Damp in loft despite breathable membrane (Ed.)

[Owen74]

Call me paranoid but I don't believe the so called breathable membrane (used instead of traditional felt) in my extension loft is really all that breathable. I've had problems with mould and condensation (and yes I've ensured soffit vents are clear of lagging). Would it be OK if I cut a few slits in this supposedly breathable membrane, to help ventilation?

 

[blup]

If its insulated there may not be sufficient ventilation between the rafters.

Blup

 

[ajstoneservices]

Fit some lap vents

 

[Owen74]

Fit some lap vents

Is it OK to fit these on the breathable membrane? I would have to cut slits for them to go in. I didn't know if I was OK to cut the membrane or if that would ruin it somehow.

I already have a few lap vents in a felted section of the roof.

 

[ajstoneservices]

Fit them where the membranes lap. The membranes will lap at either 1m or 1.5m.

 

[Dereekoo]

Breathable membrane is there to let water vapour out but prevent ingress from any rainwater/dust etc that gets beneath the tiles. By all means cut slits in it if you want.

 

[Owen74]

Fit them where the membranes lap. The membranes will lap at either 1m or 1.5m.

Felt has laps but membrane is just one continuuous sheet. (I have both - one each in 2 different roof sections)

 

[Dereekoo]

Felt has laps but membrane is just one continuuous sheet. (I have both - one each in 2 different roof sections)

https://www.roofingsuperstore.co.uk...775867&utm_content=Tyvek Breathable Membranes ????

 

[^woody^]

There is a known issue with breathable membrane not performing, and not being seen to perform as expected.

The better type is termed "Air-open" and performs better.

Also the membrane should be fitted sagging between rafters, many roofers fit it too tight.

BTW, if you have excess condensation then that normally means you have excess warm air getting into the roof void. Check the insulation or leakage from the room below or the part of the roof it's attached to.

 

[Owen74]

https://www.roofingsuperstore.co.uk/product/dupont-tyvek-supro-breather-membrane-1m-wide-cut-to-size.html?msclkid=895d258ade0b166b3e9c40dbe1c0cf7b&utm_source=bing&utm_medium=cpc&utm_campaign=Tyvek Membranes PLA&utm_term=4576098695775867&utm_content=Tyvek Breathable Membranes ????

Thanks, I think the problem is the roof section in question is possibly only around 2m in width, so possibly if there's a join it could be right near the bottom where I can't see it. So if I want to fit lap vents my only option would be to cut slits to fit them.
EDIT: Just had a closer look and I found the join! Going to shove some lap vents along it this week and see if that helps. cheers

 

[Owen74]

There is a known issue with breathable membrane not performing, and not being seen to perform as expected.

The better type is termed "Air-open" and performs better.

Also the membrane should be fitted sagging between rafters, many roofers fit it too tight.

BTW, if you have excess condensation then that normally means you have excess warm air getting into the roof void. Check the insulation or leakage from the room below or the part of the roof it's attached to.

Thanks, it's clear the membrane isn't letting out the warmer air, because the worst condensation is on the roof valley on membrane side (new roof section).

I have never noticed any condensation until this new roof section was added, possibly because i've never had any roof valleys before.

I already checked for air leakage from below a while back. Found and covered a couple holes left by plumbers (removed pipes but left holes in ceiling, duh).

What gets me though is the only place I see condensation is these plastic roof valleys. They are only perhaps 3mm thick, exposed on both sides to air of differing temperatures, it's just gonna happen, no matter what. The only places it doesn't happen is the section where the two layers of valley plastic 'overlap', giving it double thickness and therefore no condensation.

Another point - there's no vapour barrier, just lagging on plasterboard. Is that something that would help if I put a barrier under the lagging?

 

[catlad]

You shouldn't be able to see the grp valley from the inside.

 

[Owen74]

You shouldn't be able to see the grp valley from the inside.

Oh dear, I see the whole thing... here's a section

 

[noseall]

Oh dear, I see the whole thing... here's a section

Lol. There should be a (very important) run of felt beneath the valley and a lead soaker at the bottom.

Conv1

• noseall
• 6 Feb 2022

Dormerlead2

• noseall
• 17 Nov 2019

Dormerlead1

• noseall
• 17 Nov 2019

Raisedtie6

• noseall
• 15 Mar 2019

Roof Fun5

• noseall
• 17 Nov 2017

 

[^woody^]

I'll see your "Oh dear" and raise you a "WTF?"