Pitched roof

[martin88]

I’ve recently had an extension built, it has a pitched roof (vaulted ceiling internally) we have a 14.8 degree roof with a tile that can go down to 12.5 degrees (Marley melodie) we have gone for a 175mm x 50mm TR26 engineered timber rafter (Fully designed roof) with 150mm PIR insulation in between then rafters. We have a 15mm plasterboard on the ceiling with a 25mm tile lath. So in theory 50mm void.

we have no soffit vents and the membrane we have used is a breathable type. We have also put 100 rockwool in the eaves with a slight gap between that and the membrane.


I noticed water on the underside of the membrane earlier today... we are having plastering work done but I don’t know what could be the issue.

Any help it would be appreciated

 

[datarebal]

could be building drying out.. why no vents?

 

[martin88]

It’s a single storey rear extension. There isn’t suppose to be vents with breathable membrane and the pir insulation according to the internet. Although I have been about brought some soffit vents (70mm dia) to add a bit of cross flow.

Although technically it should need it.

 

[^woody^]

It was recognised many years ago that breathable felt is not as breathable as people think and condensation on the underside is a common sight, and complaint. For this reason a better performing type has been developed "Air Open", but it's not normally used unless you specify it.

But yes in this situation, the excess plaster moisture may well be the culprit, and things will stabilise later.

 

[Notch7]

Where are you seeing the condensation?

If the insulation is in I wouldn't think you can see it.

If the extension is still a "building site" i.e. Work in progress, with doors and windows in but not heated.....it's almost guaranteed you will get condensation forming - esp in the mornings when it's been pretty cold overnight, then in the morning the sun gets in a bit and warms the air up.

Moisture will be coming out of the concrete floor or screed, damp blockwork that's been rained on during the build, water trapped in the roof and any plasterwork.

 

[martin88]

Notch 7, I took a plasterboard down on the ceiling to move a cable prior to it been skimmed yesterday and when I pulled the insulation down the breathable membrane was wet on the underside.

the windows are in but no heating and it’s a bear shell. It rained a lot when the building was being built so blockwork / insulation got seriously wet.

I’ve also had a quite abit of plastering work in the rooms off the extension.

I notice on the roof lights the 4 corners have a bit of condensation on.

 

[catlad]

I'd be surprised if the bco. didn't want to see any vents.

 

[^woody^]

I'd be surprised if the bco. didn't want to see any vents.

Tyvek, maybe others too, have a BBA Cert stating that no additional vents are required. So you don't have to justify it to a BCO.

 

[martin88]

Agreed Woody, he has already passed the roof and pre plaster.

he only has completion to do.

 

[catlad]

Tyvek, maybe others too, have a BBA Cert stating that no additional vents are required. So you don't have to justify it to a BCO.

Well I would like to read that literature. All bco,s are different and I know many that wouldn't accept that.

 

[martin88]

Surely if it’s an unventilated roof.. you don’t have vents, BC can’t tell u to do that otherwise ur asking for issues.

 

[^woody^]

All bco,s are different and I know many that wouldn't accept that.

They can't ague if it's certified and installed in accordance with the manufacturer's (and certificate's) instructions