General: Mu values and breathability. Stated values and the perfect construction

1 to 30 of 87

- CommentAuthorMike George
- CommentTimeJul 22nd 2012 edited
There is much discussion here and elsewhere relating to 'breathability' of structures. Critical to how this 'mechanism' works or [or does not work] is the placement of materials based on their relative Vapour Resistance Factor [Mu value]. The understanding of this is however confused by the way in which values are sometimes stated:

Mu values (no units)
Vapour Resistance (MNs/g) or even
Vapour Resistivity (MNs/gm)

are all used as a measures of a materials reluctance to allow water vapour to pass through it/them.

A good document explaining the conversion between these can be found here http://www.builddesk.co.uk/files/BuildDesk_UK/Home/Software%20support/Vapour%20Resistances%20and%20Mu%20values.pdf

This is all very muddled :( especially when considering the placement of insulation / breather membranes / vapour control layers etc.

Manufacturers sometimes promote their products as fitting the breathability concept. This is in turn sometimes based on rules of thumb for relative 'resistance' to vapour.

I'd like to understand all of this to a level where I can competently justify insulation placement based on not just thermal properties but Hygrothermal properties as well.

A good starting point for me would be justifying the breathability rule of thumb by actual numbers for example constructions. I've read that the internal surface 'resistance' should be 3 - 5 times greater than the outside, with presumably a gradient in between.

So what is 'the perfect breathable construction/ and what are the associated hygrothermal properties?

Anyone care to raise their head above the parapet?
- CommentAuthorSteamyTea
- CommentTimeJul 22nd 2012
I suspect that it is similar to the 'thermal mass' argument and has several variables.
One being the ability to retain/release moisture, another being the air permeability (or leakiness), then there is the actual thermal inertia, which affects the evaporation rate, as does the permeability. There is also the two climate regimes to worry about, internal and external, plus the affects of wind and sun.
Feel a case of the 'solutions to partial differential equations' coming on.

Is it possible to break each and every component down into SI units and then do some algebra (jiggery pokery). JSH knows how to make a container have a fixed humidity with some copper sulphate and other colourful crystals/chemical things. That would be a good way to test how long something takes to dry out, would not be hard to make an small 5 sided pressure chamber with some pressure loss sensors in it to see how long it takes to loose pressure though the test material. Used to do that when making air filters for cars and mines with a vacuum cleaner and manometer.
Temperature drying could be done with a weighing scale and heater.
Would be a fun thing to do, you up for it?
- CommentAuthorMike George
- CommentTimeJul 22nd 2012 edited
Thanks Steamy - In my dreams - As ever you surpass me with your ability to design a test method to answer my questions but my very limited experience of testing anything in a lab are over!

I am sure that these figures are already out there. It just that there doesn't seem to be a uniform way that proponents and/or manufacturers of materials report them [if at all] Its a bit like trying to work out what your gas bill will be with various suppliers.

There is a British Standard (12524) which lists Mu values but this lacks some pertinent materials (particularly some insulations) Sheeps wool, Hemp, Flax, Recycled plastic, aerogel, multifoils etc. The latter being used in a scenario discussed here where you would expect a degree of vapour permeability....http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9371&page=1#Item_18

Since some of the above insulations are closely associated with the breathability argument it seems reasonable to me that someone has doen the legwork and come up with [what they believe to be] the perfect construction.

Soooooo....over to them.. any takers?
- CommentAuthorTimber
- CommentTimeJul 22nd 2012
Well a 1/1 wall will work, e.g. wood fibre board inside and out filled with warmcell. I can't run the numbers at the moment as I don't have my work computer to hand, but I have done something similar before and it passes a condensation risk calculation fine. You could use a breather membrane (high spec) inside and out for the air tightness layers. The reason this works is that all the layers have almost identical breathabilites (the warmcell having a slightly high resistance than the WF boards. IN this case there is no one layer as a limiting factor to moisture migration, so it just passes though without getting 'stuck'. The moment you put a low resistance insulation between studs (or wherever) and a higher resistance material on the outside, that is when the 5/1 rule starts to kick in.

