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The Shed from Hell....

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Cozzer

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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.
Fantastic.
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/melt....so 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!
 

Jameshow

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50w is 4amp hour....

Presuming you have a 100a/h battery and you discharge 50% (best practice) then you have 12hours use.
Do you have a charger or solar you would need a 100w solar or 8 amp charger to recharge at the same rate.

Cheers James
 

Fitzroy

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My outhouse roof which was corrugated iron would happily form ice on the underside of the metal that would then melt and drip amazing quantities of water. The out house is somewhat damp as it has a solid slab floor which I doubt has a dpm under it, so lots of moisture that will condense out and freeze. However, if there is frost on the ground it means the air already had enough moisture to condense at the ground temperature, and the underside of the onduline will be the same temperature as the top side.

Not much help for you but I recently replaced my roof with, OSB, 20mm Kingspan glued on, then EPDM glued on top. Fixed all the condensation issues I had. Likely just the OSB would have been sufficiently insulating but I wanted to make sure.
 

Cozzer

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50w is 4amp hour....

Presuming you have a 100a/h battery and you discharge 50% (best practice) then you have 12hours use.
Do you have a charger or solar you would need a 100w solar or 8 amp charger to recharge at the same rate.

Cheers James

All this electrical stuff has always been beyond me, but thank you anyway Jameshow!

What caught my eye was the claim that the PTC bit meant " PTC heaters draw full power initially to quickly heat up and reach the optimum temperature. As the heat increases, the power consumption simultaneously drops."

Then again, they are made in China, so....... :D
 

Jameshow

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I have 1" timber roof 50 years old... Felt on top. Never had a problem with condensation or rust.

Cheers James
 

sometimewoodworker

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But then, it would've needed moisture on the inside of the roof in order to freeze/melt....so where had that come from?
That is an easy one, physics 101, as the temperature drops the air’s ability to hold water drops. QED the moisture on the underside of the roof and inside the car came from the air in the shed and car.

Here in Thailand when you get a downpour and the temperature drops to 28C you will have 100% humidity, 2 hours later when the sun is out and the temperature has risen to 38C the humidity will drop down to 50% though there is actually more moisture due to evaporation. Simple physics.
 

Jameshow

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Simple electric tube heater should do it if you have power to it?

Otherwise some foil insulation should help.
 

heimlaga

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All unheated outbuildings have some condensation problems. It is usually worst when there should be winter but isn't. The sort of winter you had in Britain in the old days and which we are now struggling to cope with.

The best way of avoiding condensation is to nail 7/8 inch boards all over the roof. With gaps around half a centimetre so the boards can schrink and swell freely. Then put the sheet iron (or in your case onduline) on top of that. As there isn't much of a cold surface inwards very little condensation will form and what is formed is sucked up by the boards which dry out again when the spring sun is starting to shine on the roof.

Another way is to use a special membrane which lets damp through but not water. It is put directly onto the rafters under the boards.
 

Fitzroy

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I don’t think a heater will work in this situation. On a cold clear night the temperature of the roof material drops due to radiative heat losses from the roof at a few Celsius and the clear sky at effectively -273Celcius(ish) these losses can be really significant, 200+ watts per square meter. Dependant on many factors so hard to calculate accurately, the main factor is angle to the sky with flat surfaces having the most losses.

Warming the air in the shed by a couple of degrees will do not a lot, as the conductive heat transfer between the air and the roof is very limited.

The way to solve it is to keep the moist air away from the cold surface, either a barrier or a secondary insulating material, heimlaga’s boards.
 

baldkev

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Plus 1 for insulating the roof.
A tube heater or oil filled rad might add a bit after the insulation, but likely just throwing electricity away.
 

Fergie 307

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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.
 

Fitzroy

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Where exactly is this shed?

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.
 

Cooper

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Condensation? Maybe
Definitely, as agreed above.
In my VW mini bus we had two glass sun roofs, on mornings such as we are enjoying at the moment ice formed on the inside. As the sun rose the seat under the glass would be wet and unpleasant but elsewhere in the bus where there was a thin pvc inner- lining no problem. Moister condenses first at the coldest place (as in the mouldy north east corner of an exposed bedroom). Before going to the elaborate suggestions above I would pin polythene sheet to the inside of the rafters, like the thin PVC lining in our bus and see if that does the job. Two ways for it to work, first it stops the moist air touching the cold conductive roofing material, second any moist air that bypasses the sheet and freezes on the cold roof and then drips will be shed at the edge of the sheet or dried by air flow. I would only worry if the rafters stayed damp because the sheet might inhibit drying.
Insulate by all means, if you want to be warm.
 

Cozzer

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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.

Crikey!
I knew it was chilly, but....!
 

Cozzer

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Definitely, as agreed above.
In my VW mini bus we had two glass sun roofs, on mornings such as we are enjoying at the moment ice formed on the inside. As the sun rose the seat under the glass would be wet and unpleasant but elsewhere in the bus where there was a thin pvc inner- lining no problem. Moister condenses first at the coldest place (as in the mouldy north east corner of an exposed bedroom). Before going to the elaborate suggestions above I would pin polythene sheet to the inside of the rafters, like the thin PVC lining in our bus and see if that does the job. Two ways for it to work, first it stops the moist air touching the cold conductive roofing material, second any moist air that bypasses the sheet and freezes on the cold roof and then drips will be shed at the edge of the sheet or dried by air flow. I would only worry if the rafters stayed damp because the sheet might inhibit drying.
Insulate by all means, if you want to be warm.

Had to smile at this one!
I once had "a fight" with one of those plastic, cling-film-like decorating/dust sheets, bought from a pound store somewhere. For a quid, it was a bargain - God knows what size it was, but way too big for the object I wanted to cover. I unwrapped it fully, started to hold one side up above me, but managed to get my feet somehow involved with the other. A big heave to free 'em resulted in even more in the air above me, and gravity took over! Down it came, I twisted and turned, and managed to get my head totally encased somehow! I began to panic a bit - I'd got a few layers of this paper-thin stuff over nose and mouth, and it's not easy to puncture when your hands and arms are also wrapped up!
 
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