Many of these membranes are actually made of polyethylene now, not teflon. Even Tvyek brand. When we build our new house in Canada 18 months ago, the Tyvek membrane was printed with recycling information that said it was PE. I don't recall it being much different in price than any of the other brands. The builder's merchant that we purchased it from had thoughtfully printed their name on it as advertising

Paul.

Mike, the original approach was to have been insulation between rafters but that has now been changed to ceiling level. One proposal is to use a vapour check buildining paper abobe Heraklith. The breather membrane is a given. I was just interested to know whether people have views about the performance of different membranes. Tyvek seems to have become the generic term rather like 'hoover' but is that down to better marketing or is it a better product. I have tried a (very unscientific) on three or four different membranes I've used; I hold an off-cut over my mouth and try to breathe through it (I did warn you it was unscientific!) The cheapest membrane (Protec 400) was much harder to breathe through than some of the others like Corovin. I don't have any more results 'cause I passed out shortly after...

Thanks again Mike - I'll follow up that point about Heraklith and find out what they say. It would offer a saving if I could leave out the vapour barrier.

Of course the problem with your Herakliths, Diffutherms and so on is the sizing which sadly wasn't taken into account when designing the timber frame. Clearly in Germany and Scandinavia they don't go in for our 400 and 600 centres. I guess this is like getting everyone to drive on the same side of the road (i.e. totally unworkable) but it could be helpful if we standardised at say 500mm centres and then our sheet materials could be 1000x2000 or similar. Maybe I've spent too much time breathing through membranes and it's all gone to my head.

Good point Paul - I see what you're saying. I'll go and run a bath and repeat my experiment underwater. Seriously, however, there must surely be some air passage through the membrane (in the same way that Goretex type fabrics allow) for the vapour to travel - don't you feel? In contrast PU coated nylon allows neither air nor moisture through. Besides, for air to pass through the membrane it has to be forced through under a fair degree of pressure - such as in my crude test. So does all this mean that membranes with a greater resistance to air also have a greater resistance to vapour?
It seems that not all membranes have BBA Mike, but those (I checked) that don't have BRE approval. Trying to compare the figures they give for vapour permeability is a bit more dificult as they use different standards as far as I can see. For example Ruberoid Pro is tested to BS3177 and gives a figure for water vapour transmission expressed as gm/m2/24h. By contrast Protect VP400 for example is tested to BSEN1931 and gives a figure expressed as mn s/g. Very interesting - but leaving me none the wiser!

Paul In Montreal said:

In my humble opinion, breathable has become a very over-used word in so-called healthy construction.

Indeed. It Is also a complete misuse of the word. I suppose someone in advertising was underwhelmed by the prospect of using "vapour permeable membrane" as a marketing catch phrase!

You wouldn't last long trying to breathe through the rubber of a party balloon but blow a balloon up and leave it for a couple of weeks and you'll find much of the air has leaked out. If you blow the balloon up with carbon dioxide it will go down in a few hours. CO2 is a bigger molecule than N2 or O2 so you might expect the opposite. I've not tried blowing up a balloon with water vapour and have no idea what would happen, assuming you could stop the water from condensing. Maybe a wet balloon.

I have made attempts to understand the vapour permeability figures published for various paints. The Germans seem to use a different system of measurement from the British and I find it very confusing. When a can of paint says 'Breathable', without giving any figures at all, it's hard to know just what that means. Sometimes, I suspect, not a lot.

Airtightness is about movement of air in bulk under air pressure - wind, thermal buoyancy etc. Vapour tightness/breathability is about the movement of individual molecules of the vapour under vapour pressure. That may sound like the same thing but it's a quite different principle.

