Room to Breathe

It has been raining all weekend which tends to make me feel a little trapped. You can tell by my wrinkles that I am the outdoor type so I went out for a bike ride yesterday and a walk today. It is, after all, only rain and to tell the truth I enjoy braving the elements and tramping around like a nutter in my Gortex jacket and over trousers. I take a childish delight in the fact that I am warm and dry. A bit like a kid stamping through puddles in wellies. It is almost as if I am defying the jacket to leak but I don't suppose it ever will.

I have heard of Gortex leaking. I was in a branch of Cotswolds Outdoor shop in London and a bloke came in with a Gortex jacket which he said was porous along the arms. The man behind the counter said it was probably a fault and all he could do is send it back. I hesitated to say anything for a while but in the end I couldn't resist it. "Have you tried giving it a wash and a light tumble dry?"

"Oh no we couldn't condone putting it in a tumble dryer". said the shop assistant huffily.

"Well you might not but that is what it suggests in the care instructions". I replied.

The man who brought the jacket in was equally surprised so he looked on the label of a new garment which he picked off the rack. He read it out loud.

"Tumble-dry warm. The heat from the dryer will help to reactivate the durable water repellent (DWR) treatment on your garment's outer fabric. I'll give it a try. Thanks for your help".


Clearly it was the first time he had read this information. Despite having paid two hundred quid for the jacket he hadn't so much as given the instructions a glance. It is a common problem. "As a last resort read the instructions" is a national motto of many and even the bloke in the shop was more willing to pack and post the garment than read the instructions. Mind you he didn't look very pleased with me either. Nobody likes a smart arse and the definition of a smart arse is someone who knows something that you don't.

So here goes.
Gortex is made up of millions of tiny holes that keep water droplets out but let vapour through. These micro-porous holes get clogged with dirt and they need cleaning out. The repellent then needs to work its magic over the holes on the outer layer to stop the fabric becoming saturated. Also moisture that wicks through from within will condense on the outside and roll down the surface like water off a duck's back. Preventing water absorption also keeps the fabric feeling light. When it was first invented, by accident I believe, it was revolutionary but now we take breathable fabrics for granted. Gortex is even used in heart surgery though how you tumble dry it I am not sure.

The technology has since migrated. Now we have breathable fabrics in buildings as well as clothing. Breathable membranes are used under roof tiles to allow moisture to escape whilst keeping out any leaks in the roof. The membrane will keep out rain and wind but let moisture escape from the house. A miracle for sure because you don't have to ventilate the roof space and that cuts down on draughts which keeps the house warmer. But once again you need to read the instructions. Roofers have come to rely upon breathable membranes to take care of all moisture problems in the roof space and you now see fewer and fewer tile and soffit vents, so they must believe it works. Well yes and no, sometimes they ask the membrane to do too much.


A breather membrane, like a water proof jacket, is capable of dealing with a given amount of moisture per hour depending upon temperature and pressure. In the case of the house you need to stop it sweating too much. If the passage of water vapour from within is not restricted by a vapour barrier and a sealed loft hatch etc. then too much vapour will enter the cold roof space and saturate the air.
To reduce the amount of vapour in the roof space it needs to be cleared from the house by extractor fans. These are now a requirement of the Building Regulations in kitchens and bathrooms so the problem will be taken care of automatically within a new building but if a breathable membrane is being used on an older house as part of a re-roof then these points need to be looked at specifically and with no building inspector involved, who is going to carry out the survey?

As much as I love roofers they aren't given to reading instructions. So they certainly won't go around the house and check details such as sealed loft hatches and extractor fans? Like the man taking the Gortex jacket back roofers will naturally blame the product for any short fall and if the man in the merchants has also neglected to read the bit of paper he will be shrugging his shoulders and laying the problem at the door of the manufacturer. Technical reps must spend half their life looking at the same set of problems over and over. Many web sites have a category FAQ's 'Frequently asked questions' but it is FUQs 'Frequently Unasked Questions' that cause all the problems.


Roger Bisby
www.selfbuilder.net