Green Building Forum - Mu values and breathability. Stated values and the perfect construction Tue, 19 May 2026 16:46:10 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=152832#Comment_152832 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=152832#Comment_152832 Fri, 27 Jul 2012 09:16:14 +0100 SteamyTea Posted By: adwindrumLots of theory, however has anyone got the perfect construction?

Non-breathable and 100 moisture repellent :bigsmile:

Edit:
AKA Open a window]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=152836#Comment_152836 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=152836#Comment_152836 Fri, 27 Jul 2012 11:28:03 +0100 fostertom Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=152864#Comment_152864 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=152864#Comment_152864 Fri, 27 Jul 2012 22:45:01 +0100 Viking House http://www.viking-house.ie/downloads/Spokane.pdf
http://www.viking-house.ie/downloads/Tsong79.pdf

Don't get too hung up on wall breathability, concentrate on airtightness, WUFI often shows wall failure only when the humidity is increased to 80%.
The Passive House Institute say its irrelevant what the wall build up is if the airtightness is below 1ACH.
There has never been a documented case of decay in a wall due to moisture movement through a wall by diffusion alone!]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162332#Comment_162332 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162332#Comment_162332 Wed, 28 Nov 2012 20:57:41 +0000 Mike George
For those equally confused it is a measure of the material’s relative reluctance to let water vapour pass through, and is measured in comparison to the properties of air. It is calculated from other published values as in the document here
http://www.builddesk.co.uk/files/BuildDesk_UK/Home/Software%20support/Vapour%20Resistances%20and%20Mu%20values.pdf

The image below gives the values stated in BS EN 10456. Building materials and products. Hygrothermal properties. Tabulated design values and procedures for determining declared and design thermal values.

Given the details below, it seems particularly difficult to apply the 5: 1 rule to many existing buildings. Anyone care to pick their favored construction from the list below?

I'm beginning to think that there are some golden rules to be followed in upgrades for example, but the 5:1 rule is to be taken with a pinch of salt]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162356#Comment_162356 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162356#Comment_162356 Thu, 29 Nov 2012 09:43:45 +0000 fostertom Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162357#Comment_162357 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162357#Comment_162357 Thu, 29 Nov 2012 09:43:48 +0000 Timber
We know that there are potential risks with IWI on existing masonry walls if you put lots of insulation in (although this is another area where concrete proof of this is difficult to find!).]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162418#Comment_162418 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162418#Comment_162418 Thu, 29 Nov 2012 23:40:31 +0000 rhamdu Posted By: Viking HouseThere has never been a documented case of decay in a wall due to moisture movement through a wall by diffusion alone!
Can you say that again louder?]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162419#Comment_162419 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162419#Comment_162419 Fri, 30 Nov 2012 07:30:43 +0000 Mike George Posted By: fostertomAnything on EPS, plywood? Along with OSB, I'd have thought both those wd be wideish-range figures, like several of those above. Strange that cem (?) render is so different from conc.

EPS around 100,000.

I was also surprised by the cement. Noteable that it is the *same* as Lime.

This destroys one of the arguments for Lime over cement. ie that it allows the passage of vapour more easily. Clearly this will depend upon the composition of the mix.

Concrete has a higher value as it necessarily contains a higher proportion of cement (to sand) and also chippins. Hard Limestone?]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162420#Comment_162420 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162420#Comment_162420 Fri, 30 Nov 2012 07:39:31 +0000 Mike George Posted By: TimberWhy is it hard to apply the 5:1 rule in existing buildings? Un-insulated buildings kind of fall outside of that rule, as the heat loss through the structure reduces the risk of condensation. Most types of building with insulation in them generally follow the 5:1 rule or get close to it.

We know that there are potential risks with IWI on existing masonry walls if you put lots of insulation in (although this is another area where concrete proof of this is difficult to
find!).

Isn't one of the arguments for the 5:1 rule that this is the way things were done historically? Maybe I've got that wrong. Clearly it is not the case.

Cavity walls don't meet the rule
Stone buildings don't meet the rule
Timber frame buildings don't meet the rule.

Of course if you ignore facing materials such as gypsum/ plasterboard/render/lime then they all do - but it's not a rule then is it?

I have read in some sources that the rule should only be applied to materials outboard of the insulation material.]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162421#Comment_162421 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162421#Comment_162421 Fri, 30 Nov 2012 07:39:43 +0000 Mike George Posted By: rhamdu
Posted By: Viking HouseThere has never been a documented case of decay in a wall due to moisture movement through a wall by diffusion alone!

Can you say that again louder?

Why?]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162425#Comment_162425 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162425#Comment_162425 Fri, 30 Nov 2012 09:22:15 +0000 rhamdu
I don't think that can be the case.

The catch is that phrase: 'by diffusion alone'. That implies perfect airtightness. I suspect the main argument for rules of thumb such as 5:1 is that they allow a safety margin if the construction is not fully airtight, and some moisture is able to enter the walls and roof through air leaks from the interior.