As for manufacturers data, Mu is a vapour resistance compared to air, so air has a Mu of 1. The other two are usually derived from wet or dry cup moisture vapour resistance tests (and other sources). So that is part of the reason quoted data can differe... i.e. how was it tested. Once you get your head around it all, you can convert and understand units fairly well. I tend to try and remeber each of the above units for OSB, and use that as a mental guage of how breathable any particular material is.

As for perfect construction, there is none, but I do like the idea of a 1/1 wall but have never tried it in reality. Just a mental excercise.
- CommentAuthorMike George
- CommentTimeJul 22nd 2012
That's very helpful. Thanks. I was wondering about the comparison with air... need to get my head around that. [head scratching smiley]
- CommentAuthorSteamyTea
- CommentTimeJul 22nd 2012 edited
Do you have to drop into dimensional analysis here?
So Force is (Mass x Length)/Time squared

then

Vapour resistance becomes:

(((Mass x Length)/Time squared) x Time) / Mass(vapour) (think Newton's Second Law is in there F=Ma)
Reduces to
((Mass x Length) / Time) / Mass(vapour) (is this now a scalar dimension, hence leading to no units for Mu)
So
kg.ms^-1.kg^-1(vapour)

Think that gives you the speed or distance or mass or time required to push that little bit of water vapour though the material. Just depends what you know about the material you are testing.

So imagine a weight with some water vapour trapped under it pushing down on a bit of OSB of 18mm thickness. You know that if you keep pushing it will eventually come out the other side. The harder you push, the sooner it will come out. You could also imagine a heavy weight moving very fast and smashing into a bit of OSB, the water vapour that is trapped between the weight and the board will keep moving when the weight stops (assuming it does not ruin the board that is). Converts all its kinetic energy into potential energy and then releases it more slowly I suppose.
You just have to push very very hard, hence the M before the Newtons, or you will have to wait a very long time (1 million seconds is about 12 days).

Question is what happens to the OSB in that time. Is it enough time to soften adhesives, cause swelling or start mould growth?

Probably a load of old rubbish what I have typed but been out and had a couple of coffees watching the town band, so mind is tired but eyes are open.
- CommentAuthorMike George
- CommentTimeJul 23rd 2012
Hmm, thanks, I think I need a few more coffees than you to think on this
- CommentAuthorSteamyTea
- CommentTimeJul 23rd 2012
Me to, but I am sure it can be reduced to the energy needed (Joules) to force the moisture though.
We have that in the kg.ms^1 as that is a Watt.
So if we know the properties of the material, say 5000 MN.s/g
All that is saying is to push 1 gram of water vapour though 1 metre of barrier you need 5000 MN.s or 5000 Mkg.L.s^-1 or 5000 million Joules per second (quite an umph)
If the material is 20mm thick (0.02m) then
5000 x 0.02 = 100 MJ.s^-1
So over a day you would push 1157 g of water vapour though it (per square metre I think, though I think I have gone wrong somewhere but I shall carry on).
Now if air has a Mu value of 0.2 and the barrier has a value of 100 then it takes 500 times longer to push the moisture though, or takes 500 time as much effort.
The effort is going to be the same as, within a tiny bit, the pressure in the house will be the same as outside the house.
so 500 / 1157 = 0.43 g.m^-2 of vapour will pass though the barrier.
So not much at all.

Still think I have made a fundamental error somewhere, where is Ed when you need him.
- CommentAuthordjh
- CommentTimeJul 23rd 2012
Posted By: Mike GeorgeI'd like to understand all of this to a level where I can competently justify insulation placement based on not just thermal properties but Hygrothermal properties as well.

A good starting point for me would be justifying the breathability rule of thumb by actual numbers for example constructions. I've read that the internal surface 'resistance' should be 3 - 5 times greater than the outside, with presumably a gradient in between.

So what is 'the perfect breathable construction/ and what are the associated hygrothermal properties?

