Just came across this doc, suggesting that whilst XPS short-term water absorption is very good, in the long term it aborbs a lot more than EPS (18% after 15 years, reducing thermal resistance to 50% of when new):
http://www.buildblock.com/pdfFiles/Foam-Control%20EPS-Water%20Absorption%20Facts.pdf

Do we believe this? Having just filled my foundation with XPS (sirap Gematherm: http://www.sirapgroup.com/insulation/page/download/129 ) I'd like to think that it was rather better than this. That doc specifies 3% absorption (to EN12088, which tests over 28 days).

If garbage, does this mean you can tell any lie and get away with it in US - no ASA? It does sound credible to me.

Seems to be more about the cell structure rather than the material itself. This in turn will make it more or less affected by the environment it is in. So if a less than ideal EPS was used in a wet location then it is possible that water will migrate into the foam structure. This does not mean that every installation will have this problem every time, just that what was checked did. This makes it very hard to compare like for like. But if you take ABS (a type of 'solid' styrene monomer) the 24h absorption is 0.3% and the saturation figure is 0.7%, or 2 1/3rd times. Density of ABS is 1024 kg/m^3, water is 1000 kg/m^3 and air is 1.2 kg/m^3.
So the solid material can absorb another 72kg/m^3 of water (1096 kg/m^3).
Some more water will be trapped in the voids between the cells by capillary action/pressure difference, this will be dependant on the overall density of the EPS, but I would guess that it would be at least another 100kg/m^3 (but just guessing there). This would displace air that has a thermal conductivity of 0.025 W/mK with water at 0.6 W/mK or 24 times worse. ABS is 0.17W/mK or 3.5 times better than water.

So I can easily see that it is possible that what may seem like a small moisture problem can greatly affect the overall performance of the EPS.

Wookey, what measures have you taken to stop water ingress into the EPS?

Edit, EPS should read XPS
Whoops, well spotted Tom, I did, thanks

One wonders if there are significant differences between different XPS vintages/manufacturers? It's hard to know how representative the stuff put in this building in 1993 is of typical product today. Clearly they (ACH foam tech) are selling their particular brand of EPS, but I find it hard to believe that the report is simply lies. The question really is is there some kind of problem with their testing, and/or how common/current is this issue of slow long-term absorption? If the process is very slow then clearly the ISO/EC/BS 28-day test is not really representative.

http://www.exiba.org/Properties_of_XPS.asp#Water gives details of the ISO/EC/BS 28-day tests and also says that we know from 20-30 yrs experience that these tests _do_ represent a realistic upper limit.

So who's right? The most logical explanation of all this is that the stuff from Minnesota in the ACH foam tech report was not as absorption-proof as normal, but this testing probably isn't done very often, so this situation may be more common than we realise, or it may be a complete anomaly.

Saint, you say you've seen XPS fished out after 20 yrs and the lamda was unchanged. Did someone actually test it? Can you point to any docs on the subject?

ST - The XPS (not EPS!) in my cavity below DPC has no specific waterproofing features. I am relying on its advertised low water absorption. The XPS below the floor will be above the DPC so that will be protected from moisture in the ground below.

Guys,

Not wanting to hijack the thread but does anyone have any info on how XPS copes with UV exposure. Due to financial constraints our ICF build is running pretty slow and I can see its going to be summer of 2012 before we can render the outside which means the outside insulation would have been exposed to sunlight for over 2 years! Do you think this is a problem, could its thermal properties be effected? It's nearly water tight, tiling roof now but our main problem is getting the windows, hence ideally we wouldn't render until they are in?

Phil, the UV will break down the top layer or couple of layers of cells, maybe <1mm thickness or so. The damaged cells remain as a fairly tightly bound dust on the surface of the boards. This dust then serves to protect the XPS board underneath from further UV damage. Normal recommendations would be to remove dust with stiff brush once you're ready to proceeed with the job and carry on as normal. Little or no effect on the thermal properties other than typical ageing and 1mm less insulation (!). Personally though as its a reasonably long period of time I'd try and drape something over the boards if possible or simply paint with a thin coat of light coloured water based paint (no solvents!!!)