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    • CommentAuthorjczxbuild
    • CommentTimeApr 12th 2018
     
    Have a thorny issue - at least to me, which I would appreciate some help with. Extensive trawling of internet has yielded complicated and contradictory information and am none the wiser. So help from the Forum will be appreciated.

    I am renovating a 100-year old, detached two storey house. Location is a wet and windy part of UK. I have researched pretty extensively and have accepted the approach that older building need to ‘breathe’ ie allowing vapour to move in and out of the building. The house is a traditional 450mm rubble stone wall, lime mortared but had original cement harled render externally. This was in poor condition and I have had this removed and replaced with a modern cement render. (I would have preferred a lime-based render but this proved impossible to achieve locally.)

    So, from the outside we have:

    - new cement harled/roughcast render
    - 450mm rubble stone wall with lime mortar
    - 50-70mm space
    - 95mm timber frame with 100mm Pavaflex woodfibre within frame

    My conundrum is, what finishing board to fix to the timber frame.

    Cheap(er) and easy is the ubiquitous plasterboard. The alternative ‘breathable’ route advises a wood wool board with 3 coats of lime plaster – expensive and time-consuming. I am prepared to follow the latter approach if best. But my question is, is it necessary? Given that the new cement render precludes a total breathable solution is there any point in using ‘breatheable’ board and lime render to finish internally?

    Specifically my concern is interstitial condensation - and apologies for perhaps muddled thinking (blame the internet.):

    1. In a warm house, will moisture be transported out through a vapour-permeable insulation to then condense on a cold surface of the inner external wall, causing problems?

    2. Will ventilating the inner space between the insulation and inner wall solve this problem (though negating some of the benefits of the insulation.)

    3. And will boarding with plasterboard be best and help mitigate this possible problem, or perhaps make it worse?

    Forum expertise required!

    Many thx.
    • CommentAuthortony
    • CommentTimeApr 12th 2018
     
    As I see it your render on the outside will prevent moisture breathing out through the walls :sad:

    I would go for a vapour membrane on the warm side of the insulation.
  1.  
    1. In a warm house, will moisture be transported out through a vapour-permeable insulation to then condense on a cold surface of the inner external wall, causing problems?

    Probably yes

    As Tony said a VCL on the warm side of the insulation.

    too late now (unless the internal froamwork has not yet been done) but a vapour permeable EWI might have been a better option
    •  
      CommentAuthorfostertom
    • CommentTimeApr 13th 2018
     
    Not all cement render is vapour-tight, esp with large, convoluted surface like harling, and the cement base coat may well have been put on quite weak-mix.

    I don't regard old cement renders as automatically vapour-tight - there's bound to be too many cracks. Especially if harled - the whole point of harling is that it develops a web of micro-cracking rather than the few bigger ones that cause such problems with supposedly 'weatherproof' smooth cement renders.
    •  
      CommentAuthorSteamyTea
    • CommentTime7 days ago
     
    I thought, though may be wrong, that we looked at the difference between cement and lime rendering and found little difference.
    I think it is best to control the moisture in the building with decent (that does not have to mean expensive) ventilation. This is where the problem starts, so deal with the source.
    • CommentAuthortony
    • CommentTime7 days ago
     
    The problem with that approach is that if it is warm inside, warm air can hold a lot more moisture that cold air so problems will result, ventilation will reduce the problem a little but not eliminate them.
    •  
      CommentAuthorSteamyTea
    • CommentTime7 days ago edited
     
    Except that you draw in air from outside with a lower RH once it has warmed up. What RH is all all about , it is relative to temperature not AH.
    • CommentAuthorKenny_M
    • CommentTime5 days ago
     
    My understanding from my own investigations on this, and my experience of living in old sandstone houses for many years, and more recently in one that has been clad in cement render, is that there are two main ways that water is entering the wall (from outside). When the historic building types talk about breathing, they are usually referring to rain falling on the walls, and entering the porous stone, then 'breathing' out through the lime mortar pointing. This is why cement pointing causes spalling of the stone.

    In theory if the walls are covered with an impermeable cement render, then no water can enter so as long as it was dry when the cement went on it shouldn't be a problem, but in my house that theory falls apart because the house is too old to have a damp proof course, and the other main entry point is damp rising from the ground. I have though about this a lot and considered that one way to deal with this in a house that is cement rendered is to ensure that there is a strip along the bottom that is left bare stone to allow the rising damp to breathe out, but the wall surface may also need to be vapour permeable on the inside.

    The OP property is 100 year old so probably has a damp proof course and the above probably wont apply, so I would have thought the main concern would be controlling internal humidity and the design of the wall/insulation internally.

