The Shed from Hell....

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I'm curious and would like to know more, please.
In the past a typical winter around here would imply largely freezing temperatures from late November or early December to late March. The sea between Finland and Sweden froze over completely so there was very little moisture in the air. In April all accumulated moisture dried out in the spring sun and the snow that had accumulated throughout the winter melted.
A modern winter with only a little ice along the shore and temperatures swinging between frost and thaw and the snow melting several times during the winter and strong winds blowing from the sea produces lots of moisture that condenses everywhere.
You must be tough to cope with such a terrible climate.
-20 celsius and no wind is a lot more comfortable and healthy than +2 and driving slush and ice bark on everything when you work outdoors all day.
you need to put insulation of some kind in close contact with the surface where condensation forms. When the outside temperature falls rapidly, the inside of the shed can't drop as fast, so the moisture in the air in the shed condenses on the colder roof. If it was me, I would do my usual cheap solution to a problem like this and find a really low cost foam underlay for laminate flooring. It comes in various thicknesses, 2/3mm. Using a can of spray adhesive from Screwfix, I would stick the underlay directly on the the Onduline. The important thing is to get the foam in close contact with the roofing sheet. I would put on a couple of layers. By the time you finish, you will be as high as a kite from the fumes!
Thought I'd finally solved my leaky-roof shed last year by replacing the wood + umpteen layers of "guaranteed 10 year" black stuff with corrugated Onduline. Nailed to a new frame, 3 panels, overlapped as per the instructions. I even noted the prevailing wind direction so the "open edge" was covered by the next panel.
Great stuff............

I've have made numerous sheds and obviously numerous roofs, all pitched, not flat (but I don't see why flat roofs should be different}, and several leaked for reasons I could never fathom. The work was good, felt overlaps were way beyond minimum, so I did a bit of lateral thinking. The reason many shed roofs are felted is because it's a reasonable price, relatively durable and not visually unacceptable, but normally THEY NEED JOINTS so you still risk a leak, yet as a fish keeper with a large pond I keep water in successfully with a 40 yr guaranteed butyl liner. So the next roof I built on a 16' x 8' shed the first layer was pond 40 yr guaranteed butyl liner, glued to the OSB / Stirling Board roof with bitumen in one complete sheath, no joins. Not aesthetically pleasing but a waterproof envelope. I then spread copious liquid bitumen and put good quality felt on it. Not the cheapest option, obviously, but butyl liner is surprisingly cheap & temperature flexible (relatively). I haven't had a leak since. I think adhesive, rather puncturing the roofing envelope in any way seems to work.
When the sky is clear and it’s night an object on the ground effectively looks at space regards infrared radiation, space is at -273Celcius give or take. During the day Raleigh scattering (why the sky is blue) negates this effects.

This is one of the main drivers for frosts on a clear night, and enables the ground temperature to fall considerably below the air temperature.
That's the theoretical case, but not the practical realisation. Rayleigh scattering is not related to and has no effect on the sky temperature.

Deep space is around 3K (-270C) but the earth's atmosphere is a great insulator (CO2, water vapour/cloud cover, dust etc.) plus all bodies (including the atmosphere) absorb thermal radiation and if not at absolute zero emit it too.

You also need to measure an average temperature as there is far less atmosphere at the zenith (directly above you) than at the horizon.

In practice, in a dry desert on a clear night, sky temperatures of around 223K (-50C) are common whereas in the tropics with high humidity you can see 293K (20C). In the UK we get around 273K (0C) on average.