Hygrothermal properties are to do with how much water vapour a material can aborb and/or subsequently release. Permeability is to do with how much water vapour a material can transmit from one side to the other. They're different but related.

Water vapour goes from an area with high partial vapour pressure (not RH!) to an area with lower partial vapour pressure. That is generally inside to outside in winter. Summer is more indeterminate. In hot muggy conditions, water vapour may be driven from inside to outside (e.g. like the water driven from an unventilated cavity behind masonry into the OSB and timber frame in another thread)

"Breathable" is a combination of permeability and hygrothermal storage. It basically means a wall construction that allows water vapour to pass through without free liquid water forming within the wall.

You can build a "breathable" wall without any hygrothermal content at all, as long as the wall never admits more vapour into the warm side than can get out the cold side at the same time (roughly).

If you add some hygrothermal material then more water can be allowed into the wall than can get out, and the water is stored in the hygrothermal material until some time later (e.g.summer) conditions change so that the water can get out again. If too much water is allowed in or conditions never allow it to escape, then the hygrothermal material will become saturated and liquid water appears in the wall, and rot starts.

Calculating what works and what doesn't is complicated and needs something like WUFI. Traditional ways of calculating condensation risk are known not to work in some cases. The 3-1 or 4-1 or 5-1 rules of thumb are just that and shouldn't really be relied on. Equally straw bales with lime on the outside and clay on the inside violate those rules but appears to work anyway.

I've found anything written by John Straube or Tim Padfield to be very useful. Neil May also knows his onions but has an axe to grind. And there are others.
- CommentAuthorSteamyTea
- CommentTimeJul 23rd 2012
Is there a wall construction design that does not give a problem with humidity, we could try and model that as a 'standard' and then see what happens when elements are changed.
- CommentAuthordjh
- CommentTimeJul 23rd 2012
Posted By: SteamyTeaIs there a wall construction design that does not give a problem with humidity

Lots. Pretty much anything advertised by any manufacturer with a BBA cert.
- CommentAuthorTimber
- CommentTimeJul 23rd 2012
What do you mean by 'a problem with humidity'?

As mentioned, there is no prefect construction, just one who's compromises don't cause a specific problem with your building.

I ran the numbers on a 1/1 wall today using warmcel, wood fibre sheathing boards inside and out, breather membranes inside and out (to act as air barrier) with plasterboard on the inside and a cladding system on the outside. It works, unless you live in a really really cold climate with high indoor RH. If anywhere in the UK with something like MVHR to keep a control on the internal climate it works.

Is that the ultimately 'breathable'/'breathing' wall? Probably not, but it works in both heating and cooling climates. Obviously the wall would not have much racking resistance due to the wood fibre insualtion boards being used for racking resistance, but it could work on a small simple building.

Caveat - I have never built a wall like this personally, but the calculations stack up for moisture vapour movement and condensation risk.
- CommentAuthorMike George
- CommentTimeJul 23rd 2012 edited
We could call it timber's frame (sorry)
- CommentAuthorMike George
- CommentTimeJul 23rd 2012 edited
Posted By: djh
Posted By: Mike GeorgeI'd like to understand all of this to a level where I can competently justify insulation placement based on not just thermal properties but Hygrothermal properties as well.

A good starting point for me would be justifying the breathability rule of thumb by actual numbers for example constructions. I've read that the internal surface 'resistance' should be 3 - 5 times greater than the outside, with presumably a gradient in between.

So what is 'the perfect breathable construction/ and what are the associated hygrothermal properties?

Hygrothermal properties are to do with how much water vapour a material can aborb and/or subsequently release. Permeability is to do with how much water vapour a material can transmit from one side to the other. They're different but related.

Water vapour goes from an area with high partial vapour pressure (not RH!) to an area with lower partial vapour pressure. That is generally inside to outside in winter. Summer is more indeterminate. In hot muggy conditions, water vapour may be driven from inside to outside (e.g. like the water driven from an unventilated cavity behind masonry into the OSB and timber frame in another thread)

"Breathable" is a combination of permeability and hygrothermal storage. It basically means a wall construction that allows water vapour to pass through without free liquid water forming within the wall.