    P.S. I don't agree that there is little difference between cement and lime render. When mixed to the same proportions with sand, lime is much more flexible and vapour permeable. If the cement mix is very weak then it might be as vapour permeable as a standard lime mix, but most trades seem to use a standard 3:1 mix for sand cement render.
  2.  
    Totally agree with you Kenny M. Have also used the same exposed stone as a solution to rising damp and it does work. As for the difference between cement and lime when used as pointing is very noticeable. The Barn next door to me is suffering from an idiot builder who thought proud cement pointing on a sandstone building looked good. In places the sandstone has regressed over an inch in less than 30 years.
    • CommentAuthorKenny_M
    • CommentTime5 days ago
     
    I've attached a research paper that I read a while back but then couldn't find again, that compares lime and cement mixes for anyone who is interested.

    It came to a limited, caveat filled conclusion that in relation to compressive strength, flexural strength, water absorption and porosity, an 8:1 sand/cement mix, performs similarly to a 3:1 sand/lime mix. The suggestion was that with more research, 8:1 cement mixes could replace lime in some situations, but the caveat is that there may be other factors and so to do this would be experimental. However, the fact remains that it is likely that the majority of cement renders currently applied to houses will be a 3:1 mix.
    •  
      CommentAuthordjh
    • CommentTime5 days ago
     
    Posted By: Kenny_MI've attached a research paper that I read a while back but then couldn't find again, that compares lime and cement mixes for anyone who is interested.

    Interesting paper, thanks. It only investigates hydraulic lime though, so leaves a lot of unspoken questions.
    • CommentAuthorjczxbuild
    • CommentTime5 days ago
     
    Thx for comments. For clarity I haven't yet purchased/installed the wood fibre insulation. Depending on comments/further investigation it may be just as well going for a Kingspan-type solution rather than breathable wood-fibre. Thinking is that a non-permeable,well-taped 100mm insulation wall will, in effect, keep moisture within the internal shell. (internal areas can be kept ventilated.) Comments welcome.

    Re damp proof course, non existent I'm afraid, house sits higher than surrounding land and rising damp is not an issue.

    Re difference between lime and cement I tend to agree there is a considerable difference re permeability, esp with the additives such as SBR Bonding that local builders add to the mix
    • CommentAuthorlineweight
    • CommentTime5 days ago edited
     
    The whole "breathable" thing is ripe for confusion. I think part of the problem is that it gets used to mean different things in different contexts.

    Firstly, when starting out trying to understand things, it's easy to get it confused with airtightness or lack of. Because the word makes it sound like it's to do with transfer of air rather than just moisture. So people get trapped in a false dichotomy of "breathing" house vs "airtight" house.

    Then there are some who seem to promote it as a concept for letting humidity within a building make its way to the outside. Somehow. For what it's worth, some time ago I decided that just didn't seem to make sense or seem plausible.

    What does seem to make sense is building in some level of breathability as a safety measure to get rid of moisture that shoudln't be there in the first place. Either residual from construction, or getting in through leaks. The latter seems like a sensible thing to give some thought to with old buildings in particular. Or - potentially, not from leaks as such but via porous traditional materials that previously would have been paired with different materials from what they might find themselves in partnership with after a renovation.

    As a general observation - I too have been plagued by contradictory information in trying to understand the broader subject.

    The lime vs cement thing is a case in point. You might find it interesting to read through this thread which I started some time back when trying to decide what to use to repoint some old brickwork. There just wasn't a clear answer. All the evidence seemed sparse and sketchy.

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=8623&page=2#Item_30

    This thread may or may not also be of interest

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&page=3#Item_12
    • CommentAuthorlineweight
    • CommentTime5 days ago edited
     
    Posted By: Peter_in_Hungary1. In a warm house, will moisture be transported out through a vapour-permeable insulation to then condense on a cold surface of the inner external wall, causing problems?

    Probably yes

    As Tony said a VCL on the warm side of the insulation.

    too late now (unless the internal froamwork has not yet been done) but a vapour permeable EWI might have been a better option


    I'd agree with this.

    To deal with the risk of condensation on the inner surface of the masonry, you have to install a VCL.

    Once you've installed a VCL, there's no point having the woodfibre insulation because anything "breathing" through it towards the inside can't go anywhere. So might as well replace it with PUR or similar which will give you better insulation for the same thickness, and be less expensive.

    Having dealt with the condensation problem there may still be a legitimate concern about moisture that gets trapped behind the cement render, from leaks and so on. Without changing the render the only way to deal with that is potentially to ventilate that cavity, to some extent at least. But vented to the outside air of course - not the inside.

    This is pretty much where I ended up in my project described in the second link above. My concern there was about rain driven moisture in old porous brickwork with no heat from inside to dry it out.