Having recently lived in the Arabian Gulf region and done a fair bit of sleeping out in deserts, I can confirm it gets very cold at night!
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Thought I'd finally solved my leaky-roof shed last year by replacing the wood + umpteen layers of "guaranteed 10 year" black stuff with corrugated Onduline. Nailed to a new frame, 3 panels, overlapped as per the instructions. I even noted the prevailing wind direction so the "open edge" was covered by the next panel.
Great stuff.
Wind, rain in various strengths over late autumn/early winter. No drips, no leaks.
Moved some electrical and mechanical tools back in a few week ago, and banged up some 12v LED lights courtesy of a solar panel.
I was a happy punter....
Until yesterday.
Went in the shed for something and got dripped on.
Dozens of water drops clinging on to the underside of the corrugations.
Leaking? Nope.
Condensation? Maybe. It had been a frosty one the night before, but the morning sun had seen to it.
Ice formed inside the roof, and was melting? The car windscreen had had ice inside this morning, so perhaps?
But then, it would've needed moisture on the inside of the roof in order to freeze/ where had that come from?
Got the steps out this morning to have a gander at the roof top. One thing I did notice was that the overlaps weren't sitting as flush to the overlapped corrugations as before, but seeing I'd been 'generous' in overlapping 3 undulations when fitting, I don't see how these could realistically be causing a problem.
Looks like I'm going to have to revert to throwing some dust sheet-type things over the important stuff in there again...

On a tangent - and thinking of cold nights in there again - I'd spotted the clay plant pot/candle for heating ideas on YouTube. They're quite interesting to watch if you haven't a clue what I'm writing about! Some contributors even go to the lengths of measuring the surrounding temperatures after given time periods, with some surprising results!
The practice started me thinking about whether another heat source would work - the idea of a naked candle flame, albeit encased in two plant pots, didn't really appeal! - so after a quick look, I came across these :
"50w 12v Insulated Electric Ceramic Thermostatic PTC-Heating Element Air Heater".
Not really sure what the idea is, but I'm tempted to give one a go! I've got 2 12v batteries in there so hooking one up wouldn't be a problem. Even if it worked, how long a car battery would run it for, I've no idea!
What about using a diesel heater. 12volt ,small heaters that use little fuel and cost about £100 on eBay? 5 or 8 kw.
One thing you could try would be to put some breathable membrane on the underside of your roof frame. This may not solve the problem without insulation, but I think worth a try, and cheap and easy to do. I normally put this under any corrugated roof, under the battens the roof sits on, I then fit insulation under that, and a ply inner ceiling over the joists. On the subject of onduline, these have a habit of sagging if not adequately supported. I would not have your battens, at right angles to the corrugations, much more than a foot apart to avoid any long term tendency for it to sag between them.
If the battens are too far apart to support the felt as is usually the case with corrugated roofs then chicken wire stretched below the felt is an common and economical solution to sagging.
As others have pointed out, your roof is a cold bridge and its construction method is working against you which is why you are getting condensation. An insulator between the outside an in will assist but given how easily this is happening I suspect you will just shift the problem of condensation somewhere else within the shed. Still insulate but be aware that's likely not the end of it. Knowing the dew point of your shed location would assist, it can be calculated (plenty of online tools) using the air temp and relative humidity; a basic weather monitor will give you those in the first place. If you know the dew point you can plan for condensation and at least figure out what works best for you to tackle it. I also suspect you will need airflow more than an air tight seal to equalise the temperatures but again that is relative to the dew point and knowing that is a good way forward.

In the days of sub-12p Kw electric I'd tout the idea of a desiccant dehumidifier but I suspect will quickly add up and be less viable. That's likely also partially relevant to tube heaters as well unless they are well timed. Avoid the liquid fuel heaters, they will place more condensation in the air.

Personally, I'd insulate the roof and add air gaps or breather membrane if this is viable. Then get a cheap (branded) weather monitor and work out your dew point. Install a controllable vent and then reassess.
Insulation is the key. Parafin or diesel heaters pump out so much water vapour they will make it ten times worse.
Im so glad i have a nice thick insulated steel roof, it cost a few bob but i dont have any worries about damp any more!
From an environmental perspective, using energy to heat a shed even a little in an attempt to stop condensation doesn’t seem like the best approach.
There is also an environmental cost to the insulation, of course, but at least that’s a long-term solution.
Thanks, chaps!
Some interesting - and differing views! - here!
Which way to go?
Not sure....but thanks anyway!