I built a small shed for my goldwing on the side of my shop. It’s vented and unheated – recommended by Honda. I live just east of Seattle and it’s a tad moist here but I didn’t expect condensation on the metal roof because the air isn’t heated. The moisture is dripping down onto the bike and the concrete. Damp concrete in my garage pretty well destroyed my old 750 from rust. So do I put some insulation up there? Any help appreciated.
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Damp concrete in my garage pretty well destroyed my old 750 from rust.
Get a Harley.
Sorry - couldn't help it. Bump
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It takes a big man to cry, but it takes a bigger man to laugh at that man. - J. Handey
You must off installed the metal roofing over skip-sheathing or nailers.After seeing frostcicles on the underside of a roof i vowed id only put metal roofing over solid sheathing and felt.
You got the wrong guy, dude. I didn't install anything. But you're right.http://grantlogan.net/
It takes a big man to cry, but it takes a bigger man to laugh at that man. - J. Handey
Condensation forms when warmer moisture laden air hits a cooler surface.
It ain't rocket science.
Got that. I have a car port - open on 4 sides - exhibiting the same problem. Only thing I can think of is the rain itself is colder than ambient and is cooling the roof to below dew point and the outside air is holding moisture.
That metal roof is going to be colder than the air a lot of the time. I'm guesing but maybe it's the air moving over the top of the roof that cools down the steel. Is it on the north side of the house. Behind some trees. Oh, I forgot, your in the northwest. There is no sun this time of year ;^ )
I think that the cooling of the metal has something to do with "night sky radiation" but I sure ain't no scientist and I don't play one on TV.
<"night sky radiation" >
That's gotta' be it. No other way the roof could be cooler than the air.
You know, what that is is objects radiating to the 3º absolute "sink" that is the background glow of the big bang. Cool to think about!
Shade the roof with trees would work.
Forrest
That's pretty standard for un-insulated metal roofs.
Jack
you need to vent it to let that moisture out. Insulating it will only result in having wet insulation
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I am in the process of purchasing a steel shop building. I have been told that it is standard practice to specify fiberglass insulation wrapped in poly underneath the metal roof to guard against condensation. I have no idea as to what the minimum suggested R value is, but I assume that it doesn't have to be R-19 strictly to protect against condensation.
I had a client with a low-slope carport, metal roofing over skip sheathing. It dripped quite a bit on her truck. We removed the metal roofing, removed the skip, installed a solid plywood deck, covered it with Ice and Water Shield, and reinstalled the metal. It solved the problem. If your roof is steep enough (maybe 4:12 or greater) you could use felt and save some dough.
thanks david i think i'll try that. not sure why it'd work though. seems like a vapour barrier & insulation would be required
I was skeptical too but my roofer rec'd it and he is quite trusty. The carport is about 2:12 so it's too low for metal in the first place, thus the Ice and Water Shield. Replacing the skip with solid plywood decking was necessary to apply the I&WS. My theory is that if there were still issues then I'd see water dripping or staining at the plywood joints from underneath, and there is none 3 years later. The bottom of the roofing metal is far less open to the atmosphere now, although there is certainly not an airtight barrier.
Without rebuilding or re roofing, why wouldn't it work to spray foam insulation on the underside of the metal roof. If, as you describe, it is a small shed the price and bother would be alot less than reroofing/rebuilding.
As I'm picturing this, the warmer inside (moist?) air is condensing on the underside of the cooler metal roof. Educate me. Why wouldn't foam sprayed on the underside solve the problem?
I'm thinking a vapour barrier would be necessary to keep the moisture from condensing inside the foam when it hit dew point but David's method apparently works without one.
Common Problem.
Either pull the metal and lay a vapor barrier down and replace the metal
Or
Get rigid closed cell insulation and use closed cell spray foam to seal it between the rafters under the metal roof.Thickness of insulation doesn't matter, but it has to be closed cell (Dow Blue or Foamular Pink , among others).
Pole barn supply yards sell a white sheet product called "Tri Ply " (it's a vapor/water barrier) I believe. Comes in wide rolls, Might see if you can track down some that as you can easily cover a small shed with no seam.
Ask a pole barn yard for names of some contractors who do their building and see if you can't just buy the little you need form one of the contractors.
Forgot , there is also another product sold at the pole barn yards that is about 1/2 fiberglass insulation inside a vysquin envelope. It is commonly used in pole barns for just your problem. Also comes in wide widths so it can be seamless from ridge to eave.
They can't get your Goat if you don't tell them where it is hidden.
This can be reall annoying when it occurs, raising the dew point above the vapor condensing temp is what needs to happen I built a metal roofed storage building about 3 years ago. The floor is concrete and a vapor barrier was installed under the floor. The builders installed a fanfold type of insulated foam board between the purlins and the steel roofing. They un-folded it as they went, installing the steel roofing panels directly on top of the foam. The material is about 1/2" thick and green in color. I forget the name of the mfr. Where I live in Wisconsin these sorts of buildings can have a problem with indoor rain during periods of high humidity and a change in termpature. Most of the time this occurs in the spring as the days are warm and nights are cool. The vaporized moisture condenses on the cooler steel in the evening. In the morning when the sun comes up and thaws out the roof the moisture drips. I have not noticed any drips inside this building. I have a similar buldling without the foam board and do have a problem with condensation. Adding some vent louvers on the gable ends and moving some air with a ceiling fan seems to help. Roger
Thanks Roger and each of you for your input. After reading your working solutions, I think I've figured out what's happening. In this area it rains - a LOT. The condensation occurs - even in an open carport - when it rains. The big rain we just had was during a particularly warm time - in the 50's. I think the rain is colder than the air and so cools off the metal below dew point causing condensation. I think the reason felt over a wood deck works is that the air that's going up the ridges and condensing is doing so on top of the felt and so not getting inside the building. The wood deck is probably enough of an insulator to make the dew point for inside air somewhere inside the plywood but there probably isn't enough moving through to cause a problem.
I made all this up and it might be wrong but I was completely stumped until I saw descriptions of things that actually worked. Thanks for your input everyone.
PS I think Harleys rust too.