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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorJulian
    • CommentTimeMar 23rd 2007
    Timber frame extension: trusses at 600 centres, reclaimed concrete Double Roman 50's.

    Is Tyvek really worth the extra money or are you simply paying for the DuPont brand? (The nice people who make the Teflon that's sprayed on my kids school uniforms). There's a range of prices for these membranes; are the cheaper ones less breathable than the Tyvek?

    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 :bigsmile:

    Hi julian, Can I ask whether your insulation is to be at ceiling level or between rafters? If between then a breather membrane is essential, as is a vapour barrier on the warm side of the insulation.
    • CommentAuthorJulian
    • CommentTimeMar 25th 2007
    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...
    Hi Julian, I have heard that using varying density boards such as Heraklith is an alternative to breather mambranes and vapour barriers, but I have not investigated enough to really comment. Your method for checking which membrane is best seems fair enough to me.

    Have you checked their Certification? I suppose if they all carry a similar BBA? then they must all do the job okay
    • CommentAuthorJulian
    • CommentTimeMar 25th 2007
    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.
    Posted By: JulianI 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...

    A breathable membrane does not mean that it's supposed to allow air to pass - quite the opposite. It is supposed to stop air passing through but allow water vapour - just like GoreTex brand clothing. If you can blow air through the membrane, something is not right!

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

    Paul in Montreal
    • CommentAuthorJulian
    • CommentTimeMar 26th 2007
    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!
    • CommentAuthorGuest
    • CommentTimeMar 27th 2007
    Julian - you have made me really laugh!! am wondering if you can continue your unscientific experiments by holding the materials over a steaming kettle or pan of water - would be interested in the results!floaty sarah
    • CommentAuthorGuest
    • CommentTimeMar 27th 2007
    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!
    • CommentAuthorNigelpit
    • CommentTimeMar 29th 2007
    You have to differentiate between vapour permeable and air permeable membranes. The "breathe through" test mentioned therefore would have proved nothing I'm afraid.What you have to remember is the purpose they serve, that of preventing build-up of condensation in the building (more often the roof). The initial point about cost of a branded product is fair, but then again why should Du Pont be the cheapest (no I don't work for them!!). What I would urge is to check the supporting performance data of the product chosen, and to test the manufacturer for his technical knowledge of the product. Some of the suppliers do not manufacture and have a limited understanding of the way they work. As such, therefore, some of the guarantess given do not stand close inspection. As such therefore, it is invariably best to go with a product which has a track record (and there are others than Tyvek). Since changes in British Standard, BBA certification is the most common way to make comparison between products, and as I write, there are well over 40 of these. Beware too of builders' merchants selling supposedly like-for-like products, when a little examination will show that this is not the case. They often go for the cheapest, treating the item as a commodity and sell for as much as they feel the market (and often the individual customer) will stand. Products do differ in thickness and base material - polypropylene is used for example, and there are 2,3 and 4 layer materials.

    Hope that helps and sorry if I have gone over ground covered already
    • CommentAuthorbiffvernon
    • CommentTimeMar 29th 2007 edited
    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.
    • CommentAuthorGuest
    • CommentTimeMar 30th 2007
    Breather membranes usually have to fulfil 2 functions---to be able to allow water vapour molecules to pass through, and to be water resistant. They must not be air open as this would lead to convection heat losses from within the designed system. Windtightness is desirable to maintain thermal efficiency.The market leading membranes have maintained their position due to investment in R & D, many of the others have only seen a market opportunity to sell a cheaper material with little or no technical backup. With regard to the vapour transmission rate it is better to look for the gm/m2/24hr figure as this is easier to relate to the ' litres per day ' of water vapour production quoted for domestic conditions. The term 'breather membrane' has come about due to its origins in the timber frame industry where that terminology was adopted in the 1970's. It's rather like 'ventilation' in a roof- there is nothing remotely connecting ventilation as indicated in the Buiding Regulations and natural air movement which is what is required.
    • CommentTimeMar 30th 2007
    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.
    • CommentAuthorMike George
    • CommentTimeMar 31st 2007 edited
    Posted By: fostertom
    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.

