when the outside cools and the inside does not that condensation occurs?
I don't store rottable things in there. Bicycle, mower, hedge trimmer, electric saw, that kinda thing. But large amounts of water falling on them might cause rust.
I'm currently planning (form advice in here) to spray the roof with silicone spray to make the water slide to the bottom, and if that fails, add some polythene sheeting below it to catch the drips.
condensing under it, which drips onto the contents of the shed.
water in some way?
It does it (perhaps moreso) with the door left ajar. I assumed that was simply allowing damper air back in once the air inside had dried out due to condensation occurring.
It's tinted blue, but you can see through it.
Are you sure there is no condensation above the sheet causing mould?
I always use silicone (the stuff you seal your bath and shower with). No nails just doesn't work.
I never thought of condensation, and as I was never going to heat the building, didn't think to insulate it.
I thought condensation in a house was from different temperatures inside and outside, which I assumed I wouldn't have in a shed. Then again you get it in a parked car....
It's evenly distributed on the whole roof so I doubt it.
Oh to live in Spain with less damp.....
The problem is temperature differential, so condensation above the foam would be improbable. At least, nothing collapsed in the c 20 years it was in place before I sold the house.
Colin Bignell
Why is there no condensation on the inside of brick? Or does it just soak in?
Surely your roof would just have cooled more slowly, and the condensation would form more gradually over the whole night?
The brick has higher thermal mass than the thin plastic roofing sheet. The inside surface doesn't fall below the dewpoint. It also vertical rather tha facing the sky, ona clear night any heat radiated to the sky just carries on going out into the universe. Heat radiated out from a wall will probably hit something and be reflected back or reradiated so the overall loss is a lot less.
Or if it does see above... B-)
Surely the brick will eventually cool just as much as the roof, just more slowly. Both reaching the outside temperature at night.
Not really. Horizontal surfaces cool a lot faster and tend to supercool well below ambient air temperature on still clear nights. By the time the vertical brick walls are even close to cooling below the ambient air temperature (if indeed they ever do) the bulk of the water vapour will have long since have condensed out onto the much colder roof.
How is this possible?
But if I insulated the roof as well as the bricks, that would no longer apply.
supercool
See my other post, radiation into a clear night sky just disappears into the universe. Radiation from vertical wall will almost ceratinly hit something and that radiation will either be reflected back or re-radiated. Thus the heat lost form a horizontal surface is really lost but that from a vertical one may well come back. Notice how much warmer it is on cloudy night compared to clear one, those clouds reflect/reradiate the heat coming up from the ground.
The super cooling happens because the universe is really rather cold and the roof will just radiate heat out into it all night.
Are you telling me that an object can actually go lower than the surrounding air temperature? I find that hard to believe!
Can this not be used for a refrigerator, or is the difference in temperature between the object and the air not enough?
That is how the physics works. The roof is trying to reach equilibrium in an environment where below it is a room with walls at ~4C and surrounded by ground and air at 4C, but directly above it on a clear night is sky with an effective radiation temperature of about -80C. It varies a bit with the amount of haze, but on a still night the radiation losses from an isolated horizontal surface dominate its heat loss. Unless it is windy a plastic roof doesn't lose or gain much heat from the air or from its supports. Still air is a rather good insulator.
Don't take my word for it. Park your car with one side to a brick wall and all other sides exposed to the elements. The protected side may not even get frosted. Frost invariably forms on the roof and shallow sloping glass windows first.
You need to prevent saturated air coming into contact with the cold surface to avoid having condensation problems.
On a clear night you can make ice in the desert this way under the right conditions with a very clear sky. A clear sky has an effective radiation temperature of about -80C (~190K) whereas the ground is at ~290K and thermal radiation scales as T^4 (3/2)^4 = 5.
So roughly 5x more thermal radiation escapes from the ground into a clear sky than is received from it in still air this is the dominant form of heat loss. Air doesn't deliver enough heat to make much difference unless it is fairly windy.
You can do the experiment yourself on a clear still night that will be frosty. Try one shallow plastic tray with exposed water and one with a loose fitting lid on propped up a few inches from the water surface.
You might even be able to use the same largish plastic tray with only half of it covered for an even more convincing demo.
That does explain the frost on a car being completely insane sometimes. Frozen car with thermometer reading a few degrees above zero etc.
I've seen the ground frozen and the car not. And the car frozen and the ground not. I suppose the wind is what's different between those two situations.
The shed would be rather dark if I put up insulation, as the only light gets in through the roof.
I've sorted it by increasing the slope of the roof.
Thanks colonel peter / uncle scott.. Tara
Jim K
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