Hello! Just registered,and this is my first thread. I'm converting my cob barn in freezing devon,and currently fitting celotex between the rafters. I put a new roof on in the summer,retaining the elm trusses,purlins and most of the existing rafters. My concern is that today I've noticed a lot of condensation on the cold side of the celotex,which is running down and creating wet patches on the bottom purlins. The underside of the breather membrane is covered in beads of condensation too! I have not filled the gaps in the celotex eith expanding foam yet,nor have i fitted a VCL,as there's 75mm celotex yet to be fitted below rafters. Now... I understand how moist warm air needs to be kept from getting to the "cold side", by using a VCL,or taping the celotex joints, but here i am, working in minus something,..the barn is well ventilated inside due to a lack of doors and windows, ...so where is all this moisture coming from,and is this to be expected??BTW,most of it is on the colder,north side of the roof. I hope someone can put my mind at rest! Thanks.
drying concrete drying plaster wet wood bottled gas heating paraffin heating drying anything are there any open windows in the loft to vent the space ?? moisture will just condense on the coldest surface if it cant escape
Thanks big all I forgot to say i have rooms in the roof,no loft space as such. The air gap between celotex and breather membrane is unvented,which is the normal way of doing it. I just didnt expect so much condensation,even during this cold weather! I do have some new cob on the top of the walls which is drying still...maybe that's what's causing it? If i keep an eye on it and the problem persists,would it be wise to introduce some ventilation at eaves and ridge??
Hi Jules. I don't quite understand what your roof construction is - 'warm', 'cold', where there may be vent gaps. Where there ain't. As Al says, there is moisture in all air, tho' obviously more on certain days and definitely more is the air is warm. However, you can make some of that moisture condense out quite easily by simply introducing a cold enough surface - chust take a can of beer out of the fridge and watch in fascination as beads form on the shiny surface, and a droll forms in your mouth. So, more info is needed as to where, exactly, this condensation is forming, and what the construction is there. It might be that there could be long-term issues here due to the wrong 'construction' methods used, but far more likely, as Al suggests, is that it's chust down to current circumstances. You are doing this work yourself. Are you a builder? Are you following current building regs? Are you following the construction advice of an Archi?
Hi there devils! I would say it's a cold roof. Construction from outside in,is.... Slates Slate battens Breather membrane 100x25 counter battens (giving 25mm air gap,unvented) 50x75 (ish) rafters (celotex between) 75 celotex below rafters,battens(service void)VCL,plasterboard...yet to be fitted. The condensation is in the air gap between celotex and membrane.. Droplets hanging on the underside of the membrane,dripping onto topside of celotex,running down and making wet patches on purlins. This is only happening on part of the north facing side of the roof,where i have fitted celotex between the rafters,and only happened when the temp dropped below zero. I am doing all work myself. I dont consider myself a builder as such. I've done a lot of building work over the last 30years,but of course there's big gaps in my knowledge,and this is the first time ive pulled all aspects of building together to do a comlete barn conversion. I am working to regs,my BCO,is brilliant (not a clipboardy jobsworth!)..he has been very helpful and confirmed my roof design (which was done with an experienced roofer/builder) is good. I expected to need ventilation between cold side of celotex and membrane,but was assured the breather membrane would take care of any moisture getting in there. Nothing is sealed,no vcl etc yet,and the barn is unheated,with a bit of air movement inside due to large main entrance opening..no doors! I expected some condensation at this stage,but not this much! Like you said,i want to make sure the design will work long term,especially in the coldest north facing area I'm describing. Hope that expains everything! Cheers Jules
Hi Jules. Sounds like wot you have is a cold-ish roof . First off, I don't have the full info on this - I am not a roofer, nor a builder, nor a sparkie, nor a thingy. It is quite likely that what you have there will work fine in practice, and the BCO and the roofer/builder wot came up with the design must also have known what they are talking about. And in my own roof - wot was converted to provide upstairs accommodation like yours - I was told that the breathable membrane would be fine for ventilating that gap you referred to. However, I do have the 'required' 50mm minimum gap there which is also open to the bottom draughty eaves; this allows a fair bit of air flow in addition to whatever permeates the membrane. From what you describe, your gap is only 20mm and might not even be vented to the bottom eaves space? (I have Celotex stuffed between my rafters, but only to a depth that allows a 50mm gap between it and the membrane/roofing battens/slates. I then have a further layer of Celotex covering all of the underside of the rafters before p'board. Since that Celotex is foiled both sides, there will be very little condensation getting through to that void in the first place.) As I said, I can't tell you if that is right or wrong, but I think I'd be concerned about the amount of condensation forming there, even taking into account that the house hasn't been completed, and building conditions might be producing more moisture than usual. But then, it may well be that when your full layer of Celotex is fitted to the underside of these rafters, as well as the VCL (vapour control lining, I'm guessing?), there will simply be no moisture coming from inside your house and getting to that gap. So all will be well . Have you spoken to your Archi, builder or BCO about this? I would be, chust for peace of mind.
Ah,so if yer not even a thingy,what are you?!? ...anyway,yes you're right,my roof should work,in practice,and the air gap should be adequate...however,the potential problem area is cold and the sun never gets to it,so i think i will provide ventilation at the eaves,at least. Being a devon barn,the planners want to see exposed rafter ends at the eaves,like they be growing out of the top of the cob walls...so no soffits,but it would be easy to get some vents under the eaves for good measure. I will talk to my bco,fine chap he is...but he wont be in his office tonight,it being new years eve! Thanks for your thoughts on this. Happy new year! Cheers, Jules
Ok then, I am a thingy... Anyways, a 25mm air gap is considered enough? Wow - it used to be twice that. Any ventilation you provide to that space will be sensible. It will be interesting to hear what the BCO and/or your builder has to say about the visible damp (tho' I bet it's dry now being milder?!)
It's still there,thingy,but not as much. My bco has signed the roof off already,so he must be happy with the 25mm gap!?! Its worth mentioning again that the condensation had only formed on about half of the north side of the roof. None at all anywhere else.
Likely to be just down to the building circumstances - extra moisture being produced at the moment. But, still worth pointing out to the BCO and/or builder for their thoughts?
I reckon your right,thingy,lack of ventilation,damp conditions etc..it's drying up now it's milder weather. Cheers
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