this method was devised and tested by myself
Step 1: Why Metal Shed and Garage Roofs Get Condensation
It took me a while to work this out, but if you think of the way a solar still works, that is what is happening to your shed roof. The metal roof heats up quickly in the sun. This acts like a giant radiator so the air inside the shed gets very warm. Warm air has much greater capacity to hold water than cold air. Where does the extra water come from? it evaporates from the soil or through the slabs or concrete pad on the floor of the shed.
When then environment outside becomes cooler i.e. at night, the thin metal in the roof quickly cools down. When the temperature of the roof is lower than the air inside, the extra moisture in the air condenses on the underneath of the roof. The result is the roof sweating and dripping water all over your precious stuff.
(since warm air rises, the warmest -and most moisture laden air is at the top of the shed, this is also why the roof sweats and not the sides, its also why you want to ventilate as high as possible).
Step 2: How to Stop the Condensation
My solution is a combination of two methods:
1) ventilate to get the warm, moist air out
2) prevent the air picking up moisture to begin with
Step 3: Ventilate
If there is no high level ventilation already installed, install a 15cm ventilation grille at the highest point at each end of the eaves. This will provide flow through ventilation at the point where the hottest air rises to. You must have a vent at both ends, not just one.
Step 4: Prevention: a Shed Condom
Get a roll of 2mm thick plastic sheeting and spread it out over the entire floor. Overlap and tape any seams and also run it up the walls for 10 -15 cm.
Lay some type of raised floor over this; I used old pallets covered with scrap wood but you could use old carpet, lino, or anything that will protect the platic sheet from getting holes. It doesn't really matter if you cause a few pinholes in the plastic, its the overall effect that is important. But do tape up any tears.
The result will be a shed or garage with a metal roof that does not sweat.
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