New metal roof -how to prevent condensation problems

rvander1

Hi,
I am building a shop using 40*64*16 pole barn type construction in western NY, but many features will be more similar to standard home construction: will have continuous in concrete pex heating, will have interior vapor barrier all walls and ceilings-- so attic will be cold.
roof will be 9/12 pitch, Fabrel metal over purlins, continuous ridge vent, and soffet vents. Shop ceiling will be horizontal ceiling joists with bat r30 insulation and vabor barrier below that.
I'm mainly concerned about how to prevent condensation on the uderside of the metal roof-- and having that drip down onto and destroy (over time) the glass bat insulation insulating the shop ceiling.

I've read posts on this forum suggesting foam against the metal roof, but I'm wondering if there is a less expensive way to prevent the condensation.
Also, can I use a reflective foil somehow under the metal (perhaps between the roof joists and purlins so there would be a continuous space for air flow from ridge vents to the soffet vents) to cut down on heat gain in the "attic" in our hot summers/cold winters.
Thanks in advance,
Bob

RJM Build

If you have vapor barrier at the ceiling, where is the moisture going to come from? Also do you have in your design any provisions for ventilation in the attic space? Stifling the addition of moisture by vapor barrier and roof ventilation will be the key.

rvander1

I'm worried about the moisture coming from condensation on the inside of the meetal roof -- I understand from reading this forum that metal roofs can "sweat" with temp changes, and i've had friends with pole barns and metal roofs tell me it sometimes "rains" inside the barn. I don't want that since the place will be insulated at the ceiling joists and rain down into the ceiling joist insulation would make it rot/mold etc.

Chris Brink

If you have the dough and are really worried you can put a continuous layer of tile and metal underlayment over the sheathing. I have seen this done on many high end installations.

Windows on Wash

If the vapor barrier details are correct, you shouldn't have an issue.

I would up the insulation to R-50 and loose fill cellulose will be cheaper and better in this case.

I would also air seal the attic floor prior to putting in the insulation. If you did that, you can skip the vapor barrier altogether in most cases.

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