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I recently had my attic insulated with blown in Greenfiber cellulose.

dryskin | Posted in Green Products and Materials on

I had my installer put down a 6mil vapor barrier in between the attic floor joists. I have since learned this was not nessary and may even cause issues with moisture. I live in NJ. It is humid in the summer and the house is air conditioned. The job is not yet finished. Another 5″ of insulation needs to be blown in. Should I have the vapor barrier removed?

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Dryskin,

    I can't answer your question, but I'm sure someone else will give you a science-based answer in a few minutes. In the meantime...

    I'm guessing your user name is suggesting you have a problem with wintertime dry air. If so, you should be focusing on air sealing. GBA has lots of articles on this topic, such as https://www.greenbuildingadvisor.com/articles/dept/musings/how-insulate-attic-floor

  2. User avater GBA Editor
    Martin Holladay | | #2

    Dry,
    The vapor barrier installed as you describe is not ideal, but it's probably not worth removing. In most cases, it's not going to cause any problems.

  3. Jon R | | #3

    You might check for water accumulation on the attic side of the poly during humid weather with the AC on.

  4. User avater
    Dana Dorsett | | #4

    Unless you're air conditioning the space to some ridiculously low temp you're unlikely to have condensation on the vapor barrier in an NJ climate. If the AC ducts are in the attic there's some chance of condensation near ceiling register boots though.

    Taking a random NJ location such as Newark (pretty typical humidity for NJ), even during the most torrid weeks the average outdoor air dew points are less than 70F more than 75% of the time, under 65F roughly half the time:

    https://weatherspark.com/m/23783/7/Average-Weather-in-July-in-Newark-New-Jersey-United-States#Sections-Humidity

    (The orange band in the graphic is the 65F-70F dew point prevalence, the red is 70F+ dew points.

    So while that's still pretty humid, you'd have to be keeping the ceiling cooler than 65F to have significant moisture accumulation on the attic side of the vapor barrier due to air conditioning.

    Yes, there will be days/hours when dew points hit the high 70s, and if you're keeping it at 75F indoors at typical thermostat height the ceiling temps might approach the outdoor dew point temp, but cellulose would take on the moisture as adsorb, and release it when the attic ventilation air dries out. For the cellulose to become saturated would take weeks or months of torrid weather with the outdoor dew point above the temperature of the vapor retarder. That type of excessive long term humidity is possible in the Gulf Coast states, but not in NJ. In New Orleans weeks where the outdoor dew points are above 70F 80% of the time are common, but even there the dew points are "only" above 75F less than 15% of the time, even during the worst weeks:

    https://weatherspark.com/m/11799/7/Average-Weather-in-July-in-New-Orleans-Louisiana-United-States#Sections-Humidity

    But in the areas around cold supply duct boots penetrating the vapor barrier can be substantially colder than the average temp of the ceiling, and those would be the places to watch (or to relieve the vapor barrier.)

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