You can build a "breathable" wall without any hygrothermal content at all, as long as the wall never admits more vapour into the warm side than can get out the cold side at the same time (roughly).

If you add some hygrothermal material then more water can be allowed into the wall than can get out, and the water is stored in the hygrothermal material until some time later (e.g.summer) conditions change so that the water can get out again. If too much water is allowed in or conditions never allow it to escape, then the hygrothermal material will become saturated and liquid water appears in the wall, and rot starts.

Calculating what works and what doesn't is complicated and needs something like WUFI. Traditional ways of calculating condensation risk are known not to work in some cases. The 3-1 or 4-1 or 5-1 rules of thumb are just that and shouldn't really be relied on. Equally straw bales with lime on the outside and clay on the inside violate those rules but appears to work anyway.

I've found anything written by John Straube or Tim Padfield to be very useful. Neil May also knows his onions but has an axe to grind. And there are others.

Thanks Dave,

If I'm understanding you Breathability is not 'measured' by Mu values or for that matter vapour resistance/resistivity. Is that what you are saying?
- CommentAuthorSteamyTea
- CommentTimeJul 23rd 2012
Makes it analogous to thermal inertia.
So we back at the start again now?
- CommentAuthorMike George
- CommentTimeJul 24th 2012
Yes, I think so :(
- CommentAuthorSteamyTea
- CommentTimeJul 24th 2012
MVHR then
- CommentAuthorMike George
- CommentTimeJul 24th 2012
Nah, I just need someone to buy me WUFI
- CommentAuthorfostertom
- CommentTimeJul 24th 2012
You're right Mike - short of that we're all clutching at straws and rules of thumb, because it's clear that moisture isn't realistically represented by the simplistic euler model, which we in UK climate seem to get away with more by luck than science.
- CommentAuthordjh
- CommentTimeJul 24th 2012
Mike George wrote: "If I'm understanding you Breathability is not 'measured' by Mu values or for that matter vapour resistance/resistivity. Is that what you are saying?"

That's right, yes. They are part of the equation but not the whole story (I'll take one bag of mixed metaphors, please). Just like the lambda or U-value of a sheet of foam doesn't tell you the thermal performance of a whole wall, especially if the wall contains some heavy components.

There's a free version of WUFI: http://www.hoki.ibp.fhg.de/wufi/download.php?id=9
- CommentAuthorMike George
- CommentTimeJul 24th 2012
Thanks, So the 'whole equation' can only be calculated with a package like WUFI?

And there is no simplistic indication of what will and will not work based on published values?
- CommentAuthorfostertom
- CommentTimeJul 24th 2012
The rules of thumb like outer = 5x inner, and the euler programs, seem to 'work' i.e. no disasters follow.
- CommentAuthorSteamyTea
- CommentTimeJul 24th 2012 edited
Euler

e^(ix) = cos(x) + i sin(x)

Can you explain how the imaginary part works?
- CommentAuthorMike George
- CommentTimeJul 24th 2012 edited
Posted By: fostertomThe rules of thumb like outer = 5x inner, and the euler programs, seem to 'work' i.e. no disasters follow.

Hi Tom, as ever thanks for the input.

Any chance you could list a hypothetical [Part L compliant] construction based on the 5 : 1 principle for an existing wall scenario? Also when you say 5:1, what values are you relying on?
- CommentAuthordjh
- CommentTimeJul 26th 2012
Posted By: Mike GeorgeAny chance you could list a hypothetical [Part L compliant] construction based on the 5 : 1 principle for an existing wall scenario? Also when you say 5:1, what values are you relying on?