    After your thorough explanation I have to agree:shamed:
    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.
    • CommentAuthorbiffvernon
    • CommentTimeMar 31st 2007
    So models like BuildDesk are only as good as the state of knowledge of the person who wrote the software. Whoops.
    • CommentTimeMar 31st 2007
    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!)
    • CommentAuthorGuest
    • CommentTimeMar 31st 2007
    Dear Mark Brinkley

    I was very interested in your post about your home remaining at about 55% RH even when unoccupied in winter. I have been concerned about indoor air quality for some time. I can’t seem to get my house (two brick skins with filled cavity and very airtight) less that 50% even with dehumidifiers. Which seems to prove, to me at least, that the walls are allowing water vapour to penetrate as the air inside is processed. I really would like/need about 40% RH for health reasons. I now looking at the info sheets of the monocoque house from the other forum thread. It looks like this house is built from a continuous outside skin for walls, base, roof and intermediate floors, everything seems to be stiffened and supported by beams which look to be about 15mm thick and, it does actually state, 200mm deep. If the total outside skin is impermeable with no seams or joints as is claimed does this mean that with dehumidifiers I could reduce the inside moisture to a level whereby all dustmites bacteria and viruses are unable to sustain life? A greener question would be, how much vapour could be removed from a totally sealed impermeable house structure using a well designed central stack ventilation system? Also if the indoor water vapour is controllable to such a high degree I wonder how this would sit with building regulations? What damage could be caused to a house structure apart from shrinking furniture and dead dustmites? I am a building developer by the way.
    Posted By: fostertomTrue - the steady-state myth, for a start.

    We should have a vote on that one I think:devil:
    • CommentTimeMar 31st 2007
    Grrwoof! go on then
    Love to, but how do I set it up? Kieth?
    • CommentAuthorGuest
    • CommentTimeApr 1st 2007
    Guest who wants to reduce RH to below 40%.

    I think you will find it very hard to reduce your RH to below 40% in the British climate. The problem isn't that vapour is somehow forcing itself through your building fabric, it's coming in with the air changes which you have to have whether you like it or not. Fans and dehumidifiers are good at reducing RH from very high levels, over 70%, but once you get to the background level around 50% RH, then it appears further reduction just doesn't haappen.

    It seems the moment you close your windows and turn on the heating system, you up the levels of absolute humidity inside the house, whether there are people there or not. That may or may not have something to do with explaining the rising incidence of asthma and/or house dust mites - I would hesitate to jump to any conclusions just yet! As I say, these are observations taken from my own house - that's a long way short of being established science. It may not happen in other people's houses and it may well not happen in different climates.
    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.:shocked:
    • CommentAuthorGuest
    • CommentTimeApr 1st 2007
    The humidity inside a house is relative to the temperature. The ability of air to hold moisture is controlled by its temperature. In my sun lounge which faces due south.
    Over the period, 21,22,24,28 and 29th March 2007 I experienced lows of humidity of 20% with matching temperatures around the middle twenties.
    I think, this is due to my keeping the thermostat at 23.6 degrees 365/24.
    This avoids the wild temperature swings and matching humidity swings that occur in properties where the heating is turned off for periods (like overnight or when unoccupied) In these latter instances the temperature plunges - the excess moisture drops out soaking into the cold walls and running down the windows.
    Not good for the fabric of the building.
    Incidentally house mites breed very well in humidities exceeding 70%
    • CommentAuthorJulian
    • CommentTimeApr 2nd 2007
    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.
    • CommentAuthorGuest
    • CommentTimeApr 9th 2007
    Just wanted to add my two cents worth to your forum on vapour permeability and the various membranes out in the market place and in terms of being able to check one against another permeance wise you only need to go to the BBA web site and download the various BBA certs http://www.bbacerts.co.uk/query3.html on the products in question.

    There are better performing membranes out there (even better than the generic DuPont products) and yes they do come with a slightly higher price tag but you tend to pay for what you get these days.

    Read with interest too Paul's comments on the what a breathable membrane should and shouldn't do and would comment that for Canadian standards you can meet air tight requirements for code whilst having an air open, vapor open, water tight breathable membrane. Breathers therefore can be either air tight, vapour open or as before air open vapour open.

    One last point mentioned that drew interest was an unexplained moisture build up under some lower perm products. This can be explained as simply a vapour movement wanting to take the easiest route out of the construction pushing around the edges of the sheathing boards creating a heaver then normal flow / build up and where lower products may be able to cope with the vapour flow though the middle of sheathing boards they fail to handle the heavier flow around the edges etc... Seen it on site often on the US side of the pond where heavier vapour drives and higher precipitation occur...
    • CommentAuthorGuest
    • CommentTimeMay 4th 2007
    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.
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