How breathable is breathable roofing felt?

[michael]

I have had two roofs re slated in the last two years and took the opportunity to have the roofs covered with a modern breathable membrane prior to the fixing of battens and slates. Apparently this membrane acts a bit like “Gortex” i.e. it allows air to permeate through freely but stops rainwater coming in.

The old slates were unfortunately unsalvageable having been turnerised (covered with hessian and tar) some years ago. These old slates had no roofing felt underneath them, just fixed directly onto the battens and sealed up with what I think they call torching, clumps of lime mortar between the batten and the slate.

One of the roof voids has gable ventilation grilles at each end and so there is plenty of ventilation through the void. The other roof void however has no ventilation other than a few holes drilled in the access hatch at one end allowing a limited amount of air to pass up into the void from the room below. Both voids have been insulated with rolled fibre blanket laid between the ceiling joists.

Will the new breathable membrane be providing sufficient ventilation to the latter roof void which has no ventilation grilles?

 

[chuckey]

The "ventilation" holes in the loft cover are exactly what you don't want. That is warm moist air from the house going into a cold loft. Condensation will occur and cause problems. Block up these holes and start again. Have you no facia boards or sofit boards you can put vents in. Or failing these how about a gable end?
I have just had a natural limestone slate roof replaced and the lads used this sort of material and having been up there just the once I found the atmosphere rather muggy. One day I'll put in gable end vents
Frank.

 

[Bus Godden]

To check out just how breathable this new permeable membrane stuff really is, wrap a wad tightly around your mouth and see how long it takes to lose conciousness!

Good idea in theory, but how long will this wonder material remain truly 'permeable' (Hah!)? Especially after a few years worth of expanding ice crystals and general clogging up of the arteries - not to mention the 'pond-life' it will encourage, like some great piece of blotting paper peppered with Mustard & Cress seeds!!

Roofs need to 'breathe'!

Chuck in a couple of Lapvents.

 

[Nemesis]

Agree...

http://www.lapvent.co.uk/

(now whatever happened to the inventor...)

 

[Flyfisher]

Well there's breathable and permeable, even semi-permeable. Perhaps a better test would be to seal one hand in 'breathable' felt and another in a plastic bag?

More seriously, are there any studies of the long-term performance of these materials?

 

[Bus Godden]

BBA would have done the 'BBA" thing and there would also have been no doubt long-term 'in-house' tests carried out by the manufacturers themselves (awfully powerfull corporations that they are!). The telling thing is that negative feedback is appearing already - in such a short-term, that is not a very good thing for the long, long-term.

 

[Flyfisher]

The telling thing is that negative feedback is appearing already - in such a short-term, that is not a very good thing for the long, long-term.

Fair point - and a somewhat worrying. Hmmm.