Where's the romance in that?

I've finally managed to get a comprehensive source for Mu- values, which I find the least confusing method of vapour resistance

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

Why 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!).

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?

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?

Not sure why this problem popped into my head when I was driving earlier (made me slow down for some reason).
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

Posted By: Mike GeorgeEPS around 100,000

No, 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.

Nice video Nick,

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.

BS EN ISO 10456:2007 Building materials and products — Hygrothermal properties - Tabulated design values and procedures for determining declared and design thermal values

They are also listed the same in BS EN 12524:2000 Building materials and products Hygrothermal properties -Tabulated design values

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

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.

EN1745 gives many types of masonry/concrete/stone but each Mu value typically listed as 5/15

So not a lot of help I'm afraid

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. I prefer the software approach myself - my head hurts less

Yes, I've heard such claims though have yet to see published evidence (other than that funded by vested interests) Anything is worth a look though if you have links?

Had a quick look though that paper, not so different to what I did for my BSc (except I was looking at solar gain and thermal mass). Nice summing up saying the best at reducing swing is impractical and don't treat it, shall go and get all the paint of my walls this weekend with a steamer

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