flat roof sweating urgent help

[NATHAN01]

iv just finshed insulating and plaster boarding my flat roof and iv just gone back into the annex today and there is wet patches across the ceiling some iv taken a sheet of pb down and the void between the insultion and the ply is boiling hot do you think i need to put some kind of vents in to let the air breath any advice would be great

 

[NATHAN01]

any advice please do you need to know any more info im worried to death here

 

[^woody^]

What kind of construction is it (warm or cold deck), and is there a vapour check?

You don't need to vent the void just because it is hot, but you do need to prevent any condensation formation

 

[NATHAN01]

well its iv used 100mm kingspan in between each rafter and then used foil backed pb fixed to ceiling and i was told that i did nt need a vapour barrier by a builder friend of mine also i put the copper heating pipes above the insulution in the void but i put pipe insultion on them and wrapped rockwool around them to stop them frezzing was it the right thing to do

 

[^woody^]

You've done this wrong

If you have a void beneath a cold deck, then you need to ventilate it completely from end to end - 25mm continuous vent.

The other way, is to completely insulate between the ceiling and deck. In particular fit insulation tight up to the underside of the deck so that there is no cold surface for any air to condense on.

You can use kingspan and no vapour barrier, but you should tape the joints and over the joists too with foil tape too so that no vapour can get through.

You may want to take the Kingspan down, then fill the void above it completely with some quilt insulation (make sure no air voids as above), and then refit the kingspan.

Or fit a vent strip to each end of the joists

 

[NATHAN01]

oh right only thing is now iv fitted all the plaster board its all ready for skimming could i get away with fitting circular vents at each barge board up the sides to get a cross ventilation in its quite a big ceiling to take it all down

 

[^woody^]

You need to have the equivalent of a 25mm continuous strip of ventilation

This works out to 100mm2 between joists which are at 400mm centres.

So for circular vents this will be between three and five between the joists, depending on their size

And make sure you have a proper through air-flow. You wont see the joists or deck start to rot, until its too late

 

[NATHAN01]

only thing is i cant get to the end of the joists because of the guttering is in the way to vent it like that and the annex backs on to another shed so i can only vent the void from above the insultion on the barge board sides if this makes any sense as it is a sloped roof as well i cant get to each joist

 

[noseall]

only thing is i cant get to the end of the joists because of the guttering is in the way to vent it like that and the annex backs on to another shed so i can only vent the void from above the insultion on the barge board sides if this makes any sense as it is a sloped roof as well i cant get to each joist

Oh dear!

Did you use breathable membrane?

 

[NATHAN01]

No i was also told i did nt need it is there any way of getting around this problem

 

[noseall]

.........if this makes any sense as it is a sloped roof as well i cant get to each joist

Is it a flattie or a pitched roof?

If it is a flat roof then the simplest way is build a 'warm deck' roof, meaning the insulation sits completely above the joists...... well apart from the 25mm they are now asking to be fitted betwixt, but tight up against the deck anyhow.

 

[NATHAN01]

its a flat roof but its got a slight slope on it iv got it all ready for skimming at the mo ie. electrics are in and plumbing every thing is there any way of just fitting vents up the edge of the barge boards to vent the gap