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General: breathability of EWI and interstitial condensation risk

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    • CommentAuthorMarkyP
    • CommentTimeJul 28th 2017 edited
     
    had a chat with BCO about my EWI plans, he was interested but raised some concerns about EWI over existing CWI and risk of interstitial consdenation within the existing cavity. I find this hard to understand, as it stands I have a 50mm cavity with glass wool, outer leaf is cold. Any amount of EWI will raise the temp of the cavity and the outer leaf, and should surely only push the dew point further out in the build up? Does it even matter if the dew point appears somewhere out in the build up? On a very cold winter's day, my existing cavity must be at risk of condensation anyway. My point to the BCO was that EWI can only improve the situation. I'm even considering adding some EWI to the extension which has a much wider fully filled cavity, it's being rendered along with the existing building, and wouldnt be any great amount more work for me to EWI it at the same time. The BCO wasn't saying no, I think was just thinking out loud, got the sense this might be the first EWI he'd dealt with.

    And on a related note....I've got 100mm grey EPS on site and I am aiming to make the whole show as vapour open as possible, all the EWI system manufacturers claim their buildups breath and yet most use cementitious adhesives and base coats, are these different in nature to the bulder's standard sand/cement based renders, which are generally accepted to hinder passage of vapour through walls? I'm actually planning a lime system - lime green and licata both offer a hydraulic lime base/mesh/base system which can be finished with a thin coat (yuck) or, as is my preference, base finished with float and then topped with mineral paint. I do wonder though whether this is really necessary. Cementitious base coats are ubiquitous, and seemingly used up adn down the UK without reported issues with breathability. These would be much cheaper than lime per m2, somewhat easier to install and would still take a mineral paint finish.
    • CommentAuthorgoodevans
    • CommentTimeJul 28th 2017 edited
     
    As I understand it - Brick, mortar, cementitious adhesives etc are all vapour permeable (suprisingly so) - EPS is not as permeable as these - but it will still allow moisture to evaporate should it get in. I would say an eps outer coat could not make things worse than the existing make-up. I think the trick is not to let the rain into the structure, keep the absolute humidity inside the house (and therefore within the structure) as near to outside levels as possible by good ventilation using MVHR ideally or constant volume extraction from wet rooms if not (trickle vents get closed in winter). Finally, not to trap or slow the movement of vapour with vapour barriers (this last bit will cause some concern with BC's).
    • CommentAuthortony
    • CommentTimeJul 28th 2017
     
    Ensure that the render and paint finish are both breathable, I would use coloured lime finish in preference to paint

    The dew point will be very near or on the surface on cold nights, a few millimetres into the eps as a worst case.
    • CommentAuthorgoodevans
    • CommentTimeJul 28th 2017
     
    Yep - agree with Tony - but the eps will be holding so little moisture in the form of vapour - and is rot proof there is no issue.
    • CommentAuthorMarkyP
    • CommentTimeJul 28th 2017
     
    planing to use mineral paint for that reason Tony. Also, think it will offer a more traditional finish which I much prefer. I just dont like the look of the finish on either mono/scrape back or the silicone paste thin coat renders at all.

    On the condensation, this is what I thought, that it can only push the dew point out, and whether the EPS or beind the render, it doesnt really matter anyway.

    are cementitious renders breathable? The manufacturers say so but I'm struggling to find data to compare with lime. LIme will be more expensive (I need to show a BBA to the BCO, so need a system lime product rather than site mixed) and will require care with managing the drying post application, the cementitious less demanding in that respect. Both can be mineral paint finished. EPS I've learned following this forum is somewhat vapour open, is lime worthwhile over an insulation layer that is only moderately breathable? I can see the argument for lime over woodfibre or mineral wool, less sure over EPS.
    • CommentAuthorgravelld
    • CommentTimeJul 28th 2017
     
    Can you use Therm or something similar to give you some support?

    I'm expecting a question like this.
    • CommentAuthortony
    • CommentTimeJul 28th 2017
     
    Cement tender will cause the eps to sweat and could produce wep patches behind the render, if these freeze problems could result, notbthe case with lime (or silicone)
    • CommentAuthorMarkyP
    • CommentTimeJul 28th 2017
     
    gravelld, yes a while back I did use an online tool to produce an intersitial condensation risk analysis, it came back fine. Only I cant remember what tool I used, it was free and produced a report page which I hoped might be enough. I will have to find it if the BCO pushes the point.

    Tony - but doesn't every EWI system provider uses cementitious base coats over EPS? maybe not with mono/scrape back but in the thin coat systems only the top 1.5mm paste is silicone, the rest is two coats of cement/polymer with mesh. And for the mono, often a base coat used to dub out as is much cheaper than using the finish to flatten a poor substrate. I'm just puzzled how this is all touted breathable while it's basically a coat of cement.
    • CommentAuthorgravelld
    • CommentTimeJul 28th 2017
     
    Would be interested in that tool if you can find it...