5:1 may not be necessary if a high standard of airtightness is built in, and subsequently maintained. But - tell me if I am wrong - my impression is that most people only go for high airtightness if they intend to use MVHR, which of course makes breathability irrelevant.

The irony is that you can be in a 'breathable' house but unable to breathe:bigsmile:]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162432#Comment_162432 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162432#Comment_162432 Fri, 30 Nov 2012 09:57:20 +0000 SteamyTea But is this not the 'leaky bucket' producible when it come to modelling.

Imagine a bucket that can hold 10 kg of water. It has a hole in the bottom that has a leak rate of 2 kg per hour.
Now if you fill it up at a rate lower than 2 kg per hour, it holds nothing, but if you increase the rate (the humidity if you like) to greater than 2 kg per hour then the bucket starts to hold water until it if full.

Now imaging that there is a second bucket that it can overflow into (a bucket in a bucket), that has a volume that is larger (it does not have to though) and a leak at a slower flow rate, say 1 kg per hour, but the volume is larger, say 50 kg of water.
Now as the first bucket overflows the fill rate of the second bucket equals the fill rate of the first, it then has to wait till a while till that second bucket is full and it overflows into a third buckets and so on.

So this is then the model but instead of buckets we have a material that can absorb water, an initial flow rate of water that is based on the humidity in the building, and a resistivity which is the flow rate out of the bucket.
Seems to me that all the elements are there to calculate this. Just use the output from one bucket as the input to next bucket.

I would do it but got to write about cows instead, so shall leave the Excel work up to others.

I made a video of the concept as that is faster than working it out or even typing it up (going to take a couple of hours to upload, so check back if you are interested, and does anyone know how to get grout clean, I keep meaning to change those horrible 80's tiles, but much more important things to do):
http://youtu.be/ufmB4C_vofI]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162437#Comment_162437 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162437#Comment_162437 Fri, 30 Nov 2012 11:21:42 +0000 Ed Davies Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162438#Comment_162438 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162438#Comment_162438 Fri, 30 Nov 2012 11:31:40 +0000 fostertom Posted By: Mike GeorgeEPS around 100,000No, that's solid polystyrene i.e. plastic moulding.

Posted By: Mike Georgecement. Noteable that it is the *same* as Lime.
Concrete has a higher value as it necessarily contains a higher proportion of cement (to sand) and also chippins
Hm - obviously aggregate, whether sand or chippin(g)s is impervious if flint, granite etc, tho less impervious if limestone. That leaves a) airspaces and b) cement slurry as the path for water vapour, in both render/mortar and concrete. Is cement slurry (set) permeable?

I suspect that any of render/mortar and concrete, if composed of well-graded aggregate i.e. selection of grain sizes that can compact together (dry) to leave a minimum of voids between, would be most vapour-impermeable, regardless of cement:sand (or cement:aggregate generally) ratio. Very high render/mortar/conc strengths are attainable with minimum cement content, the better-graded the aggregate is. These would I guess also be highly impermeable. Typical more lowly mixes of poorly graded aggregates would be more permeable.

Maybe the answer is that modern readimixed conc is in fact v well graded, for economical strength with minimum expensive cement content; whereas typical mortar/render mixes are v poorly graded.

With poor grading of aggregate grain sizes, lots of voids are left between the (dry) compacted particles (incl sand), which either remain as airspace or have to be filled with cement slurry. As cement slurry has v poor strength when used as gap-filler, but high strength when used like superglue l.e. in minimal thickness between close-fitting surfaces, that explains why well graded aggregates can make strong render/mortar/conc even with low cement content.

And explains why poorly graded aggregates may make less impervious render/mortar/con, regardless of cement content.]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162449#Comment_162449 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162449#Comment_162449 Fri, 30 Nov 2012 13:34:50 +0000 Mike George
Expanded polystyrene is in fact listed as 60
Extruded polystyrene 150

others listed :

PUR 60
Mineral wool 1
Phenolic 50
Perlite 5
Wood wool 5
Poly beads 2

Interesting comments about the cement. This is the main area I have been trying to research. The BS just states Render and Lime mortar both 10

Why do you think the cement content makes no/little difference?

I thought the main school of thought (of conservationists for example)was that rich cement renders = impervious renders. Are you saying its more to do with the aggregate than the cement content?]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162451#Comment_162451 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162451#Comment_162451 Fri, 30 Nov 2012 13:37:50 +0000 Mike George
Yes it is a good way to demonstrate the vapour movement. Though as Ed says there are many more factors in play. Not least water ingress form the outside.]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162454#Comment_162454 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162454#Comment_162454 Fri, 30 Nov 2012 13:41:28 +0000 djh Posted By: Mike GeorgeI was also surprised by the cement. Noteable that it is the *same* as Lime.
Yes. It flies in the face of the other papers I've read and given links to some of on here. What is the source?]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162459#Comment_162459 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162459#Comment_162459 Fri, 30 Nov 2012 13:47:46 +0000 Mike George
They are also listed the same in BS EN 12524:2000 Building materials and products Hygrothermal properties -Tabulated design values]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162463#Comment_162463 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162463#Comment_162463 Fri, 30 Nov 2012 13:54:03 +0000 djh Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162466#Comment_162466 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162466#Comment_162466 Fri, 30 Nov 2012 13:57:47 +0000 Mike George Posted By: djhHi Mike, Does the BS give a source for the numbers? Presumably it is quoting some experimental measurement somebody did somewhere at some point.

