Venting a breather membrane and condensation

[polkablues]

Hi guys,

I currently have the following pitched roof construction, mono pitched lean to roof, marley modern tiles, 50x38 battens, cromar vent 3 light membrane, 50mm air gap, 100mm cellotex, 50mmc cellotex over rafters taped, vcl membrane, 12.5mm plasterboard, there are no vents in the roof. this was installed on a new extension.

We are getting a lot of condensation on the underside of the felt which is dripping down at the lowest point.

The builder is planning to install over facia ventilator and vent tiles further up the roof, I am concerned as he wants to vent the 50mm airgap where as my research suggests that the ventilation should be to the top of the membrane and essentially, not in the 50mm void which is effectively inside the building.

would appreciate some advice

thanks

 

[Alastairreid]

The ventilation is to the cold side ie above the celotex! your builder is correct.

 

[hardmetalking]

The most important part of the build up is the vcl membrane!

Most builders don't know a vcl membrane from a breather membrane.......very very different things.

Oils you be more specific about the vcl membrane?

 

[hardmetalking]

To create an adequate vented roof you need continuous vents at the eaves and ridge to create a positive flow of air. The eaves vent needs to be 1/1000th of the overall roof area. The ridge vent then needs to be 15% larger than the eaves to force the air flow up to the ridge.
By installing a few tile vents it's more of a guess to fix the problem rather than a calculation.
If you have a decent vcl membrane then you shouldn't have a problem.......but you have. My guess is you have a bit of polythene stapled up there that's got a thousand holes in it when they fixed the plasterboard.

 

[polkablues]

To create an adequate vented roof you need continuous vents at the eaves and ridge to create a positive flow of air. The eaves vent needs to be 1/1000th of the overall roof area. The ridge vent then needs to be 15% larger than the eaves to force the air flow up to the ridge.
By installing a few tile vents it's more of a guess to fix the problem rather than a calculation.
If you have a decent vcl membrane then you shouldn't have a problem.......but you have. My guess is you have a bit of polythene stapled up there that's got a thousand holes in it when they fixed the plasterboard.

Hi, Its going to be difficult to vent at the ridge as its monopitched. an over facia ventilator will work at the eaves.

The VCL membrane was specific for the job but yes would have been punctured by the plasterboard fixings. There is also a taped layer of kingspan insulation after the VCL so its almost doubley protected however there is still condensation.

I dont want them to vent under the brether membrane if its meant to be vented above

thanks for the reply

 

[Guitar_man]

Does the same principle apply to a sloping flat roof?
Ie 50mm gap, celotex/vcl membrane 12.5mm plasterboard? In my case there are no ventilators, do I need the air gap in this case?
What vcl are you using? The room is going to be a bathroom.
Many thanks in advance.

 

[hardmetalking]

Yes the same applies Jimbo.