Last year I built a barn out of full dimensional timbers. The roof is a simple 7/8" corrugated over purlins with no moisture barrier. The underside collects moisture and drips! I'm thinking of attaching rigid foam insulation to the underside of the purlins and keeping the gap on the upper side of the rafters so that moisture can dry. Any suggestions?

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Mr. Antonioli,

You built a barn. And you need suggestions on moisture control. Would it be appropriate to ask for more information? 

Are you living in this barn? Does it have a floor? Have you considered an air leakage test to determine where outside air may be entering?

So, without more information, just using my imagination, I see a barn over a dirt floor, no foundation, no vapor retarder over the dirt, wood frame walls with metal sides/board and batten sides, no attempt at sealing the envelope. It's a barn, right? Alternatively, if I built a barn in Vermont, say, I would be darn sure to make it airtight, concrete floor, 2x6 wall framing with R-19 insulation, tongue and groove paneling interior, with a blower door test to verify air leakage control. I might also have rigid foam over the purlins on the roof, and screw the metal to the purlins and framing through the foam.

If you are looking for a quick fix, try to control the moisture inside the barn first. Seems like it always condenses on a cold surface, which is what your metal roof sounds like.

Corrugate steel roofs tied directly onto purlins are very common here....for outbuildings, wood sheds, barns, etc. The barn does not need to be conditioned space and I have no interest, let alone budget, for a tight envelope....such would be antithetical to the purpose of the structure. Moisture inside the barn is not the issue, it's the condensation on the underside of the metal roof. It collects and it drops. 

Dan wrote: "Moisture inside the barn is not the issue, it's the condensation on the underside of the metal roof. It collects and it drops."

Are there animals (breathing) in the barn? That moisture has to come from somewhere.

I don't think installing rigid foam as you describe will help unless you ventilate the roof via continuous soffit and ridge vents. With adequate ventilation, you don't need to foam boards so much for insulation as a means of creating a channel for the ventilation. Otherwise, I don't know how you could passively ventilate the roof surface.

Without venting, insulation below the purlins would likely make it worse. On the other hand, installing insulation in good contact with the roof would eliminate the condensation surface. Given the corrugated profile, spray foam is probably the only way to achieve good contact.

David Butler, you are not understanding the dynamic, at all, and your suggestions are so far off the mark that I suggest you revisit the post. "Barn" means many things and I've listed a very specific condition. 

Dan - you asked for help without providing even a modest amount of information to allow people to assist you.  And then you have insulted one of the smartest people on the forum.  Well Done!

Well as I am reading this --- Nothing is going to stop moisture from forming under said roof as long as it has air from the outside or inside the barn able to get to it (as it contains moisture & this is a nice condensing surface). The only fix is drying everything out, priming it & then spraying closed cell foam over all of it including the purlins, etc...

If someone had a clue when they built it & knew that they didn't want moisture dropping down then they would have installed some sort of drainage plane to get it to run outside, to a gutter system, etc... like we do with decks

Some basics for you: http://thehtrc.com/2015/faq-window-wall-condensation-iced 

To be more gracious to Mr. Antonioli, everyone starts without the most basic understanding of building science. To plug into this forum in that state, provide less than minimal information, and ask for help should have told us all there is someone with a real need for understanding. So I will try.

First, please give us more information about the structure you built, where it is, the climate zone, direction it faces, building materials, as much information about the structure as you can provide. I can't remember the last time I had too much information about a structure...

Secondly, did you build it yourself? What is your construction experience? What training? I built homes for more than 10 years without benefit of any building science training at all. Once I started learning the basics, I got this real deep uneasiness about the houses I built, like I had done something horribly bad. So, I learned very quickly how to repent of those practices, and started building using both construction science and building science. Only then did I start on the road to real quality. Even then, it took a long time...

I hope that is what you are asking of us on this forum. How can we help you make the building you built a better building using those building science principles? To do that, we need the tools we asked for. Mostly information, but sprinkled lightly with patience and attention. Please, Mr. Antonioli, help us help you. Tell us what you know, listen when we ask questions, and be patient with us as we struggle, because we all struggle and it's better when we do it together.

Walter

Walter, 

If you think that a barn is subject to a BPI analysis I suggest that you find a profession elsewhere. 

You are missing the point. 

Barn, Mr. Walter, Barn. 

Perhaps if the person who built the barn and roof had even a basic understanding of building science or even barn building, they would have understood that this is common condition in open air barns with metal roofs and that some form of drip protection is required.  In my region, most agricultural barns are installed with a full membrane above the roof structure and under the metal roof sheathing.  The best designs also incorporate a drainage mat between the metal roof and membrane.

It is a bit late to try and address this when the barn is complete.

This video discusses the common problem associated with barn metal roofs

https://wickbuildings.com/blog/how-to-prevent-condensation-under-a-...

this vide

What's interesting is that the all metal building we use for the outdoor kitchen has minimal condensation and the drops never get big enough to drop. Same conditions, different dynamic. Also, the barn has a south facing wall that's also corrugated and there is zero condensation. 

Wow for people trying to help you. you certainly seem like you know it all. and are not very professional in your replies.. 

condensation is always worse on north facing roofs in the northern hemisphere because those roofs do not see the sun and are cooler. Outdoor kitchen is probably more open than the barn meaning the metal roof has less chance to operate independently from the surrounding air.  The metal is a thermal mass.  It will condense moisture in the air when ever its temp is below the dewpoint of the surrounding air.  This happens more in semi-closed in spaces because the air temp can change faster than the metal can keep up.

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