Oi

I heard that.

What exactly is the point of breathability then?

I'm open-minded about 'breathable' techniques. I'm a fan of straw bales and of Kooltherm and aerogel, each in the appropriate place. But nobody else seems to have tried to answer this question so here goes. I think there are two reasons:

(1) Historically, this is how buildings were built, before the invention and widespread use of impermeable materials. So if you want to maintain old buildings, you have to understand their design principles to avoid the problems that can otherwise occur (cement renders and mortars on stone, for example)

(2) If you want to build modern buildings from 'natural' materials, perhaps because of moral or philosophical concerns, then there can be problems with condensation because many such materials can rot and it can also be difficult to provide vapour barriers to protect such materials. And expectations about insulation and about restricting air movement are greater today. So you need to understand how to create buildings that allow vapour transport without bulk air movement.

Well, that's my take on it anyway.

>What exactly is the point of breathability then?</blockquote>
I'm open-minded about 'breathable' techniques. I'm a fan of straw bales and of Kooltherm and aerogel, each in the appropriate place. But nobody else seems to have tried to answer this question so here goes. I think there are two reasons:</div></blockquote>

I agree with you Djh, I said pretty much the same thing on page 3 of this discussion:

* bot de paille
* CommentTimeSep 25th 2009
quote
Breathability is a very valid term for use by those who work on renovating old buildings and new buildings that use "natural" building materials. When working with old buildings problems can often be created when modern materials are used which block or trap moisture inside a wall that could originaly breathe out moisture with variations in temperature and humidity.

Posted By: Paul in Montreal

Posted By: skywalkerWhat else are going to kill off today!

*cough* multifoils *cough*

Eh? Who? Where?

Posted By: TunaSurely the opposite is true? If you start off assuming there is no route for internal vapour to be released (ie. VCL is intact), and design the building to manage that (with passive ventilation, MHVR etc. etc.), then the case where the VCL fails is more than adequately dealt with?

I don't understand this Tuna, as far as I can see the whole point about breathability seems to start from the question 'what happens when some water does get in there?' If you assume that it will, then you will want hygroscopicity, vapour openness and capillary openness, so that the water will be spread out and be dispersed to the outside (or inside).

If you don't do that, how are you protecting against the inevitable ingress of water?

if you assume that water will not get in, what happens when it does?

Peter

Posted By: fostertomKnow that they will significantly fail sooner rather than later and that the building's vapour control strategy will therefore fall apart.

Precisely.

Posted By: fostertomKnowing that, what do you do instead of relying on strongly resistant membrane VCLs?

The same thing that has allowed timber frame buildings in the UK climate to survive for centuries, assume the worst and arrange for the building envelope, the walls and roofs themselves, to deal with the inevitable aquatic assault, through the materials they are built from and the detailing.

Of course, it would be good to update the traditional approach to take account of the requirement to stop burning loads of fossil fuels to keep the place habitable. That is the challenge as I see it.

Tom, Peter, taking into consideration what you've just said, I have a scenario I would like you to pick holes in [in the context of breathability being necessary]

This is my standard sloping ceiling scenario. From Inside to outside

Plaster, painted with matt emulsion
12.5mm standard plasterboad fixed with 45mm galvanised clouts at 400mm centers. All board edges fixed into timber noggins
Cheap polythene lapped accross at least 2 rafters and fixed with standard mild steel staples. Polythene lapped down walls to ensure continued air barrier at walls
30mm PUR insulation fixed with 65mm clouts to underside of rafters. Joints filled with expanding PUR foam
150mm x 50mm rafters at 400mm centers. Voids filled with 100mm PUR flush with rafter undersides and deliberately cut to leave 10mm short of rafters. The 10mm filled from above and below with expanding PUR foam.
Clear 50mm cavity between rafters above insulation
Breathable roofing membrane
Slates/Tiles on treated roofing batten
Eaves vented with continuous overfascia vents
Ridge vented with dry ridge ventilation system.

Some would argue that some parts of this construction make-up are unnecessary [such as a breather membrane]. But other than varying the insulation thickness to improve u-value, I believe it to be a belt and braces approach - Is it?

Posted By: Mike GeorgeI believe it to be a belt and braces approach - Is it?

I don't know, probably it is, my house has roofs constructed in this way.

The general problem is that IF some water gets in to that construction it may be hard to get out again. That might be all right too, depending on how much gets in and where.

If a tile slips and is not noticed for a while, water could run onto the breather and through into the rafter/insulation. Might the damp have a detrimental effect on the rafters? If the insulation in between and below the rafters was hygroscopic, it would protect the rafters. If it had good capillarity, the water could run throughout the structure and not pool. If it was very vapour open in addition, it could dry out faster. This would happen especially if the 5:1 ratio of vapour openness was present.

If a hole is created in the plaster/vapour barrier, eg by picture hooks, water vapour could be getting in there, at a very low rate, but maybe continuously, day and night, for months on end, each year, year after year.

A sloping ceiling/roof is probably the safest bet, as I am sure you were aware when you chose it. What about the same kind of construction for a wall – with electrical sockets and service penetrations through to the outside?

It all depends on the plastic vapour retarder being installed perfectly, is this realistic? What about the joints between the plastic sheets? Will they be perfect? How long will the adhesive last?

Just my thoughts, I would be glad to hear what anyone else thinks.

