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General: Vapour permeability of EPS polystyrene

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  1.  
    A slightly vague question, but does the idea that EPS is to some extent vapour permeable have any truth in it?

    Essentially, the idea that it's meaningfully more permeable than, say, foil faced PUR, and meaningful in the sense that it's something that might become a relevant consideration when choosing a wall buildup that has *some* degree of "breathability" to allow, say, construction moisture to gradually escape from timber over time.

    This is a notion that I'd somehow got into my head at some point (I think maybe reading comments on here some time back) but something I'm guilty of not having really gone and fully checked out yet myself.

    Is this simply nonsense? And if so is it a common misconception? Or is it one of those half-truths that completely depends on the context?
    • CommentAuthortony
    • CommentTimeApr 9th 2018
     
    No, eps is made up of polystyrene balls squashed together and there are continuous air paths between each side of the sheets and so they are vapour permeable.

    Closed cell sheets are not nor ate ones covered in foil.
  3.  
    Posted By: tonyNo, eps is made up of polystyrene balls squashed together and there are continuous air paths between each side of the sheets and so they are vapour permeable.

    Closed cell sheets are not nor ate ones covered in foil.


    So are certain types of EPS "closed cell" or do you mean that other types of insulation are closed cell?

    I get confusing results looking up a few materials. For example, this graphite EPS:

    https://www.insulationshop.co/10mm_grey_polystyrene_ewi_graphite.html

    is listed with a Vapour resistivity of 20 MN.s/g.m (although descrbed as "closed cell" in the blurb?)

    On the Jablite EPS tech PDF, vapour resistivity for EPS70 is given as 145 MN.s/g.m and EPS250 as 238 MN.s/g.m.

    Celotex PIR in comparison, on their BBA cert give vapour resistivity for the core as 300 MN.s/g.m.

    And if I look at the table here

    https://www.engineeringtoolbox.com/vapour-resistance-d_1807.html

    Vapour resistivities of EPS (100-750) and Phenolic (150-750) seem to occupy similar ranges.
  4.  
    XPS (extruded polystyrene) is closed cell. Effectively it does not need a VCL since the insulant is its own VCL.
  5.  
    Posted By: Nick ParsonsXPS (extruded polystyrene) is closed cell. Effectively it does not need a VCL since the insulant is its own VCL.

    What about the gaps where the sheets of XPS butt together? Power cables coming through a VCL is often quopted as a problem so why would the butted joints between sheets not be an issue - or is it suggested that the joints are sealed?
    •  
      CommentAuthorfostertom
    • CommentTimeApr 10th 2018
     
    You've done good researching lineweight. All these insulants, different samples differ greatly so results often overlap or contradict. But even so, hard to explain.

    Hold a piece of different insulants to the mouth. If you can get a good airseal, suck hard (hygene warnings apply!). You'll be able to pull air through EPS - it's not airtight. If it's not airtight it's most certainly not water vapour tight, in other words is vapour permeable. Compare with other samples.
    • CommentAuthorlineweight
    • CommentTimeApr 10th 2018
     
    Posted By: fostertom
    Hold a piece of different insulants to the mouth. If you can get a good airseal, suck hard (hygene warnings apply!).


    Not a test one can really do at a specification stage!

    My main interest in the question stems from the argument often used in favour of eg. woodfibre based insulation; ie. that it's significanty more vapour permeable than foam type insulations.

    The principle that a construction buildup should give any moisture trapped within it a chance to escape seems a valid one to me. And therefore vapour permeable insulation seems like a good thing to aim for in certain circustances. Of course the downside of mineral or wood fibre insulation is that you need a greater thickness of it. If EPS can provide some vapor permeability as well as better performance...then it seems like it has a lot going for it.

    But I've never really been able to find anything conclusive about how we should expect it to perform in real life. Are there any examples of real world tests to examine whether a thick layer of EPS can allow moisture trapped behind it to escape? Such a test wouldn't necessarily need to prove that it performed in an equivalent way to other insulation materials - just that it was sufficiently permeable to allow, say, a reasonable level of construction moisture to escape within a time period short enough to avoid that moisture starting to cause problems.
    •  
      CommentAuthordjh
    • CommentTimeApr 10th 2018
     
    Posted By: lineweightNot a test one can really do at a specification stage!

    I've seen people doing stranger things in the DIY sheds :bigsmile::bigsmile:

    If EPS can provide some vapor permeability as well as better performance...then it seems like it has a lot going for it.