Vapours travel and pass through apparently solid things even when there's no air movement and no air pressure - and can even move 'upstream' against an air movement. Like humans, molecules of vapour 'see' only their own kind - and are repelled by them! So any local concentration of a vapour - a bathroom full of water vapour, or a tiny drop of evaporating perfume - wants to get away from itself, to disperse away from the centre of concentration, to even itself out. In a space, every single different kind of vapour that's present sees only its own vapour pressure - a measure of the local density of those particular molecules at any given point in the space. The molecules respond to their own private vapour pressure gradient and move away from high vapour pressure towards low vapour pressure areas. Volatility is a measure of how fast each type of molecule moves for given vapour pressure gradient - thus highly-volatile perfume reaches you with incredible speed, even when there's no air movement to carry it bodily.

Of course the many individual vapours that make up air are doing all of this - but air tightness is about the mass movement of air. The vapour pressure action happens within that air movement - the centre of concentration and the vapour dispersal pattern will be carried along bodily with the airstream, like a hot air baloon rushes overhead but to the occupants it seems they're in still air because they're moving with the breeze.

You can have membranes that do one or or both or neither. E.g. polythene is air-tight and water-vapour-tight (but more transparent to other vapours); Tyvek is fairly air-tight and almost vapour-transparent; anything open-weave is more or less transparent to both. As Biff says, an apparently impervious balloon puts up considerable resistance to many of the air vapours, but not much to CO2.

You can roughly assess air tightness by blowing through a membrane (I prefer to suck!) but that tells you nothing at all about its vapour resistance/breathabilty.

I believe there are still many aspects of the behaviour of water vapour in buildings which have yet to be explained. For instance, vapour permeable membranes often don't work as anticipated: many people have witnessed condensation forming on the underside of Tyvek and similar. In theory, this shouldn't happen. In practice, it's surprisingly common. Why? The only scientific explanation for this is that there is little or no vapour pressure difference across the membrane, which may be the case, but is also, in theory, unlikely.

Another strange phenomenon, which I have picked up from several years of home observation, is that the absolute humidity levels inside a house are consistently much higher than those outside. This is routinely explained by the addition of water vapour into the air by the actions of the occupants: indeed this is the principle reason why we fit vapour barriers into timber frame walls and roofs. But if this was the case, then you would expect the absolute humidity levels to fall to a very low level when the house is unoccupied. But they don't. Relative humidity levels settle at around 50% to 60% in our house all through the winter, whether there are people in residence or not. It's as if a warm house in winter acts as a vapour sponge and draws water vapour into it. I think this runs somewhat counter to our conventional understanding of how vapour pressure should work, which is that vapour pressures will tend to equalise and that vapour will travel through all manner of materials in order to achieve this.

It's a very simple experiment to run, although its getting a little late in the year to do it now. You just need a few three quid hygrometers scattered around the house, and then note what happens to your RH levels.

True - the steady-state myth, for a start. BuildDesk as a consultancy are leading lights in the committee that runs BR443, so embody those concepts that the multifoil guys have called into question. (light blue touch-paper and stand back!)

Posted By: fostertomTrue - the steady-state myth, for a start.

We should have a vote on that one I think

Love to, but how do I set it up? Kieth?

The last guest comment was from me, forgetting to log in. Must say, I find this new system rather daunting. Or maybe I am just getting too old for all this new fangled technology.

Firstly thanks very much to everyone who contributed to this - it's been fascinating, informative and very comprehensive. Secondly, may I ask a follow up? The timber frame has been going up this week (2x18mm vertical overlapped WRC cladding, 25mm batten, 9mm Panelvent, 150mm stud, 150mm damp sprayed warmcell, Paneline and service gap with vapour check and lining). At present there is no vapour permeable membrane on it and I have had conflicting advice over whether to use one over the panelvent. Excel said not needed while others have said I said I should use a vpm.

i have just been searching like mad until trying to find a comparison chart published as an advert for permavent. they used data available through bbacerts and collated it to form a chart comparing performance but not cost.
was published in professional builder magazine that is feee in builders merchants. publishers - hamerville.co.uk

anyway if you want it i can forward a scanned copy to a document dump if any knows of one or has a server for this kind of thing.

hey for keith - how about the ability to post related links and upload documents that can be listed alongside the forum string - theres a big void on the left side of the page below all your own stuff for a couple of boxes. and a bit of code for voting shouldnt be that hard to get.