# Damp Shed



## gc (27 Jan 2011)

I went out to my shed today and noticed that the roof was pretty damp and actually had some mould/furry stuff on it. The shed is just standard shed with t+g panels and osb roof covered with felt. Is there anything i can do to help prevent this or treat it?
It mainly seems to be the roof that is affected.


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## 9fingers (27 Jan 2011)

If the felt is in good condition, then the problem could be condensation and just needs a bit more ventilation.

However if the felt is old or wind might have driven some water in, then that needs sorting out.

hth

Bob


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## jimi43 (27 Jan 2011)

This time of the year the last week has seen (relatively) mild temperatures which means the air contains higher levels of water vapour. A sudden drop...it just started snowing here....means that the dew point is reached rapidly and the cold surfaces inside with no moving air form large condensation ripe surfaces for the moisture to return to liquid form.

Ventilation is the way to go..air it out a bit during the day. A balance of air temperature will reduce the chance of condensation.

Jim


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## gc (27 Jan 2011)

The felt was replaced in the summer so should be okay. With regards to the ventalation there are quite a few gaps at the top of the door as the door was never a great fit (better after the move) however maybe its not enough.

Would it be advisable to wait until better weather for it to dry out and then put some sort of insulation layer inside to help? I saw some aluminium foil rolls in b+q and wondered if something like this would help a bit, then even cover with a layer of osb. Any suggestions would be great.


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## 9fingers (27 Jan 2011)

Insulation only changes the speed at which heat flows from the warmer side to the cooler side. Unless you are heating the shed, then it will have no useful effect. In fact it can still allow condensation to form but where you can't see it so you think it might have stopped but is instead allowing the timber to decay.
Ventilation is the answer so that warmer, moisture laden air cannot become trapped and then cool overnight thus releasing water. Instead it can escape so that air inside is at the same temperature and humidity as outside.

Bob


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## jack55 (27 Jan 2011)

I have a shed which I store wood in and it is open at the eaves so there is plenty of ventilation, but condensation still drips from the inside of the roof. I have a theory that because the felt absorbs the heat when the sun does shine. It warms up the air in the roof space which in turn draws up the cold air and when the roof cools the moisture condensates. But then I had a theory i would win the lottery and thats not proved to be right either.


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## srp (27 Jan 2011)

It's radiant heat loss. Always worse from a roof surface because it's pointing at the sky, so the roof radiates heat (especially to a clear night sky) and the temperature falls below that of the surroundings (walls, floors, surrounding ground etc). This is then a magnet for water vapour which condenses on the cold surface. Insulation and a vapour barrier on the inside will definitely help, even if you don't heat the shed. Ventilation can help, but it won't stop the radiant heat loss at all, so this will happen even in an open sided shed (and it does so in my log store).
The walls (and floor maybe?) would benefit from the insulation and vb treatment as well, along with a coat of something waterproof on the outside - single skin shiplap will be almost saturated at this time of year and that is where a lot of the water vapour will be coming from inside the shed, so you need to stop the water getting in through the wood.


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## bubbabear (27 Jan 2011)

I get the condensation on the inside of my log shed roof which is single skin steel even though one side is yorkshire boarding and one side is open.It is always worse when the sun has been out in the day and the weather slightly milder, a cold night always results in drips. My shed that has kingspan insulated roof never has this problem and that has very little ventilation, I even have draught excluders round the door, this maybe because the inside layer of steel is not subject to the variations of temperature that single skin sheets are. A dehumidifier may help in gc's shed


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## jumps (27 Jan 2011)

gc":2u1xf6jc said:


> The felt was replaced in the summer so should be okay....



I know it's obvious but if there has been a tree bough/branch in contact it can wear through very very quickly...I found out the hard way of course!


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