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Community and Q&A

Ceiling Vapor Barrier Condensation

rickymn | Posted in General Questions on

I’m remodeling a house built in the 70s with a flat roof that has minimal ventilation in some of the eaves. While removing the ceiling drywall I decided to also take out the old fiberglass R19 and replace with mineral wool R30. I had to do this during a heat wave of 95° for several days. My wife came over to check on the progress and insisted we turn on the AC since it was also about 95° inside the home. When I went back to finish installing the vapor barrier there was condensation everywhere on the inside of the barrier that had already been installed and also on the exposed mineral wool that hadn’t yet been covered with plastic. 

The roof assembly is EPDM/half inch fiberboard/roof deck/2×10 joists (now with 7.5” mineral wool) Certainteed Membrain and eventually new drywall. 

I’m in Minnesota and the vapor barrier is required by code. I’m officially out of my depths here on what to do and wondering if I’m totally screwed. I have a drywall scheduled to be installed next week and am worried it will be trapping all that moisture in there. 

The certainteed membrain is supposed to change its permeability and allow drying to the inside but I don’t know how long that would take. 

Thanks for any guidance.

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  1. user-2310254 | | #1

    You want to avoid air leaks in the roof assembly. Do you have recessed lights or electrical boxes in the ceiling? This might be one of those situations where filling the joists with an HFO closed cell foam is a safer approach. But let's see if an expert will weigh in.

  2. Jon_R | | #2

    > 2×10 joists (now with 7.5” mineral wool)

    I wouldn't use a 2" vent for a flat roof (well, except with powered smart ventilation).

    Air sealing and blower testing to verify is vital. Avoid designs with excessive exterior to interior perm ratios and/or no exterior side air barrier (a condensation problem with AC).

    I'd say that things will improve some once the AC gets interior humidity low. But I don't want to encourage your risky path.

    +1 on using some spray foam in an unvented design.

    1. Expert Member
      Dana Dorsett | | #11

      >"+1 on using some spray foam in an unvented design."

      Make that +2.

      Flat roofs aren't a great idea in snow country, and even in less extreme climates venting provides remarkably little benefit compared to what venting can do for steep sloped roofs.

      If the local code officials will give it a pass, consider using a "smart" vapor retarder such as 2-mil nylon (Certainteed MemBrain) or Intello Plus rather than polyethylene sheeting. When the proximate air to the vapor retarder is dry (as in winter, when the cold roof deck takes on the bulk of the moisture of the entrained air in the insulation) the smart vapor retarder has class-II vapor retardency- less than 1 perm (it would meet the Canadian code definition for "vapour barrier"). But when the proximate air to the vapor retarder is humid (like when the ceiling is much cooler than the roof deck due to air conditioning) they become more vapor open than standard interior latex paint (3-5 perms). That allows the assembly to dry to the interior, while providing significant protection against wintertime interior moisture drives while still allowing the assembly to dry toward the interior.

      During the very peak summertime humidity when outdoor dew points are north of 70F (rare, but it happens) the venting is actively working against you if air conditioning the room to 70F or cooler, but fortunately those events are usually hours or days in duration, not weeks & months.

  3. Expert Member
    BILL WICHERS | | #3

    If you have a vapor barrier part way installed, my guess is that the A/C was able to cool things off enough that moist air on the "other" side of that vapor barrier condensed out on the vapor barrier when the temperature of the air on the conditioned side dropped below the dew point of the air on the unconditioned side. That's probably not a huge problem in a half-built assembly, since you should be safer once things are sealed up and you don't have as much migration between areas of the assembly.

    All that fun stuff said, flat roofs can be more risky. Venting solves a lot of problems by providing an escape path for any moisture that sneaks through by way of small leaks. You could try cupola-style vents to get some extra airflow in your vent channel.


  4. rickymn | | #4

    Thanks for the replies. I mistakenly said 7.5” mineral wool, it’s actually 7”, not that 1/2” makes much of a difference. The previous fiberglass was 6.5” thick so I tried to get close to that thickness for some air movement across the top. I would have much rather had spray foam but the cost was far out of our budget. The mineral wool was already an expense not prepared for (of course).

    Do you think I should shut off the AC and wait to see if the condensation goes away before buttoning everything up with drywall, or keep the AC running and turn on a dehumidifier to get the interior air drier? We’re looking like 90° temps and high humidity for the foreseeable future.

    1. Jon_R | | #5

      Turn on the dehumidifier (to cause more drying vapor drive) and as much as tolerable, don't run the AC (it's causing the problem),

    2. Expert Member
      BILL WICHERS | | #6

      You need to try to get that humidity down, but it will always be a loosing battle until you have the air barriers in place. Until the house is sealed up, every air leak is still going to bring in humidity. Just do the best you can and try to avoid actual liquid water in assemblies as you close them up.


  5. rickymn | | #7

    Thanks. I went back today and all the condensation drops that were on the mineral wool are gone and have now pooled on the vapor retarder. I turned off the AC and brought a dehumidifier and have that running. I don’t know how long it would take for those little pools to work themselves through the “smart membrane” or if I should cut slits to drain them and then tape the slits. I couldn’t have picked better weather to be doing all this. Lol

  6. Expert Member
    NICK KEENAN | | #8

    I don't think interior humidity is the problem. It sounds to me like hot, humid exterior air is leaking in and condensing when it hits the cool vapor barrier. That's why it only started when you turned on the AC.

    The conventional wisdom is that you put vapor barrier on one side of an assembly, the side that is the warm side most of the time. Moisture tends to flow in the same direction as heat. You want the other side to be vapor open so that the vapor can dry. In cold climates that means vapor barrier on the inside, and on a roof that means the roof underside is vented, because presumably the roofing material is waterproof. The assumption is that when the vapor drive is in the other direction --as it is right now for you -- moisture will accumulate, but it will be driven out when the drive reverses.

    If you don't buy the conventional wisdom, it is possible to do what's called a wrong-side vapor barrier. That's what an unvented roof is. The assembly is sealed on the cold side and vapor open on the warm side. For this to work there has to be impermeable insulation -- like closed cell spray foam or continuous exterior insulation -- on the cold side, so warm humid air never has the chance to contact a point cold enough where condensation can occur. This assembly is designed to need little drying, and to get that drying when the outside is warmer than the inside.

  7. rickymn | | #9

    That makes total sense to me. It was definitely a recipe for hurdles as the house had no insulation for a day with roof vents allowing plenty of humid air in, plus a window left open. MN has the best of both worlds, freezing temps half the year then a few months of hot humidity. With my current assembly having the small vents on the underside of the roof and vapor retarder on the inside behind drywall, it sounds like what I have is the best I can hope for without installing a completely new roofing system? I can only get the barrier sealed up to a certain degree but with the eave vents still letting warm air in, it sounds like I will always get that warm air to cool surface exchange?

    I really appreciate all the feedback. Trying to make more energy efficient improvements to this house has proven challenging, but even if small gains are made I feel it’s worth it, and hopefully for the next owner as well.

  8. Expert Member
    BILL WICHERS | | #10

    I would poke a hole with a nail to drain any pools, then tape the hole closed with some flashing tape once things have drained.

    I agree with DC that the moisture is coming in from outside too, which is why you'll always have higehr humidity than you'd like until you have things sealed up.


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