Looking at the column for vapour permeability in a PDF entitled Breathability Matters that I think I downloaded from the natural building website it says that gypsum plaster has a value of 50 and lime plaster 75.
I have some areas of wall that have been plastered with gypsum and I was thinking of removing it but if the vapour permeability is lower for gypsum should I bother and why is lime plaster said to be better?

Many thanks

Dunno about those numbers. Here are some others by Straube who also quote Minke:

http://members.westnet.com.au/ejt/pdf/Straube_Moisture_Tests.pdf

[ng/Pa s m]

Cement:Sand 1:3 datum 1.7
Cement:Lime:Sand 1:1:6 datum 10.3
Lime:Sand 1:3 Datum 18.9

Gypsum appears to be in the range 1.62 - 3.5 according to http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=05930938

It may also be interesting to read:
http://www.conservationphysics.org/wallbuff/wallbuff.php

Anyone become any wiser on this issue over the last couple of years? Seems a lot of hassle getting plasterers to use traditional plasters so is there anything really wrong with gypsum? I'm thinking for example on the inside of pavatherm used as internal insulation on brick walls.

here's some tables from the above discussions

fRom links above
http://builddesk.co.uk/wp-content/uploads/2013/01/vapourResistances.pdf

-value (“mu-value”) of a material is also known as its “water vapour resistance factor”.

To convert a m-value to a vapour resistance (MNs/g)
1. Multiply by thickness in metres
(this gives the “equivalent air layer thickness” in m)
2. Divide by 0.2 g.m/MN.s
(this is a typical value in the UK for the vapour permeability of still air)

To convert a m-value to a vapour resistivity (MNs/gm)
Divide by 0.2 gm/MNs
(this is a typical value in the UK for the vapour permeability of still air)

The µ value of a material is its vapour resistivity relative to that of still air. The vapour resistivity of still air is said to be 5 MN·s/(g·m). So multiply the µ value by 5 to get the resistivity (or divide by 0.2 if everything in your life is so simple that you need to make the arithmetic look at tiny bit more complicated).

To get the resistance of a particular layer multiply the resistivity of the material by the thickness of the layer.

Posted By: jamesingram(this is a typical value in the UK for the vapour permeability of still air)

Will the vapour permeability of still air in Scotland change in the event of a “yes” vote?

Seriously, is this why people use µ factors - because they scale with some other parameter? Temperature I would guess?

just reading that myself :)

On that bombshell I'm off to the pub

Posted By: Ed DaviesFirst table: “Calculated Mu = Vapour Resistance x 0.2". I think “Resistance” should be “Resistivity” here. It's what makes sense logically, given that µ factors are stated for materials in general rather than particular thicknesses of layers, and how the numbers seem to be calculated.

Yes, you are right Ed. My mistake :) The conversion figures in the tables are correct but resistance should read resistivity

Mike, in the particular case to which I refer, the wall had been about 90% re-skimmed with gypsum, but the vast majority was still lime behind - only 2 'eyes' about 400 dia had been re-plastered right back to the wall in gypsum. These 'eyes' 'held the damp'. The householders thought they were being watched!

The (c3mm) gypsum skim seemed to be able to breathe enough to allow the lime to still 'do its lime thing', while full-thickness gypsum just 'sucked'.

Posted By: delpradocould I add to this debate? It seems to me there is nothing wrong with gypsum, per se,

I was just about to start a new thread but this one came up in search so I'll revive it a bit. From my research it seems that there is a dearth of research and understanding regarding gypsum and its functional properties within buildings, especially in regard to how it performs from a breathability perspective. Popularly it get a pretty bad rap with lots of claims it isn't breathable.

From recent research however, it seems that gypsum has a number of properties that are very useful and can outperform both lime and clay, for example.

Part of the problem is the general understanding of breathability and how it functions rather than the material per se. I think it's also important to understand what you're looking to achieve in the use of materials. In my case I do want a healthy building fabric but indoor air quality is also important, as is moisture buffering because I'm using natural rather than mechanical ventilation.

In older buildings it appears that the whole wall/building buildup prefers vapour permeability, e.g. a lime render/plaster which doesn't display particularly good moisture buffering/hygroscopic capabilities due to its pore structure. Gypsum however has been found to have both 'some' vapour permeability together with airtightness, and moisture buffering so as a solution to managing indoor air quality, it looks like an excellent candidate providing it isn't covered with some plastic paint.

I have now decided to use gypsum plaster and plasterboard in my build in preference of both lime and clay plasters which had previously been recommended. In part because I've had enough of the world of lime snootiness that I seem to have experienced so much in the last couple of years, and because they don't appear to provide additional benefit given what I'm looking to achieve.