It states

This International Standard provides such tabulated information based on the compilation of existing data (see
reference documents listed in the Bibliography).

This one seems to be the most likely from the list

EN 1745, Masonry and masonry products — Methods for determining design thermal values]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162470#Comment_162470 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162470#Comment_162470 Fri, 30 Nov 2012 14:40:46 +0000 SteamyTea The main point though when modelling this is to use the output from one as the input to another, if you want to include flows in the other direction, multiply that side of it by -1 and then add it to the input of that element. You can imagine it as pulling and replacing the plug.]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162482#Comment_162482 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162482#Comment_162482 Fri, 30 Nov 2012 15:23:40 +0000 fostertom Posted By: Mike GeorgeWhy do you think the cement content makes no/little difference?I'm deducing and speculating. Seems that (if not air voids) cement slurry (set) must be the main route for water vapour, between and around impermeable aggregate particles, whether sand or stone; therefore that cement slurry must have high permeability - anyone know?

Posted By: Mike Georgemain school of thought ... that rich cement renders = impervious renders
Doesn't that mean impermeable to liquid water, which can be unrelated to water vapour permeability?

More cement, given the same aggregate in a render, would more fully gap-fill the spaces between the particles, so less airspace, more cement slurry (set) as water vapour route. Obviously cem slurry will be more resistant than air, hence more cement (if it reduces air voids) may mean higher vapour resistance.

But I'm guessing that cem slurry (set) has far lower resistance than the aggregate - tho that might not be true if the aggregate is limestone (incl shell), rather than flint/granite/sandstone.

Modern strong conc will have well graded aggregates, meaning a v small part of its volume will be airspace or cem slurry.]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162483#Comment_162483 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162483#Comment_162483 Fri, 30 Nov 2012 15:29:05 +0000 fostertom Posted By: Mike GeorgeExpanded polystyrene 60
Extruded polystyrene 150
PUR 60
Mineral wool 1
Phenolic 50
Perlite 5
Wood wool 5
Poly beads 2All v interesting.
So most plastic insulations (except XPS - much higher) are on a par and have similar resistance to timber and OSB.]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162488#Comment_162488 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162488#Comment_162488 Fri, 30 Nov 2012 16:09:22 +0000 Mike George
So not a lot of help I'm afraid]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162491#Comment_162491 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162491#Comment_162491 Fri, 30 Nov 2012 16:17:45 +0000 Mike George Posted By: fostertom
Posted By: Mike Georgemain school of thought ... that rich cement renders = impervious renders
Doesn't that mean impermeable to liquid water, which can be unrelated to water vapour permeability?

I've always taken it to mean both vapour and water, though now i'm not so sure :confused:

Either way though, the arguments against cement in this regard seem unfounded, as it will allow vapour passage as easily as lime. I still think the mix is a significant factor though:-

If you compared say a mix of 20 parts sand to 1 part cement against 3 parts sand to 1 part cement I would suggest the former will clearly allow passage of both water and vapour more easily. If you accept this assertion then it follows that mixes in between these proportions will have progressively greater 'resistance']]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162492#Comment_162492 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162492#Comment_162492 Fri, 30 Nov 2012 16:18:56 +0000 Mike George Posted By: SteamyTeaYes there are variables, but to a certain extent changing the flow rate can account for that so you can get an intuitive feel for the problem.
The main point though when modelling this is to use the output from one as the input to another, if you want to include flows in the other direction, multiply that side of it by -1 and then add it to the input of that element. You can imagine it as pulling and replacing the plug.

You are too good with numbers for me. :cry:I prefer the software approach myself - my head hurts less]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162496#Comment_162496 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162496#Comment_162496 Fri, 30 Nov 2012 16:39:06 +0000 djh Posted By: Mike GeorgeEither way though, the arguments against cement in this regard seem unfounded, as it will allow vapour passage as easily as lime.
I think the evidence from the likes of historic buildings and straw bale buildings is that there is a significant difference between the two, though I don't believe it is entirely straightforward. And there are certainly vested interests on both sides who muddy the waters.]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162497#Comment_162497 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162497#Comment_162497 Fri, 30 Nov 2012 17:03:41 +0000 Mike George Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162513#Comment_162513 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162513#Comment_162513 Fri, 30 Nov 2012 23:33:47 +0000 Viking House http://www.viking-house.co.uk/images/web-pages/Downloads/Moisture%20Buffering%20Effect%20of%20Wood%20Products.pdf]]> Mu values and breathability. Stated values and the perfect construction https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162518#Comment_162518 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&Focus=162518#Comment_162518 Sat, 01 Dec 2012 06:29:52 +0000 SteamyTea

The underlying physics/maths is the same. Exponential growth and decay. So well understood.]]>