Peter

Posted By: Tunawhen we are talking about vapour ingress as opposed to condensed water

I may be missing something.

As far as I can see vapour ingress is only a problem if it condenses once it is in there, if it does not condense, probably there is no problem?

But if it does condense, even if there is only a tiny hole, or lots of tiny holes, if it goes on getting in for months day and night and condensing, year after year? And there is no 'breathabilty' to let it get out again?

I certainly won't be 'intervening' (as you put it) for you in any future subjects djh!

I believe you have just used my polite enquiry on your behalf to obfusticate the discussion at hand.

Posted By: djh

For the record, I asked you to ask them on my behalf why I hadn't received the documents. Subsequently, a senior man from Kingspan said "Having picked up your concerns via Keith Hall, I did some digging and found that indeed your request had not been acted on. Through oversight rather than malice. Please accept my apologies and a copy of the CAR report. I can not force you but would ask that you respect the caveat that you do not onwardly circulate this document. If anyone else wishes to have a copy, please refer them to me on this email address. The reason for this is that Kingspan wishes to track to the best of its abilities, who has a copy."

So I believe intervention is an entirely appropriate word to use and I fail to see why it offends you. I for one am not questioning either their integrity or their environmental concerns but I am questioning their standards of customer service.

I don't like being used.

...but he asked you politely to help... then you helped... then you told everyone you'd helped... then Dave told everyone how helpful you were...

don't think any harm was intended...



J

.

OK so I'm over-sensitive.

Sorry Dave

Picture a plastic bag full of water. It's got a pinhole but someone says don't worry, it's only one tiny one. But overnight the bag's nearly emptied itself and the carpet's seriously wet. That's what a pinhole or hairline in a VCL is like. Any hole at all ruins it. Over a period that's very short compared with the length of a winter season, that pinhole will pass great quantities of water vapour - quite enough to cause real trouble to an un-robust design of wall-insulation sandwich, i.e. a design that relied on the (false) assumption that the VCL would function forever as intended.

The bag empties itself because the hydrostatic pressure on one face of the pinholed bag is higher than atmospheric pressure on the other face. The differential pressure drives a flow through the pinhole.

In the case of the pinholed VCL membrane, it's an indoor/outdoor differential in the 'partial vapour pressure' that the water vapour molecules 'see' - the water-vapour PVP is higher on the indoor face than the outdoor face of the pinholed VCL membrane. The PVP differential drives water vapour molecules to squirt through that pinhole. This happens even if atmospheric pressure is identical on both faces and there is nothing that would cause airflow through the pinhole.

You may say that such a PVP differential is small compared to the hydrostatic pressure differential in the bag of water example, so wouldn't drive such a flow - but then the viscous resistance of water vapour is also small compared to that of water. So that small PVP differential can drive a powerful 'squirt' of water vapour into where it may cause trouble, just as the drip-drip of water will empty the bag overnight and soak the carpet.

For each individual gas/vapour in a mixture of gasses, the PVP for each gas/vapour is proportional to (or a function of) the local concentration of that particular gas/vapour. Thus, in air, which is a mixture of gasses, nitrogen, oxygen, argon, CO2 and water vapour (and many more) each have their own PVP. Each gas/vapour molecule 'sees' only its own kind, and is repelled by them!- while ignoring the others.

So when some water evaporates to vapour, there's a local concentration of water vapour, which creates a local peak in water vapour PVP, relative to surrounding ambient water vapour PVP. In other words a gradient arises in water vapour PVP and the water vapour molecules are powerfully moved by that gradient, i.e. they repel each other, away from the centre of the local mollecular concentration. This continues until the local concentration is completely dispersed and evened-out with the surrounding ambient concentration. And while this is happening, all the other gas/vapour molecules are completely oblivious and unmoved by the repulsive drama that the water vapour molecules are playing out amongst and between the other molecules!

Water vapour movement is neither caused nor ameliorated by air movement. It's driven by PVP, almost completely independent of air movement aka ventilation. Once water vapour is created, in no time at all it's spreading and dispersing through airspaces and deep into vapour-permeable 'solids', blocked by VCLs but squirting through any pinholes. To repeat, water vapour migrates fast through any permeable medium, without disturbance to that medium. The air may be static or moving, x airchanges per hour may be happening, makes no difference, the water vapour 'flashes'outward until it's completely 'mixed' and equalised with surrounding ambient water vapour.

The only way that air movement can prevent that process is to create such a hurricane-velocity of airflow, that it exceeds the vapour dispersal velocity. This is attempted in chemistry lab fume cupboards and catering cooker hoods. Usually that results in a very high airchange rate but that's not the point - it's the velocity that does the job. And even so there are plenty of extra-volatile vapours, both in the lab and in the kitchen, whose dispersal velocity exceeds the air velocity and so still work their way 'upstream' and escape, causing smells.

No normal ventilation system attempts that trick. The idea that ventilation 'carries away' water vapour (and body odours!) as they're produced is completely false. What ventilation does is bring in fresh air of 'ambient' vapour content, thus slowly diluting the higher concentration that remains as post-dispersal equilibrium. PVP in the room slowly falls, the PVP gradient reverses, and the vapour that initially dispersed may return the way it came.

Any learned paper that discusses water vapour movement as a function of air movement, is either ignorantly unscientific, or is written by the marketing department.