Mu values and breathability. Stated values and the perfect construction

[Anonymous]

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?

[Mark Siddall]

Hi Mike,
Is there a reason for this being posted in the refurbishment forum? Are you seeking a perfect breathable retrofit, or is the question somewhat generic?

In terms of the debate about what is the 'perfect' breathable construction I would argue that these days you would need to move from Glazer method towards dynamic analysis. The advantage of such analysis is that reverse diffusion characteristics can be assessed; this is particularly important when considering options for retrofit. The old “1:3-5” ratio, based upon Glazer method, may, in certain conditions, be found wanting.

A number of threads that relate to the broader debate are listed below (some of which you have already contributed towards):

https://aecb.net/forum/index.php/topic,3493.0.html
https://aecb.net/forum/index.php/topic,2787.0.html
https://aecb.net/forum/index.php/topic,3547.0.html
https://aecb.net/forum/index.php/topic,3391.0.html

Mark

[Anonymous]

Apologies if I have posted this in the wrong place but I thought the topic was narrow enough to warrant its own discussion.

IE. 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?

Your third link is somewhat relevant, but none of them really focus on the numbers used to back up the breathability 'concept'

So to answer your question, yes, I would like to target the discussion towards refurbishment, as this is the market commonly targeted by manufacturers and individuals advocating the 'breathable' method/solution. It's a research exercise really – I find using forums for such a purpose is very productive.

[Mark Siddall]

Mike,
If breathability (a clumsy and in appropriate term really – better to stick with ) in retrofit is what you are looking at then, for UK construction, you really should be looking into WUFI/Delfin. With regard to moisture the over riding issue is whether the insulation is located on the inside or the outside.

I personally would not advocate internal insulation (there are to many percieved risks with uncertain consequences). The reason for m suggesting more advanced tools arises in case you should be thinking of internal insulation and applying it to existing masonry (this risks of reverse diffusion and saturation of the masonry are risks which can in turn lead to frost decay of the masonry – never mind rotting joist ends). If considering IWI then firstly you would want to limit the rain water that can ingress to the masonry – lime render or a microporous paint should help. Only once you address this issue (and it is not straight forward if using microporous paint) can you afford to thin about the more substle aspects of moisture transfer. As for golden ratios. At the moment I tend to think that they no longer exist. What works in the winter (say 1:4) may not work in the summer.

https://aecb.net/forum/index.php/topic,3604.0.html
https://aecb.net/forum/index.php/topic,3601.msg13102.html#new

Mark

[Author]

Posts