    Also interested in this contradiction I sometimes take from whether cement render is breathable or not (or to what extent it is breathable).
    • CommentAuthorgoodevans
    • CommentTimeJul 29th 2017
     
    The whole area of condensation is full of contradictions and misinformation - it's so difficult to work out what is a problem make up or not with little objective research in the area.

    There are so many causes of moisture - rain getting in, humidity infiltrating/exfiltrating from the property, humidity transported by convection currents, vapour created by sunshine warming wet walls, plain old diffusion (aka driving differential vapour pressure to sound more scary). Each country/region has its own rules and customs - sometimes based on sound reasoning - sometimes not.

    It is possible that cement based top coats have an issue with cracks that let in rain - but taking a relatively long time to migrate/evaporate out (compared to the rate of rain take up). Over time (or over a season) moisture builds up. With a brick substrate that moisture would get out through the brick and via your cavity - with EPS substrate it may not get out fast enough - if it doesn't someone looking at the problem might conclude interstitial condensation (it sounds good).

    To be safe go for lime - (rapid evaporation/drying) or silicone - (good waterproofing and reasonable evaporation)
  11.  
    Given the industry standard solution for EWI (EPS with cement based adhesive and render and thin film acrylic top coat) has been in use over here for in excess of 30 years and no show stopping issues/problems have arisen and until problems are shown and documented (as against a theoretical 'could happen') I would go, and have gone with the industry standard solution which is easy, readily available and understood (= within the comfort zone of the trades people). If it ain't broke why try to fix it !!
    • CommentAuthorPeterStarck
    • CommentTimeJul 29th 2017 edited
     
    Posted By: MarkyPgravelld, yes a while back I did use an online tool to produce an intersitial condensation risk analysis, it came back fine. Only I cant remember what tool I used, it was free and produced a report page which I hoped might be enough. I will have to find it if the BCO pushes the point.


    I used a free 30 day trial version of Builddesk U which calculates condensation risk analysis amongst other things.
  13.  
    Concentrate on Airtightness its 100 times more important than Diffusion.
    •  
      CommentAuthorfostertom
    • CommentTimeJul 29th 2017 edited
     
    Builddesk U and other such only do a Glaser-theory check, which has often been shown to be simplistic. Therm doesn't attempt moisture calc - WUFI (or now several equivalent 'modellers') is the one to use - but as Peter says it's all pretty bombproof, and others above have added precautionary advice e.g. as VH says, airtight is beneficial - but still it's pretty safe anyway.

    'Cementitious' is not at all same as a cement render or mortar.
    The latter may or may not be vapour permeable, by a wide variance, depending on composition (cem:sand:lime ratio) and application (e.g. spatterdash vs tight trowelled) of its several coats - so not reliably controllable (unlike full-lime renders).
    Patent 'cementitious' adhesive/base coats/finishes may have little cement, many additives like acrylic, and carefully graded aggregates, and can be readily designed to have high vapour permeability - trust the manuf's data.

    Adding more outboard insulation (assuming all is vapour permeable) can only, beneficially, move the dew point outward as inner layers will be that much warmer. Just don't put trad cem render over it (unless as a back-ventilated rainscreen). Even winter-long interstitial condensation in outermost layers of EWI won't cause any problem to it or to EWI-designed vapour-permeable patent acrylic or mineral renders, whether 'cementitious' or not. Generally likewise with trad full-lime renders - but the many variants of that need careful checking for compatibility with the EWI.
    • CommentAuthortony
    • CommentTimeJul 29th 2017
     
    Unless it freezes in or under the outer surface, local climates vary.
    •  
      CommentAuthorfostertom
    • CommentTimeJul 29th 2017
     
    Well, it does seem to be proof against such freeze - winter condensation in the outermost layer and the render thickness is normal - and seems no ill effects - maybe all the components are stretchy enough to accomodate ice expansion.
    • CommentAuthorgravelld
    • CommentTimeJul 30th 2017
     
    Thanks for that overview Tom. Although I'm not sure if I can trust manufacturers claims these days.
    • CommentAuthorCWatters
    • CommentTimeJul 30th 2017 edited
     
    Posted By: MarkyPAny amount of EWI will raise the temp of the cavity and the outer leaf, and should surely only push the dew point further out in the build up? Does it even matter if the dew point appears somewhere out in the build up? On a very cold winter's day, my existing cavity must be at risk of condensation anyway.


    The existing cavity should be ventilated so any condensation that occurs at present can escape. The BCO is probably concerned that when EWI is added the dew point might not be far enough to the outside so that condensation can still form in the outer leaf - only now it won't be able to escape as the ventilation is blocked.

    I suppose one option is to ensure there is enough EWI to move the dew point outside of the existing outer leaf entirely. Perhaps you need a proper condensation risk analysis. See if the EWI companies will do one for you?
    • CommentAuthortony
    • CommentTimeJul 30th 2017
     
    Anything more than 50mm of insulation on a house would do that but it would be mad to put on that little anyway.
    • CommentAuthorgravelld
    • CommentTimeJul 31st 2017
     
    Posted By: CWatters
    The existing cavity should be ventilated so any condensation that occurs at present can escape. The BCO is probably concerned that when EWI is added the dew point might not be far enough to the outside so that condensation can still form in the outer leaf - only now it won't be able to escape as the ventilation is blocked.

