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Community and Q&A

Metal building and condensation

BobHr | Posted in General Questions on

I got a call from a friend Friday. He was asking me about a metal commercial building that was having a problem with condensation on the underside of the roof. He had not been out to see it yet so he did not have all the details.

From what I gathered it didnt have insulation against the underside of the roof. There was an insulated flat ceiling with an air gap above. Supposedly not much room in between.

Does a metal roof require the insulation in contact with it to prevent the condensation. Or if the air barrier is improved will venting do the trick.

Nearly forgot…..Climate zone 5a. We have had snow on the ground sine mid December, only a few days last week that melted a lot of snow but back to single digit lows.

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  1. GBA Editor
    Martin Holladay | | #1

    Ideally a metal roof is installed over solid roof decking. The best installation requires roofing underlayment on top of the plywood or OSB roof sheathing, and then purlins or furring strips, and then the metal roofing. That way, if you get any condensation on the underside of the roofing, it drips on the roofing underlayment and evaporates before it reaches the insulation layer.

    There are two ways that you can get this type of condensation. In some cases, the source of the moisture is warm outdoor air. If the roof is covered with snow, you'll get condensation on the underside of the metal roofing on warm days.

    The other way that you can get this type of condensation is when you have a leaky ceiling (no air barrier) that allows warm interior air to contact the underside of the metal roofing.

    Before you can determine the best solution, you have to figure out where the warm air is coming from. Is it coming from outdoors or indoors?

  2. Oak_Orchard | | #2

    I have 3 buildings topped with galvalume/steel. Their tops are frequently drenched with dew during the summer and fall dew season.

    Two of these steel roofs are capping plywood or OSB decks; horizontally strapped with 1 X 4 purlins and 30 lb felt, so the steel is separated from the deck by a 3/4" air space. The temperature (and maybe humidity) of the air under these steel roofs is probably tempered by the proximity to the decks

    The third building is a shed with no deck; the steel is also screwed to horizontal purlins, 24" OC, but they run atop the rafters, so the underside of the steel is visible from indoors.

    In the shed with no deck, for a few minutes, on some mornings during the non heating season, a small amount of water runs down the underside of the steel. .

    This is a different phenomenon from the normal deposition of dew out of the air onto the top surface of the steel/roof (i.e. like dew on the grass, or truck body; etc).

    Where the steel roof is close to ply/OSB decks, the underside condensation is not apparent. So there seems to be a microclimate operating. This process is short-lived; only a couple of minutes in duration and the quantity of the condensation is small; drying quickly.

  3. davidmeiland | | #3

    This is a common problem around here, where a lot of metal buildings have low-slope metal roofs over purlins, and "bag and sag" fiberglass insulation with a plastic sheet under it, up against the roofing. As far as I can tell, air leaks through the seams in the plastic, causes condensation against the metal, and eventually water ponding in the low spots in the plastic. These buildings are invariably on slabs, and I think in most cases they have no VB under them.

    What I tell folks is that there are two ways to fix this. One, remove the metal roof, install a solid deck, install rigid foam over the deck, then reinstall the metal, or two, remove the fiberglass and spray the bottom of the metal with closed cell foam.

    I have fixed a couple of open carports with this dripping problem by removing the metal and the purlins, installing a solid deck, installing continuous sticky membrane, and reinstalling the metal. Both of these carports are attached to houses, and I think the presence of a heated wall is part of the problem, in that it raises the temperature of the air under the roof.

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