Breathable Membrane

Can't go wrong with tyvek, having said that i have used the screwfix one and its been fine.

They are a doddle to work with - they don't rip so don't need as careful handling as traditional felt.

Price - not cheap!

 

tyvec give you most options - tyvec supro or supro plus.

venting the top of a lean-to roof is usually unsightly, a dry ridge on an apex roof is a good solution, and eaves level vents are a doddle. however in your situation i would reccomend the tyvec, and eaves vents at your discretion.

there actually is under tile/slate ventillation through the cracks and gaps in the covering which will help to disperse vapour, this is recognised by the manufacturers, and together with a vapour sealed ceiling and breathable underlay roof space ventillation can be dispensed with completely (in most domestic situations with suitable felt).

please note vp400 (sfd) requires eaves vents as a minimum, even though it performs as well as tyvec supro in all measurable ways.

 

I've seen condensation in a few roofs fitted with breathable felt, it might not be as good as it's claimed, so I always put in ventilation to the roof space.

It's nice to work with but I wouldn't rely on it working with out any other ventilation.

 

The BCO on the last job i used Tyvek on said that vents made the condensation problem worse!

 

What's the BCO saying, if you have condensation with tyvek and no vents, adding vents will cause more condensation.

If he thinks that then he should insist that tyvek is not used, as you can get condensation when using it. Airflow is the accepted way of keeping the roof space dry but if tyvek is causing condensation with that airflow then it should not be used.

Then again BCOs do sometimes come out with stupid comments, I've had one tell me arches are not structural.

 

Yep - thats basically what he was saying. In fact he has a point. A breathable fabric works on the different pressures of water vapour. Thats why they don't allow water vapour in - there will always be more pressure inside than out - your goretex jacket works the same way