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Shibby

Damp after frost

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Hi, I've been trying to tackle this problem for a few weeks now. I'm finding that, after a frost, damp patches are appearing on the ceiling of my (still incomplete) observatory. Worried that the felt was not up to scratch on the shallow pent roof, I've actually painted the whole roof in roof sealant, which goes over the felt. However, this morning, after an other frost, I was pretty frustrated to find new damp patches. I can't have this, as there will be electrics for the lighting running through the ceiling.

I can't figure out where moisture is getting in. I've found that nothing gets through after heavy rain - it seems to be only when there is frost sitting/thawing on the roof. Is it possible that it's just condensation? Bearing in mind that I'm not heating the obs yet, although the floor is insulated. I'm about to add vapour barrier, which would solve the problem if it is condensation, but of course once all that's on I won't be able to tell for sure as it'll be hidden! Any thoughts?

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Is there any ventilation in your shed ? ... it could be trapped moist air that is condensing onto the underside of the cold roof panels.

I have plenty of ventilation in my obsy. and have never noticed any condensation on the under side of my roof.

Just a thought.

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46 minutes ago, SlimPaling said:

Is there any ventilation in your shed ? ... it could be trapped moist air that is condensing onto the underside of the cold roof panels.

I have plenty of ventilation in my obsy. and have never noticed any condensation on the under side of my roof.

Just a thought.

I really, really hope for it to be condensation, because the vapour barrier would solve that - however, the obsy is fairly well ventilated - where the sliding roof forms an overlap. So, I'm not too sure...

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Sounds like condensation.. make sure you have adequate ventilation.

 

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That will be condensation, from the slightly warmer air on the inside condensing on the underside of the roof, you need more ventilation that is the key, there is no point in just warming up the inside it will make it worse, just put more ventilation in the shed and all will be good with no heat required at all.. :)

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46 minutes ago, Shibby said:

I really, really hope for it to be condensation, because the vapour barrier would solve that - however, the obsy is fairly well ventilated - where the sliding roof forms an overlap. So, I'm not too sure...

Vapour barrier on the inside of the shed will NOT stop condensation forming on the inside of that, if the air is warmer inside, and hits the cold spots and condenses, then condensation will form, and run down the walls, its all about ventilation and air flow.. :)

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You say you have a gap where the roof rolls off, this is higher up and so you need ventilation low down on the walls then you will get a natural flow of air in through the bottom and the air will warm, rise and go out through the gap you mention, thats the airflow you need to stop this issue.. :)

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Thanks for the comments all. What I neglected to actually say is that this is occuring on the warm room side, so it will be heated with insulation, vapour barrier and dehumidifier. Some good advice though for the other side, I must certainly add a low down vent to sort out the air flow...

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8 minutes ago, Shibby said:

Thanks for the comments all. What I neglected to actually say is that this is occuring on the warm room side, so it will be heated with insulation, vapour barrier and dehumidifier. Some good advice though for the other side, I must certainly add a low down vent to sort out the air flow...

You still need ventilation on the warm room side, even more so if you are having heat and vapour barrier as the warm air has nowhere to go and will condense on the inside of the roof which will be colder.. it’s so much easier and healthier  just to add ventilation, rather than messing around with dehumidifiers.. :)

Edited by LightBucket
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Have you got fresh (<2 year old) concrete in your obsy? If  there is there  is a good chance that's where the moisture is coming from.

More air flow will help or seal it up and run a dehumidifier. Dessicant types work down to 0 degrees, compressor types down o about 10 -  12 degrees.

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Thanks all. I guess my confusion is as to why there'd be so much condensation when I'm not yet heating or using the observatory and there is (some) ventilation. However, this...

On 25/11/2017 at 17:13, Skipper Billy said:

Have you got fresh (<2 year old) concrete in your obsy?

... may well explain it - the concrete was poured ~6 months ago.

Util I can do more ventilation-wise, I'm leaving the dehumidifier running as often as possible to keep the moisture levels down. I'm still a *little* paranoid it might be the flat roof so may look to slap some extra waterproofing around a bit here and there ;)

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5 minutes ago, Shibby said:

Thanks all. I guess my confusion is as to why there'd be so much condensation when I'm not yet heating or using the observatory and there is (some) ventilation. However, this...

... may well explain it - the concrete was poured ~6 months ago.

Util I can do more ventilation-wise, I'm leaving the dehumidifier running as often as possible to keep the moisture levels down. I'm still a *little* paranoid it might be the flat roof so may look to slap some extra waterproofing around a bit here and there ;)

It wont be the flat roof causing condensation, you could put 20 more layers of waterproofing on and it will make no difference to condensation, its the conditions inside that need altering.. :) not on the outside

Edited by LightBucket

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I'd install a couple of fans to keep the airflow going. You could rig them up with a solar panel so they work all the time.

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4 minutes ago, LightBucket said:

It wont be the flat roof causing condensation, you could put 20 more layers of waterproofing on and it will make no difference to condensation, its the conditions inside that need altering.. :) not on the outside

No, I meant I wasn't 100% confident there isn't a leak somewhere!

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