dpcvt | | #7

    Thank you for the reassurance Martin.

    One more question: Do you think I need to wait until the weather warms back up before I install sheetrock to allow all of the walls to fully dry? They are not dripping wet, but have about 1/8" of frost evenly distributed.

    I assume moisture will be able to diffuse our through the zip system sheathing and inward through the drywall, but I am not sure how long that might take.

    I'm not sure if starting sheetrock while it is still cold is a bad idea or not.

    If nothing else, this has been a really good hands-on lesson on moisture and air movement.

  8. Expert Member
    Dana Dorsett | | #8

    A 2-pronged moisture content meter (even a cheapie $30 version from a box store) would tell you when the wood is dry enough. Well under 20% m.c, something like 10% or less would be ideal, but sub-15% would still be good enough.

  9. Dpcvt | | #9

    I have a moisture meter that I used to use to monitor the logs in the previous house that stood here. I assume I will find over 20% since the sheathing becomes damp to the touch when the insulation is pulled back and the frost is allowed to melt.

    My question is: If I just put sheetrock up over the wall as it stands, how long might it take for the frost to dry once warm weather comes and warms the sheathing up? The only other option I see is to hold off on sheetrock for several more months and wait for the cavities to be verifiablly dry before installing sheetrock.

    This might be obvious to those with more experience, but I am fairly novice. I honestly am not sure if the wall will dry fairly quickly and be no problem, or if I would be making a terrible mistake and guaranteeing mold growth.

    To review, the walls are 2x6 studs, R23 Roxul in between the bays, zip system sheathing with taped seams and fiber cement clapboards. I plan to install drywall using the Airtight drywall approach.

  10. Expert Member
    Dana Dorsett | | #10

    If insulated it'll have to wait for warmer weather. If it's uninsulated, heated, and open to the interior it will dry much faster.

  11. Dpcvt | | #11

    We are now in the middle of a major January thaw in VT. As expected, all the frost behind my insulation has melted. A couple cavities have dripped small pools of water out onto the floor. I have the heat pump water heater and dehumidifier running to help remove some of this moisture.

    If I pull any Roxul out of a bay, it is predictably wet back there. Most are just moist, a few are actually wet. If I pull a batt out, things dry out quickly.

    Currently my plan is to just keep on as I am with the bays being frosty until spring and getting wet once every couple weeks when the temps come up outside.

    It is very disheartening to see my new walls wet, and I do worry about compromising the sheathing because of this frost and moisture.

    Do I need to change my approach, and pull insulation out of all the cavities to expedite the drying so I do not damage the Zip sheathing? It does seem like it will dry once the temps stay above freezing, but that will be several months from now.

  12. GBA Editor
    Martin Holladay | | #12

    I don't have much new information to add, except to inform other GBA readers that there's a reason why I advocate the use of continuous rigid insulation on the exterior side of wall sheathing. Such a layer of continuous insulation keeps sheathing warm and dry during the winter. Builders who encounter frost and moisture on their sheathing when building during the winter are able to have a direct experience of what happens when moist interior air contacts cold sheathing.

    Conservative builders will probably pull out all the insulation and wait until warm weather to continue work.

    Builders with a higher tolerance for risk will soldier on, banking on the fact that the damp sheathing will probably dry to the exterior eventually.

    If you want to walk down the middle of the aisle, you should know that south-facing sheathing dries faster in the springtime than north-facing sheathing.

  13. Dpcvt | | #13

    Thank you Martin. I really appreciate the time to respond, and have learned much from reading through your articles and forum posts.

    I do not have the opportunity to add foamboard at the exterior at this point. The building is sided. This has been a great lesson for a novice such as myself.

    I am not sure I have the option of pulling the insulation right now, since I live here with my wife, and the cost to heat may become prohibitive. Although, that just may be something I have to deal with.

    I am hoping that I can get my sheathing dry by leaving the sheetrock off and allowing the frost to melt and dry once spring arrives. I am no longer considering installing sheetrock until the walls are dry.

    My only remaining concern is: How dangerous is allowing this pattern of frost and melt to continue for the rest of the winter. I don't have a good feel for how bad this is for the Zip Sheathing.

  14. GBA Editor
    Martin Holladay | | #14

    One winter's worth of condensation or frost won't hurt your Zip sheathing or lumber, as long as everything dries out by May or June, and as long as everything is buttoned up before next winter.

  15. Dpcvt | | #15

    Thank you. It will certainly be buttoned up before next winter. What a treat that will be.

    We had a log house standing on this spot up until May 1st this year. Its been a long road with both working full time jobs and working on the house!

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