    Yes, especially graphite EPS seems to hit a sweet spot for me.

    But I've never really been able to find anything conclusive about how we should expect it to perform in real life. Are there any examples of real world tests to examine whether a thick layer of EPS can allow moisture trapped behind it to escape?

    I expect there are but I don't remember any specific results at the moment.

    Such a test wouldn't necessarily need to prove that it performed in an equivalent way to other insulation materials - just that it was sufficiently permeable to allow, say, a reasonable level of construction moisture to escape within a time period short enough to avoid that moisture starting to cause problems

    It's the last bit of that statement that makes life difficult. Problems come in various sorts, such as spalling, but a very important class is decay of organic components, such as timber. And in order to determine how long it takes before rot sets in, it isn't sufficient to know the vapour permeability. You also have to consider the hygroscopic behaviour of the materials. And of course it also matters whether the insulation itself is susceptible to decay.

    So EPS has the advantage that it doesn't rot, and the disadvantage that it isn't really hygroscopic. So it's ideal for use on the outside of organic components, so keeping them warm enough that RH doesn't get high enough for rot to start.

    Contrarily, woodfibre has the disadvantage that it can rot, but the advantage that it is hygroscopic. So it can be used where both it and, say, some timber will experience condensation. It will help to buffer the moisture and protect the wood from decay. But the overall design must ensure that there is sufficient drying time, compared to the wetting time. Hence systems like that get more complicated, and depend on the external exposure and weather etc.
    • CommentAuthorlineweight
    • CommentTimeApr 10th 2018 edited
     
    Posted By: djh

    So EPS has the advantage that it doesn't rot, and the disadvantage that it isn't really hygroscopic. So it's ideal for use on the outside of organic components, so keeping them warm enough that RH doesn't get high enough for rot to start.



    Agree with all that, but with the niggling concern remaining about whether there's enough evidence to show that it will allow any trapped moisture out, where there's timber present. The concern's not about moisture resulting from interstitial condensation, but about moisture that gets introduced at construction stage, or perhaps resulting from small leaks.
    • CommentAuthorTimSmall
    • CommentTimeApr 10th 2018
     
    Slightly idiosyncratic, but some useful rules of thumb here I thought:

    https://buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh
    •  
      CommentAuthordjh
    • CommentTimeApr 10th 2018
     
    Posted By: lineweightAgree with all that, but with the niggling concern remaining about whether there's enough evidence to show that it will allow any trapped moisture out, where there's timber present. The concern's not about moisture resulting from interstitial condensation, but about moisture that gets introduced at construction stage, or perhaps resulting from small leaks.

    I believe it does allow moisture through, for the reason Tom gives, among others.

    But note that if the insulation is outside the 'rottable stuff', then the moisture doesn't need to get out, because the RH should be low enough that the 'rottable stuff' won't rot. It's still much, much better if it can get out of course, and all designs should be such that it can get out, preferably in multiple ways. The source of the water doesn't really matter; it's all water.
    •  
      CommentAuthordjh
    • CommentTimeApr 10th 2018
     
    Posted By: TimSmallSlightly idiosyncratic, but some useful rules of thumb here I thought:

    https://buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh

    Fascinating article, thanks. It doesn't really mention EPS, except right at the end, where it says it is permeable enough.

    I hadn't really thought about hydrostatic pressure as an issue in wallcoverings before. I think I'm lucky, because where I have some render over timber frame, I do have a drainage plane behind it consisting of some polypropylene mesh as he mentions, plus I expect that the render doesn't fill the full depth of the render carrier board so water can drain through that space. And where the render is bound directly onto the straw, the straw itself is open to liquid water enough to allow drainage in smallish quantities, as well as dispersing it by capillary action. The bottom of the wall is designed with drainage for liquid water.
    • CommentAuthorlineweight
    • CommentTimeApr 10th 2018
     
    Posted By: djh
    Posted By: lineweightAgree with all that, but with the niggling concern remaining about whether there's enough evidence to show that it will allow any trapped moisture out, where there's timber present. The concern's not about moisture resulting from interstitial condensation, but about moisture that gets introduced at construction stage, or perhaps resulting from small leaks.

    I believe it does allow moisture through, for the reason Tom gives, among others.

    But note that if the insulation is outside the 'rottable stuff', then the moisture doesn't need to get out, because the RH should be low enough that the 'rottable stuff' won't rot. It's still much, much better if it can get out of course, and all designs should be such that it can get out, preferably in multiple ways. The source of the water doesn't really matter; it's all water.