For anyone interested, I've recently got hold of a PHD thesis covering the topic from an indoor air quality perspective which I found quite enlightening.

https://www.researchgate.net/publication/351687590_Understanding_moisture_buffering_effects_in_the_indoor_environment

Here is another earlier one looking at the Measurement and Modelling moisture transport processes within porous construction materials which looks specifically at different gypsum plasters and their constituent layers:

https://core.ac.uk/reader/82971367

Posted By: delpradocould I add to this debate? It seems to me there is nothing wrong with gypsum, per se, but that you have to apply a load of pva to get it to stick, which is basically impermeable?

This is the problem I feel, they apply pva to everything.

Is there a breathable alternative?

Posted By: wholaa@SimonD,

Is this true of just gypsum skims or plasterboards too? If true for plasterboard I guess not for moisture resistant bathroom plasterboards. I think a big factor is a lot of skims are very thin, while a lime putty finish probably has a lot more bulk. Is your project for partition walls or external facing?

I honestly don't have any figures for moisture resistant plasterboards, but I would assume it wouldn't have particularly good moisture buffering values. But plasterboards do provide moisture buffering. My project is both partition walls and external facing where I'll be using plasterboard for partition walls and gypsum plaster on the external facing masonry walls. I'm going to be finishing with a clay paint to retain open porosity.

For reference, in the PHD thesis I linked to above (Understanding moisture buffering effects in the indoor environment), I'll quote the explanation in why lime, clay, gypsum and plasterboard function differently and the results of testing:

"Clay and lime presented mainly macro-pores, which have an average diameter of around
125 nm. Gypsum also had macro-pores of a significant bigger size (365 nm average),
but it also presented micro-pores, as shown in Fig. 3-2b. Gypsum plasterboard had a
more accentuate micro-pores presence and a significantly higher average pore diameter
(631 nm) than standard gypsum. Overall, the gypsum and plasterboard showed a
significant higher pore volume than clay and lime. Due to the more complex pore
structure of gypsum and plasterboard, both vapour and liquid transport take place
into the materials, when exposed to a RH and vapour pressure gradient, while in clay
and lime only vapour transport occurs. Water vapour transport can take place in
the macro-pores and its driving potential is the water vapour pressure, whilst liquid
transport takes place in the micro pores, where the driving force can either be the
relative humidity and the capillary pressure."

Test results:

"The moisture buffering capacity of the plasters within the 30 m3 in-situ volume is
shown in Fig. 4-49 and Table 4.18. In the full-scale testing, gypsum was the better
performing material, as it presented higher moisture adsorption (88.63 g/m2), whilst
lime exhibited the lowest capacity (33.56 g/m2). Plasterboard had lower values than
gypsum (76.16 g/m2) and similarly to the laboratory testing the material presented a
plateau. Differences with the laboratory can be seen in Table 4.18. The variations
between laboratory and in-situ testing were due to the variable fluctuation of the
humidification system and errors in the measurements. Gypsum and plasterboard
presented 7% and 15% higher sorption capacity than in the laboratory, whilst clay
and lime showed 30% and 22% lower values, respectively. The reason of the higher
moisture buffering values for plasterboard and gypsum are related either to the higher
sensibility of the materials to moisture variations and to the possible uneven
distribution of the moisture on top of the materials."

The figures from the same PHD in lab tests shows:

"The moisture buffering performance of the materials can be seen in Fig. 3-8. After 8 hours of
humidification they reached respectively a moisture buffering capacity of 60.90 and
81.190 g/m2, whereas lime reaches only 43.26 g/m2. Plasterboard shows a good
moisture buffering capacity (65.52 g/m2), but the shape of the curve is significantly
different compared to the other cases. Plasterboard started presenting a plateau,
which indicates that the thickness of the specimen was too small to use all the
moisture buffering potential of the material. This would explain the lower moisture
buffering capacity of plasterboard than gypsum, despite of its low density and
permeability, and high porosity and moisture capacity (Table 3.7), which usually
indicate a high moisture buffering potential."

There are some caveats here.

Moisture buffering values vary with temperature, the higher temperature usually increases moisture buffering. But it also depends on the whole buildup of the wall. For example, in a different study (https://www.sciencedirect.com/science/article/pii/S0360132315300597?via%3Dihub) using a hemp-lime wall assembly, the best internal finish to retain its excellent moisture buffering value levels was a lime plaster. The plasterboard with a service void still provided a 'good' buffering value which still utilised the hemp-lime assembly's moisture buffering capability.

The value of moisture buffering typically lies where there is good aritightness and low ventilation rates. Where you have higher ventilation rates the moisture is typically removed by the ventilation.

Another caveat is understanding moisture transport through the whole wall assembly and time. From the study above, it's looking at short term buffering over a number of hours, possibly a day whereas for moisture to move through and entire wall it can take anything from days to months, possibly even longer.

Posted By: djh
I've mentioned previously that it seems it took five years or so for the moisture in our house to stabilise. Internal RH was around 40% for the first five years and has now switched to around 50%.

That doesn't surprise me with your bale house. I've seen some figures somewhere stating 3 years for some masonry walls.