water on underside of osb

[TANK]

Hello guys. 

 

Been out this morning to do a bit of work inside my new block shed when I felt a drip hit my head. looked up to find all the osb was wet all the way across. It is covered by epdm, has no leaks (had loads of rain last month), so I'm assuming this is condensation? 

it has been incredibly frosty all week, and I'm assuming a build up of ice is melting now that the sun has hit the roof this morning. Haven't noticed it wet or felt dripping until today though which seems odd. 

I haven't got around to putting the insulation out plaster board up yet which is why it is bare. 

 

The floor is yet to be concreted, so I suppose could be damp rising from there but thought the fact there is a huge amount of ventilation around the building and the roof especially (the soffit isn't in yet) would negate any of this. 

 

I'm new to this and at a bit of a loss. Any ideas would be amazing. 

 

thanks

 

Tank 

[Mr Punter]

Yes it is condensation.  Moist air from the warmer ground rising to hit the freezing underside of the OSB, condensing and freezing.  Flat roof will tend to suffer more than pitched.  No wind currently, so the ventilation is not doing much.

[TANK]

thanks for that. so this should be remedied once I have the vapour barrier in under the concrete yeah? 

I don't suppose it is doing too much harm for now (I can't get the floor in until the spring). 

The thing that confused me was that there is no water on the undersides of the joists... 

[Conor]

I've a small desk fan running 24x7 in my shipping container to keep condensation at bay, works really well when it's weather like this.

Edited January 17, 2022 by Conor

[Radian]

What are your plans for the ceiling insulation? Post up a photo of the underside of the OSB as it is now...

[TANK]

I have a load of 50mm xtratherm been lying around, was just going to put that up between the joists. Then cover with plasterboard.

I'm out at the moment, I'll get one when back. 

[TANK]

sorry I don't know how to upload photos unless they have to be through a hosting site? it doesn't show a great deal tbf anyway. 

[SteamyTea]

 

 

Or just copy and paste the in.

[TANK]

when I click choose files to attach it literally does nothing at all. 

[steveoelliott]

3 hours ago, TANK said:

when I click choose files to attach it literally does nothing at all. 

Can you drag the files in?

[Dave Jones]

On 17/01/2022 at 14:11, TANK said:

I have a load of 50mm xtratherm been lying around, was just going to put that up between the joists. Then cover with plasterboard.

I'm out at the moment, I'll get one when back. 

 

yup that will make it worse.

[TANK]

On 18/01/2022 at 17:17, Dave Jones said:

 

yup that will make it worse.

Why bother post that?!

How about something along the lines of 'doing that would just make it worse because ...'

 

isn't this normal cold roof construction? I won't put it up until the floor /vapour barrier is down (which I'm assuming has been the source of the condensation).. 

[TANK]

On 18/01/2022 at 15:01, steveoelliott said:

Can you drag the files in?

I'm on the mobile site not the pc

[Dave Jones]

On 22/01/2022 at 11:21, TANK said:

Why bother post that?!

How about something along the lines of 'doing that would just make it worse because ...'

 

isn't this normal cold roof construction? I won't put it up until the floor /vapour barrier is down (which I'm assuming has been the source of the condensation).. 

 

nope.

 

you need an air gap under the osb. vap barrier is pointless in a shed.

[TANK]

I said I'm going to put it up between the joists. Never did I say there would be no gap. 

The joists are 6x2, the xtherm 50mm = Big gap.... 

 

And again 'vapour barrier pointless in a shed', you are merely blurting out a simplistic statement without backing it with reasoning, explanation, or anything which is of any help to me. 

[Dave Jones]

the issue will be where the dew point ( condensation) forms, for example outside is 0c then inside will be between 0c and whatever the temp is inside which will be higher. so condensation will form inside the building its just a case of where. you want it above the insulation so the air gap takes care off it. problem is you dont have a uniform covering just stuffing it between the joists.

 

vapour barrier will do nothing unless you put a warm roof on which will stop it completely.

[redtop]

49 minutes ago, Dave Jones said:

the issue will be where the dew point ( condensation) forms, for example outside is 0c then inside will be between 0c and whatever the temp is inside which will be higher. so condensation will form inside the building its just a case of where. you want it above the insulation so the air gap takes care off it. problem is you dont have a uniform covering just stuffing it between the joists.

 

vapour barrier will do nothing unless you put a warm roof on which will stop it completely.