I'd suggest looking at the wall constructions on the NBT site - both the Pavaclad & Diffutherm - to get an idea. Typically a 'breathable' wall has a fairly vapour resistant inner layer (blocks or a board) then some insulation and then a fairly vapour permeable outer layer (board or render etc). The insulation is often the most vapour open layer. In that case, the 5:1 refers to the ratio between the inner and outer layer. If the insulation also significantly affects things, or if there are yet more layers, then you probably need to look more closely than a simplistic rule of thumb.
- CommentAuthorfostertom
- CommentTimeJul 26th 2012
Posted By: djhthe 5:1 refers to the ratio between the inner and outer layer
You mean the layer(s) in between don't matter (as long as 'below the curve' in resistance) - so it's not necessary to achieve a fairly smoothly curving resistance gradient? I'd say prob not necessary.

I still am not clear whether we're taking about Resistance, or Resistivity of the 2 layers. I'm inclined to think it's Resistivity i.e. about the local gradient of the curve, not the area under the curve. What do you think?
- CommentAuthordjh
- CommentTimeJul 26th 2012
Posted By: fostertomYou mean the layer(s) in between don't matter (as long as 'below the curve' in resistance) - so it's not necessary to achieve a fairly smoothly curving resistance gradient? I'd say prob not necessary.

That's my understanding, too, but I don't claim to be an authority.

I still am not clear whether we're taking about Resistance, or Resistivity of the 2 layers. I'm inclined to think it's Resistivity i.e. about the local gradient of the curve, not the area under the curve. What do you think?

I think the issue is resistance rather than resisitivity. What matters is the rate that vapour enters and leaves the wall, which is determined by the resistance. It doesn't really matter how thick a piece of material you use to achieve the resistance (typically though the thicker materials will also be hygrothermal, which complicates the picture).
- CommentAuthorEd Davies
- CommentTimeJul 26th 2012 edited
Isn't the thing that matters the ratio between the thermal and vapour resistivities of the layers? I'd have thought you want the inner layers to have a higher vapour/thermal resistivity ratio than the outer layers so the initial vapour pressure gradient is steeper than the initial thermal gradient so keeping the RH in the wall no (or not much) higher than indoors.

Doing a condensation risk analysis for steady state internal and external temperatures and relative humidities and a one-dimensional construction is pretty simple - just graphing the temperatures and humidities and seeing that the temperature doesn't drop below the dew point at any point in the wall (that is, the dew point in the material in question - e.g., condensation in wood starts to happen when the RH reaches about 95% I believe). Doing this in 2D or 3D (e.g., for studs or rafters in insulation) is a bit more computationally complicated but not conceptually difficult, though graphic display of the results would also be a bit trickier.

Without having looked, I assume that Ã¢â‚¬Å“properÃ¢â‚¬Â CRA (BS, WUFI, etc) has to take into account more dynamic conditions, at least externally to the house. Is there a standard for this? Also, do they allow occasional condensation in cases where evaporation is likely to happen pretty quickly so having to take more account of the hygroscopic properties of the materials?

Are there any freely available documents on the subject?
- CommentAuthorWillInAberdeen
- CommentTimeJul 26th 2012
+1 what Ed said

The water content of wood depends on the RH. See graph http://en.wikipedia.org/wiki/Equilibrium_moisture_content

(This bit is about RH, not partial pressure, although as was said previoulsy, partial pressure is what determines everything else in the moisture transport process, so water can diffuse from inside to outside, even when its 100% RH and raining outside, so long as its warmer inside than out).

So to keep the wood dry enough to avoid rot, you need to keep the RH at the wood layers of the construction below about 80% (IE keep all the wood bits quite a good margin warmer than dewpoint temperature).

None of the above seems to apply in my old granite house, where the moisture mostly comes inwards through the porous walls due to rain/wind, not outwards through the walls due to diffusion. Only in one wall where somebody put a cement render on the outside, which has cracked and lets water into the stone when it rains, but wont let it diffuse out again.
- CommentAuthoradwindrum
- CommentTimeJul 27th 2012
Lots of theory, however has anyone got the perfect construction?

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General: Mu values and breathability. Stated values and the perfect construction