    Not sure I follow that. Isn't it the moisture content of the rottable stuff that matters, rather thsn the RH of any air next to it?
    • CommentAuthorgravelld
    • CommentTimeApr 11th 2018
     
    In practical terms, don't we need to consider what's outside the EPS too? S+C render surely less VP?
  15.  
    Nick Parsons 1 day ago edit quote
    XPS (extruded polystyrene) is closed cell. Effectively it does not need a VCL since the insulant is its own VCL.

    Peter_in_Hungary 23 hours ago quote
    Posted By: Nick Parsons
    XPS (extruded polystyrene) is closed cell. Effectively it does not need a VCL since the insulant is its own VCL.

    What about the gaps where the sheets of XPS butt together? Power cables coming through a VCL is often quopted as a problem so why would the butted joints between sheets not be an issue - or is it suggested that the joints are sealed?

    P-in-H, you have highlighted a weakness, and one that is particularly true of laminated insulation boards, whatever the insulant. It's why I no longer use laminated boards except in very rare situations. What I used to do was to run a couple of lines of silicone down the joining faces, and 'rub the boards in' as I fitted them. Not very easy, and only 'better than nothing'.
    •  
      CommentAuthordjh
    • CommentTimeApr 11th 2018
     
    Posted By: lineweightNot sure I follow that. Isn't it the moisture content of the rottable stuff that matters, rather thsn the RH of any air next to it?

    The moisture content and the RH are related by the sorption curve, with large time lags of course.
    • CommentAuthorlineweight
    • CommentTimeApr 11th 2018
     
    Posted By: djh
    Posted By: lineweightNot sure I follow that. Isn't it the moisture content of the rottable stuff that matters, rather thsn the RH of any air next to it?

    The moisture content and the RH are related by the sorption curve, with large time lags of course.


    I still don't really follow. If there's moisture in timber, yes it's related to the humidity of any air surrounding it. But if the humidity in that air can't escape because it's trapped by vapour-impermeable materials then it's just going to sit there isn't it? It doesn't really matter what temperature that part of the construction is at: the moisture is in the timber and can't go anywhere.
    •  
      CommentAuthordjh
    • CommentTimeApr 11th 2018
     
    Posted By: lineweightI still don't really follow. If there's moisture in timber, yes it's related to the humidity of any air surrounding it. But if the humidity in that air can't escape because it's trapped by vapour-impermeable materials then it's just going to sit there isn't it? It doesn't really matter what temperature that part of the construction is at: the moisture is in the timber and can't go anywhere.

    Yes, if you wrap the timber in plastic. That's why you wrap it with hygroscopic and/or vapour-open materials instead.

    I'm not claiming you can hermetically seal everything, add as much water as you like and everything will be fine. Sorry if I gave that impression.
    • CommentAuthorlineweight
    • CommentTimeApr 11th 2018 edited
     
    Posted By: djh
    Posted By: lineweightI still don't really follow. If there's moisture in timber, yes it's related to the humidity of any air surrounding it. But if the humidity in that air can't escape because it's trapped by vapour-impermeable materials then it's just going to sit there isn't it? It doesn't really matter what temperature that part of the construction is at: the moisture is in the timber and can't go anywhere.

    Yes, if you wrap the timber in plastic. That's why you wrap it with hygroscopic and/or vapour-open materials instead.

    I'm not claiming you can hermetically seal everything, add as much water as you like and everything will be fine. Sorry if I gave that impression.

    It was this comment that I'd picked up on, but maybe misunderstood -

    "But note that if the insulation is outside the 'rottable stuff', then the moisture doesn't need to get out, because the RH should be low enough that the 'rottable stuff' won't rot."
    •  
      CommentAuthordjh
    • CommentTimeApr 11th 2018
     
    Posted By: lineweightIt was this comment that I'd picked up on, but maybe misunderstood -

    "But note that if the insulation is outside the 'rottable stuff', then the moisture doesn't need to get out, because the RH should be low enough that the 'rottable stuff' won't rot."

    Out meaning outside, it can escape to the inside. But because the insulation is on the outside, there won't be any condensation because the vapour doesn't get cold enough. Sorry I wasn't clearer about what I meant.
    • CommentAuthorlineweight
    • CommentTimeApr 11th 2018
     
    Ah, ok, I get you now. Yes, if construction moisture can dry to the inside, assuming the 'rottable' stuff hasn't got some other impermeable construction layer to the inside of it.

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