So simple warm roof, someone correct me if wrong. Rip of epdm, stick vapour proof membrame to OSB. Stick insulation to that. Re fit epdm stick to insulation. No insulation between joists

[Jenki]

37 minutes ago, redtop said:

So simple warm roof, someone correct me if wrong. Rip of epdm, stick vapour proof membrame to OSB. Stick insulation to that. Re fit epdm stick to insulation. No insulation between joists

For me it would be

joist

OSB or thin ply

Vapour Barrier

Insulation 

T&G OSB 3 Boards - fixed with long screws into joists

EPDM

this way you can use the PVA type adhesive for the majority of the roof - allows the removal of creases much easier. then contact adhesive for the perimeter / upstands

 

Otherwise you have to use contact adhesive for the whole area, and any crease will be there for all to see.  Additionally Firestone would not guarantee direct bond to insulation.

Lastly I create a timber edge around the roof the thickness of the insulation (4x2 / 5x2) so you have something to screw trims, drips, gutters etc.

 

 

[redtop]

5 minutes ago, Jenki said:

For me it would be

joist

OSB or thin ply

Vapour Barrier

Insulation 

T&G OSB 3 Boards - fixed with long screws into joists

EPDM

this way you can use the PVA type adhesive for the majority of the roof - allows the removal of creases much easier. then contact adhesive for the perimeter / upstands

 

Otherwise you have to use contact adhesive for the whole area, and any crease will be there for all to see.  Additionally Firestone would not guarantee direct bond to insulation.

Lastly I create a timber edge around the roof the thickness of the insulation (4x2 / 5x2) so you have something to screw trims, drips, gutters etc.

 

 

Agree that works as well ? we used the special insulation for flat roofs which has the covering you can glue to. Whole lot will be a green roof eventually, but yes you can see the creases

[Gordo]

14 minutes ago, Jenki said:

For me it would be

joist

OSB or thin ply

Vapour Barrier

Insulation 

T&G OSB 3 Boards - fixed with long screws into joists

EPDM

this way you can use the PVA type adhesive for the majority of the roof - allows the removal of creases much easier. then contact adhesive for the perimeter / upstands

 

Otherwise you have to use contact adhesive for the whole area, and any crease will be there for all to see.  Additionally Firestone would not guarantee direct bond to insulation.

Lastly I create a timber edge around the roof the thickness of the insulation (4x2 / 5x2) so you have something to screw trims, drips, gutters etc.

 

 

I wouldn’t recommend timber between insulation and EPDM. It will get damp from condensation and be vulnerable to decay.

[Dave Jones]

1 hour ago, Jenki said:

For me it would be

joist

OSB or thin ply

Vapour Barrier

Insulation 

T&G OSB 3 Boards - fixed with long screws into joists

EPDM

this way you can use the PVA type adhesive for the majority of the roof - allows the removal of creases much easier. then contact adhesive for the perimeter / upstands

 

Otherwise you have to use contact adhesive for the whole area, and any crease will be there for all to see.  Additionally Firestone would not guarantee direct bond to insulation.

Lastly I create a timber edge around the roof the thickness of the insulation (4x2 / 5x2) so you have something to screw trims, drips, gutters etc.

 

 

almost but you want the vapour barrier to seal to the inside walls so needs to be underneath the joists.

 

also waste of money top decking, you can buy glass fibre backed insulation that takes the epdm spray.

[Jenki]

3 hours ago, Dave Jones said:

almost but you want the vapour barrier to seal to the inside walls so needs to be underneath the joists.

 

also waste of money top decking, you can buy glass fibre backed insulation that takes the epdm spray.

Yep. ?. Wrong place.

 

I've used the insulation you mention but difficult / impossible to get flat with the spray glue. Horses for courses ??

 

[redtop]

33 minutes ago, Jenki said:

Yep. ?. Wrong place.

 

I've used the insulation you mention but difficult / impossible to get flat with the spray glue. Horses for courses ??

 

Why would you put the vapour layer underneath the joists for a shed? I would be hanging all sorts off the joists. And you would get the OSB and the joists exposed to the damp produced from the dewpoint being somewhere in the insulation with no ventilation. Just Search warm roof build up, 

Edited January 26, 2022 by redtop

[redtop]

[Dave Jones]

2 hours ago, redtop said:

Why would you put the vapour layer underneath the joists for a shed? I would be hanging all sorts off the joists. And you would get the OSB and the joists exposed to the damp produced from the dewpoint being somewhere in the insulation with no ventilation. Just Search warm roof build up, 

 
pretty much all wrong.

 

the vcl is there to stop moist air from inside migrating into the insulation. How you going to do that without lapping it down the walls?

 

you could have a parapet and lap it up that but don’t see many sheds with a parapet.

 

the whole point of a warm roof is ventilation is not required as it’s a sealed sandwich by the vcl from below and the covering above.