    I should hope it isn't ventilated because there'll be little point in adding the EWI. (Unless I misunderstand what is meant by cavity here, I assumed Marky meant a masonry cavity).

    Posted By: CWatters
    I suppose one option is to ensure there is enough EWI to move the dew point outside of the existing outer leaf entirely. Perhaps you need a proper condensation risk analysis. See if the EWI companies will do one for you?

    I think you're right there, it's about moving that dew point as far as possible outside, and best way of doing that is more insulation. Trouble is if the existing insulant in the cavity is too (!) performant, it will require much more EWI right?
    • CommentAuthortony
    • CommentTimeJul 31st 2017 edited
     
    Probably not though the morevthe better, dew point is rarely anywhere but in the very outside few millimetres
    • CommentAuthorCWatters
    • CommentTimeJul 31st 2017
     
    In the case of a warm flat roof most of the insulation is usually above the joists. I remember reading somewhere that if you want to put some insulation between the joists as well then it should be no more than a third of the total. On that basis 50mm in the cavity and 100mm EWI would seem to be a reasonable starting point.
    • CommentAuthorgyrogear
    • CommentTimeJul 31st 2017
     
    <blockquote><cite>Posted By: tony</cite>Probably not though the morevthe better, dew point is rarely anywhere in the very outside few millimetres</blockquote>

    er, should this read, "anywhere BUT " ?

    gg
    • CommentAuthortony
    • CommentTimeJul 31st 2017
     
    Yes but don't they mean the same with or without the but?
  25.  
    Posted By: tonyYes but don't they mean the same with or without the but?


    Not for me.

    However you could also say
    .............dew point is rarely anywhere " other than " in the very outside.............
    • CommentAuthorgyrogear
    • CommentTimeJul 31st 2017 edited
     
    <blockquote><cite>Posted By: tony</cite>Yes but don't they mean the same with or without the but?</blockquote>

    well, basically, no...

    "dew point is rarely anywhere in the very outside few millimetres"
    means that it is rare for the dew point to occur in the external few millimeters.

    whereas "dew point is rarely anywhere but in the very outside few millimetres"

    means that generally the dewpoint will occur in those last few outside millimeters.

    Which is the total opposite to the 1st version
    (and which, I suggest, is what you really meant...)

    Apologies if I appear to be splitting eggs or whatever !
    (my habit, having raised two dsylexic children...).

    gg
    • CommentAuthortony
    • CommentTimeJul 31st 2017 edited
     
    Sorry I will edit it

    The dew point is rarely anywhere but in the very outside few millimetres
    • CommentAuthorgyrogear
    • CommentTimeJul 31st 2017
     
    I like it !

    gg
  29.  
    Am interested why do you think that, Tony?

    For inside air at 20C 70%Rh, the dewpoint is 14C. In winter, this temperature will occur halfway through the wall thickness, not in the outer few millimetres. As was noted the outer leaf of a filled cavity wall could all be below internal dewpoint.

    This is not a problem if the outside of the wall is more vapour open than the inside, as moisture will permeate out faster than it can build up. But Marky the OP has this the other way round, with EPS outside of wool. A vapour barrier on the inside should 'fix' this, if its really a concern**

    You can check this by modelling the insulation as lots of thin layers instead of 1 thick layer, to see whereabouts through the insulation thickness you first hit dew point. (the stars in the pic below, http://www.vesma.com/tutorial/uvalue01/uvalue01.htm )


    **Many GBFers advocate that water in walls is more to do with rain and airleaks, rather than condensation, but that's for Wufi etc
      IMG_20170801_203227.jpg
    • CommentAuthorgoodevans
    • CommentTimeAug 4th 2017 edited
     
    I'm going to stick my neck out here...

    A house with 70% humidity at 20 deg C inside when it is 0 deg C outside has ventilation problems - 12g/m3 inside, 4.8g/m3 outside - A continuous ventilation system in all wet rooms would be able to keep the average down to a difference of 2g/m3 (a MVHR does just as well but without the heat loss) - Even trickle vents do a lot better than this scenario (but I accept that occupants can and will do strange things to houses).

    A vapour barrier will help the calcs because of the insanely high Sd values on vapour barriers - only valid if there is absolutely no holes - but as most houses have a blower test above zero m3/m2Env at 50Pa you can assume the calcs using those Sd values would not be valid.

    Even taking into account the massive internal humidity in the scenario above - if you slice the inner layer of eps again you will find the dew point inside the eps not on the inside surface. In reality there will be so little moisture (because of the high Sd value of the EPS) that there will be no issue - and it gets even more complicated because as the vapour condenses out heat is released into the eps slowing down the condensing action. And once the vapour has turned to liquid it will move through the eps in a different way due to capillary action.

    But I could be wrong - is there a rash of EWI problems out there with interstitial condensation that can not be attributed to poor ventilation or from rain ingress reservoirs. (If so I'd like to see some eps sectioned and see the a dew point half way through i.e. one side dry the other side saturated).

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