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Community and Q&A

Condensation on Vapor Retarder Over Mineral Wool Insulation

JimmyCoro | Posted in Energy Efficiency and Durability on


noticed something odd today. Might not be odd, but I thought it was. I have a few bays of my exterior wall (above grade) that have mineral wool insulation in 2×6 wood framing. This differs from the rest of the bays that have dense-packed cellulose – the reason for this was that these few bays used to be where a window was located – but I filled it in since we are making some changes to that part of the basement. Didn’t want to mess around with cellulose for literally like 16 square feet of fill.

Anyhow, I noticed that these bays have some visible moisture on the back side of the VR (Certainteed Membrain) today where the neighboring cellulose bays do not. What is this all about? This was a recent change (filling in the window) so i am very curious if this is expected or if I should make a change prior to closing it up?

My wall assembly – vinly siding, Tyvek wrap, 7/16 OSB, cellulose / mineral wool, VR-Membrain.

Was a very mild day, about 70F, very low humidity in central WI.

Edit – that day about 7PM, the visible moisture disappeared. The next day was warmer and more humid outside (we ran a/c) and it appeared again during the day. Later that night, it disappeared again. Is this cycle something i should expect forever? Is it dangerous? Why mineral wool bays exhibit this behavior but not cellulose?

Thank you!

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  1. JimmyCoro | | #1

    Is the solution just very obvious, or did I manage to stump everyone? Thank you!

  2. Expert Member
    BILL WICHERS | | #2

    The reason the moisture got in there in the first place would be that conditions were right, which means the right combination of indoor and outdoor temperature and humidity levels to cause a bit of moisture to get into your particular wall assembly. To pin down exactly what the “right combination” of conditions is can be a bit tricky and would require a lot more info.

    The reason you only saw the moisture in the bays insulated with mineral wool is because mineral wool is hydrophobic and doesn’t absorb water. Cellulose has a moisture buffering effect, which makes it act a bit like a “water battery” — small amounts of moisture will be absorbed and held by the cellulose and released later. This effect would prevent the moisture from showing as water droplets on the interior side of the vapor retarder. Since mineral doesn’t have this moisture buffering effect, bays insulated with mineral wool WILL show the condensation.


    1. JimmyCoro | | #3

      Hello Bill, thanks for the reply.

      Since there is no difference between the bays other than the insulating material, I feel confident saying that overall, roughly the same amount of moisture is present in both bays. This was not due to a leak, it seems that this is just due to moisture in the air. One simply manifests itself as water droplets on the backside of the VR. Would you agree?

      So then:

      Is this something to worry about? If I had an entire house with MW instead of cellulose, I assume then that this would be happening in every bay? Homes must exist out there that have MW insulation in exterior walls, and they aren't doomed to rot and crumble?

      Safer to get those bays filled with Cellulose, or is that a waste of time?


  3. Expert Member
    BILL WICHERS | | #4

    If you’re seeing this happen during times you run your air conditioning, then the moisture is probably coming in from outdoors. This probably isn’t a huge problem as long as the moisture clears later in the day, but it’s probably indicative of air leaks. I would double check your air sealing.


  4. JimmyCoro | | #5

    The moisture appeared two days in a row, and has not happened since. Those two days were warmer ( weused air conditioning) and had some relatively elevated humidity (but not uncomfortable, don't recall the exact RH). Also, the wall in question is on the south side so it gets direct sunlight. Since those two days, temperatures and humidity have been lower (and it's been cloudy) and it has not happened again.

    Two additional details, not sure if it helps or not:

    1.) This wall area I filled in with mineral wool has house wrap that is the Kingsgard Rain Drop. Everywhere else is Tyvek. I taped the vertical seams where the two overlap. There are holes all over the house wrap due to staples, siding nails, etc. This isn't unique to any one area on the house.

    2.) The VR has *zero* holes in it (apart from staple holes). The region above grade has no receptacles in the wall or any similar penetrations. Again, this isn't unique to the area in question.

  5. Expert Member
    BILL WICHERS | | #6

    Chances are you had high enough outdoor humidity levels, and a long enough air conditioning run, to get that vapor retarder to drop below the dew point (which is what has to happen for condensation to form). Wall areas with cellulose buffered the mositure levels in the wall, so you didn't see condensation in those areas.

    It's possible that enough moisture made it through the house wrap (which is intended to be vapor open) that you had enough mositure in the wall for some condensation to form, but I'd suspect air leaks. It doesn't take much air leakage to allow quite a bit of moisture through. This is one of the reasons air sealing walls is so important for the longevity of the walls.

    The good news is that if the conditions you had when this condensation occured are fairly unusual, and the wall dried out later in the day anyway, then your wall is probably still safe and you don't really have anything to worry about. If the moisture built up over time, getting worse and worse instead of clearing up, THEN you would have a problem you'd need to worry about.


  6. Jon_R | | #7

    The sun heating a wall with accumulated water will drive the dew point inside the wall very high. But given dew points in your area, it's unlikely that outside air was the cause of this accumulated water (compare weather reports of max daily dew point to interior temperature).

    +1 on mineral wool making the effect much more obvious.

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