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Is condensation under metal barn roof an issue when ceiling is insulated?

mikeysp | Posted in General Questions on

Hi everyone, I would appreciate your advice. 

zone 4a. I am building a 28×80 shop building.

Will I experience condensation dropping issues with my detals?

Normally in our area, if you build a barn you need to use bubble foil insulation or the condensation collects on the metal roof and drops onto your items.

My details are not normal. I have a slab with 2″ of recycled XPS unde rit. Good drainage in all directions away from slab. and I will have a ceiling with blow in insulation on it with a vented attic.

I will NOT have any roof sheathiing. Just purlins 24O.C. with the metal roof deck on it. 

I did not know where all the humidity comes from that condenses on the underside of the barn roofs that causes all the inside rain drops. If it is a sort of green house effect, where the moisture comes from the ground and is trapped by the roof and there condenses, I would think I do not need the bubble insulation. However, as it is a very wet and humid area with plenty of temerature swings, I also thought that the air entering at the soffit vents would enter the attic and cause the same problem, thus neccesitating my using some insulation layer under the metal roof to prevent condensation dropping all over my blowns in fiber insulation on the ceiliing.

Of course I will spend the extra $600-$800 for the double bubble if needed, but I am trying to avoid being wasteful since my budget is tight. I am just not sure of the building science.  As for buying specialty metal with the underside coated, that is not an option as I already have some recycled metal roofing. 

Thank you for your advice.

-Mike

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Replies

  1. Expert Member
    PETER G ENGLE PE | | #1

    If you pay attention to making your ceiling airtight, you shouldn't have any significant condensation issues. Most of the condensation is from moisture in the building condensing on the cool underside of the roof. If you have an airtight ceiling, the warm, moist air can't get into the attic in teh first place. The small amount of moisture that does get through the drywall gets vented out with the attic venting.

    There is some risk of smaller amounts of condensation caused by the attic vents themselves. In some weather conditions, the metal roof can drop below the local air temperature due to radiational cooling towards the night sky. During these times, warmer, very moist air entering the soffits can condense on the chilled roof panels. In most attics, this isn't enough moisture to cause any real trouble. It drips down onto the insulation and evaporates the next day.

    All of this changes based on the use of the barn. If you are keeping animals, growing things, or doing other very humidity-inducing activities, then you need to pay more attention to your vapor management package. But if the barn is to be used as a workshop, dry storage, or other low-moisture activities, you should be fine.

    1. mikeysp | | #2

      I am indeed planning to have an airtight ceiling. I happen to have a ton of OSB and was planning to use it on the ceiling and use some sort of sealant caulk on all the seams, or whatever inexpensive option for airsealing the seams I can find.

      I had wondered about where most of the humidity came from. It will be a small fabricating facility. I may run some exhaust fans during certain seasons or for an occasional operation that I want to get any odros or fumes out of the shop area., but the attic will be completely air sealed fromt he shop work area.

    2. Jon_R | | #3

      > It drips down onto the insulation

      Conceivably, radiational cooling combined with a sloped roof that concentrates the condensate into a small area could be a problem.

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