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Post Frame: Metal Roofing on Purlins – Condensation…Rot Prevention Solution?

mikeysp | Posted in General Questions on

Zone 4a. 

Hi, Post frame metal roof attached to purlins seems like a condensation/purlin rot problem int he making. Anysimple suggestions to mitigate this problem?

Details: The metal roof will sit on a bubble wrap to eliminate condensation dropping onto ceiling blow in insulation. The bubble wrap will sit on the 2x purlins. I would think the underside of the metal will condensate and follow those screw holes + time = rot.


Thank you.


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  1. Expert Member


    Why bubble wrap as opposed to a high heat underlayment designed for metal roofs?

  2. mikeysp | | #2

    I am not married to the bbble wrap. It is the common solution around here on pole barns to keep condensation from falling from barn roofs. It is about $150 for 750 sq ft.

    I have not heard of high heat underlayment being used in this situation.

    Note: I will not have any roof sheathing. It will be metal roof on purlins with something to prevent condensation from dropping onto ceiling cellulous insulation and an OSB ceiling.


    1. Jon_R | | #3

      Sounds like you will have an attic. Ventilating it well will reduce condensation to much less than you have probably seen with uninsulated/exposed roofs.

    2. Expert Member
      MALCOLM TAYLOR | | #4


      From your concerns I'm assuming you are using an exposed fastener metal roof.

      I'm curious as to what happens where the panels are screwed down through the bubble wrap into the purlins. They must either collapse the nearby bubbles and leave you with a dimple in the panel, or rely on the air bubble to maintain a small gap which if it later collapses may well let the gasketted fasteners leak. Neither seems desirable.

      1. mikeysp | | #5

        Malcolm, that is a VERY good point. So, can you provide a link to the underlayment stuff you were ttalking about as abetter alternative?

        I googled it, but came up with different materials.


  3. mikeysp | | #6

    Hi everyone. Since I am only a day or two from putting on the roof, I wanted a last sanity check.

    I attached an image showing the general details. Ground is sloped away on all four sides as well.

    In my zone, in barns, humid air condensates under metal and drips onto equipment parked underneath, unless foil bubble wrap is placed between metal roof and purlins.

    However, that is with exposed buildings and buildings without the attic insulated. I have no one to compare with who has an insulated building, air tight, and vented attic with metal roof on purlins.

    I am sure that my design will have less humidity in attic because of the ground and indoor humidity being eliminated from a tight envelope. However, considering the air is so humid, I worry that it will still create a dripping problem onto my blow-in cellulose insulation.

    Help please. Thank you. -Mike

  4. bloedelbuilders | | #7

    Hi Mike,

    In our experience, in Zone 6, the condensation and dripping problems are usually in buildings with high humidity and no thermal break/insulation/air barrier - as in when a farmer is using a forced air combustion heater to temporarily heat a metal-skinned pole shed for short term mechanical equipment work. If your attic is properly ventilated (balance intake/exhaust at an appropriate rate for the volume), and your air/moisture barrier is carefully detailed under your blown cellulose insulation, then there should be no meaningful (hazardous) accumulation of moisture in your attic. I would think that the condensation due to temperature extremes in zone 6 would be exaggerated compared to your zone 4, but I have no practical experience in your climate.

    The air sealing, the insulation layer, and the ventilated attic system have all been developed to address the specific concern that you are asking about, so if you do them thoroughly, there should be no problem. That being said, we are primarily a residential contractor in a rural setting, so we occasionally build pole buildings as well as do some insulation retrofitting in them, so we have seen many installations that were not very well done, and those have had some issues, though nothing catastrophic. Typically the most demanding use case is a livestock operation - cheaply built, corrosive fumes, massive humidity. What is the intended use for the building?

    Good luck on your build. - Nick

    After reviewing the whole discussion, I see that you are intending to address everything I had mentioned. As long as the purlins are able to dry, any rot will be minimal. The paint on your steel will probably fail before the purlins will from condensation in that application. The cellulose will handle dripping much better than fiberglass would, so if you are concerned about a little compression/settlement on the drip lines, blow a few extra inches of insulation as buffer toward your intended r-value. Enjoy the building!

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