How to keep condensation from forming on shed ceiling?
October 23, 2013 11:28 AM   Subscribe

This is the first question in what will become my WE BOUGHT A HOUSE!!! series. WE BOUGHT A HOUSE!! It comes with a good-sized aluminum shed. The seller told us that condensation forms on the ceiling during the winter (we have cool, rainy winters), so he kept everything in there covered with a tarp. I inspected the shed yesterday (WE GOT OUR KEYS!!), and did see some mildewy spots on the ceiling indicative of condensation. Is there anything I can do to prevent the condensation from forming?

My first thought was to install some sort of vent so that the inside and outside temperatures can equalize. Is there a better option? (And would that even work?)


posted by mudpuppie to Home & Garden (13 answers total) 3 users marked this as a favorite
Flat ceiling or sloped?
posted by Etrigan at 11:29 AM on October 23, 2013

Response by poster: It's peaked in the center. Here's a bad cell phone photo.
posted by mudpuppie at 11:33 AM on October 23, 2013

Best answer: Vent! Condensation forms on the warm side of a warm-cool interface, because the cool surface cools the warm air, meaning that the now colder air can't hold as much moisture as it used to. This is why attics have lots of venting (and the more modern the attic, the more venting), and flat roofs have lots of integrated insulation.

Pop some vents in that structure, if it's a peaked roof make sure you've got some good louvers on the walls underneath the peaks, if it's shingled, consider putting in ridge vents.

And congrats on the house! Welcome to a life of fine tuning and tweaking your environment to fit your lifestyle!
posted by straw at 11:33 AM on October 23, 2013 [2 favorites]

Best answer: Vent!
posted by MrMoonPie at 11:34 AM on October 23, 2013

Response by poster: Thanks!

One last question -- will a jigsaw with a metal blade be sufficient for cutting the holes for the vent?
posted by mudpuppie at 11:37 AM on October 23, 2013

Best answer: This is one of those corrugated metal prefab sheds? Yes, I'd drill out a starting hole and hit it with a jigsaw or sawzall.

(BTW, MeFi mail inbound)
posted by straw at 11:46 AM on October 23, 2013 [1 favorite]

Best answer: I am betting it is not aluminium, more likely gal iron (steel plate).

Vent will help, but may not be a total cure. If the problem persists, you will need to install a vapour barrier against the under side of the roof, preferably with insulation underneath that.
posted by GeeEmm at 2:05 PM on October 23, 2013

Best answer: Ventilation is definitely going to be your first line of defense here. You could start with just a passive ventilation system (a louvered grate at the peak of the roof on each end of the shed, with chicken wire behind to keep critters out) and then if that doesn't work you could upgrade to an active system with a fan that runs either constantly or, better, whenever it senses a large temperature differential between the inside and outside of the shed.

Another thing you could do is install radiant barrier insulation on the underside of the shed's roof. This is fairly inexpensive and easy to do. Radiant barrier insulation is just a thin layer of material (oftentimes it comes in rolls that look like fancy tinfoil) that reflects infrared radiation. This keeps radiant heat out of the structure and reduces the greenhouse effect inside. (You may also consider installing this under the roof of your house in the attic, it might be a cost-effective way to reduce your cooling bills.)

As far as a vapor barrier, I'm not sure what installing one on the underside of the roof would do. Someone please correct me if I'm wrong, but I don't think that roofs are a major point of entry for vapor. Depending on the construction of your shed though, the floor may be. If your shed is on a concrete slab (looks like it from the photo) or bare ground, you are probably getting a lot of moisture coming up from the floor and into the shed. You could consider laying down some polyethylene sheeting (available at your local hardware store) and then installing some kind of flooring (I would go with just plain old half-inch plywood) over that. This would help prevent moisture from entering the shed.

Honestly though I would first just do like everyone else is saying and cut some holes under the peaks of the roof and put some vents in there. I bet that'll solve your problem. If that doesn't work though then what I've described above should provide you with some additional options.
posted by Scientist at 2:36 PM on October 23, 2013

Best answer: Our shed is a flat roof design, but the solution my father-in-law used might work for your peaked roof. He installed ceiling acoustical tiles by glueing them directly to the inner metal surface. They've been there for several decades and condensation has not been a problem.

I concur with the venting idea, whether active or passive. Obviously, passive will be cheaper to install.

I frequently use a metal blade in my jigsaw without problems, but I heartily recommend hearing protection.
posted by Kitist01 at 2:59 PM on October 23, 2013

Best answer: And eye protection.
posted by mightshould at 4:19 PM on October 23, 2013 [1 favorite]

You're going to need two vents on either side, and depending on your climate, that may not be enough. We have a metal shed with a metal roof--two vents, east and west, and a north facing door with a louvre. Still drips on occasion. The neighbor has the exact same shed--two vents, east and west, plus spray on foam insulation. His shed seems to equalize better and doesn't get condensation.
posted by BlueHorse at 5:34 PM on October 23, 2013

Venting might improve things. If not insulating the ceiling will do the trick for sure. All you need to do is glue 1/2 foam board directly to the roof on the inside. Use the pink or blue extruded stuff not the white expanded stuff that is all tiny crumbly balls.
posted by Mitheral at 6:52 PM on October 23, 2013

If you're going to glue up insulation, rather than the pink or blue stuff, I'd pay the extra for the foil faced of-white polyisocyanurate foam. For two reasons: Fire resistance, and it's a really good insulator.
posted by straw at 8:36 AM on October 24, 2013

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