    The fact is, internal insulation of old fabric causes a lot of headaches! Much easier to do it externally if possible or acceptable aesthetically - which it in't always.

    My feeling is that woodfibre insulation really only makes sense in an external-insulation context.
    •  
      CommentAuthordjh
    • CommentTime5 days ago
     
    Posted By: lineweightOnce you've installed a VCL, there's no point having the woodfibre insulation because anything "breathing" through it towards the inside can't go anywhere.

    That depends on the VCL - if it is an 'intelligent' one then there is still an escape route for vapour in the correct conditions.
  3.  
    Posted By: lineweightMy feeling is that woodfibre insulation really only makes sense in an external-insulation context.

    Providing you can guarantee that it won't get wet via a failure of any rain screen that will be installed.

    Enter from stage left - EPS - which is cheaper and not affected by any water coming through the final layer.
  4.  
    Posted By: Peter_in_Hungary
    Posted By: lineweightMy feeling is that woodfibre insulation really only makes sense in an external-insulation context.

    Providing you can guarantee that it won't get wet via a failure of any rain screen that will be installed.

    Enter from stage left - EPS - which is cheaper and not affected by any water coming through the final layer.


    Yes. Then we enter into the question of how breathable EPS is. Which also doesn't seem to have a clear answer.
  5.  
    Posted By: djh
    Posted By: lineweightOnce you've installed a VCL, there's no point having the woodfibre insulation because anything "breathing" through it towards the inside can't go anywhere.

    That depends on the VCL - if it is an 'intelligent' one then there is still an escape route for vapour in the correct conditions.


    Yes true. Had forgotten that's an option.
    •  
      CommentAuthordjh
    • CommentTime5 days ago
     
    Posted By: lineweightThen we enter into the question of how breathable EPS is. Which also doesn't seem to have a clear answer.

    I think it's clear that it's breathable enough for EWI, plus of course EWI doesn't necessarily need to be breathable. As Tom said, it's possible to breathe through it, so it must be pretty breathable.
    • CommentAuthorKenny_M
    • CommentTime4 days ago
     
    With the caveat that I understand breathable is a poor term, what I mean when I say this is presumably vapour permeability or vapour transmission?

    I'm still not convinced about EPS being right for historic buildings. I'm not convinced that its not either mind you, and Tom's statement that you can breathe through it has given me pause, but I posted before some figures for breathability and EPS was a lot less vapour permeable than for example lime mortar and wood fibre. I also contacted Historic Scotland and somehow managed to get their senior technical advisor, and while they were all on for insulation, and they they were not at all convinced about EPS being sufficiently breathable for old buildings. They were promoting products like aerogel, cellulouse, wool, wood fibre etc. There is also that word, hydro something, that means the ability to hold on to water then release it later, which I think EPS does not have, and wood fibre might? I definitely remember hearing that lime plaster had this ability to get very damp, and then as long as it eventally dried out, was fairly unaffected.

    However, having thought a lot about this before and after, I agree that if a wall is dry already, putting EPS on it, or anything for that matter shouldn't matter, because if no water gets in then it doesn't need to breathe out.

    I too agree that water vapour is unlikely to transfer through a 600mm masonry wall from inside to out, in any meaningful quantities, but I think that the inside surface/insulation probably needs to be breathable so that water that does enter, can get back out. I've heard Tom discussing this idea before, that over a period it needs to be able to handle the rise and fall of humidity. The models that I have seen for old buildings suggest this too, and is probably where the term breathability comes from, because its like the building breathing in the moisture then breathing it out again.

    The one thing that I do think is continually forgotten though, is the lack of damp proof course in old buildings. My house has been covered in cement render on 3 sides for presumably around 30 years. Apart from some penetrating dampness in one spot, caused by a chimney leak there is no sign of any dampness, anywhere upstairs, or in fact anywhere in the house above about a foot from the ground. All of the serious problems are at ground level, and seem to be because the moisture is being drawn up from the ground and is unable to breathe out due to impermeable surfaces inside and out, and ultimately just rots the wood at the back of the skirting boards etc.

    Having just had the cement render taken off the side of the house the stone underneath looks fine, and didn't seem damp. I've started to think that, other than issues at the base which can be dealt with by leaving a gap, and impermeable render over sandstone might not be a problem. Cement mortar definitely is, because if the sandstone is still exposed then it will take in water and when it can't breathe out through the mortar it causes spalling of the stone.
  6.  
    Posted By: djh
    Posted By: lineweightThen we enter into the question of how breathable EPS is. Which also doesn't seem to have a clear answer.

    I think it's clear that it's breathable enough for EWI, plus of course EWI doesn't necessarily need to be breathable. As Tom said, it's possible to breathe through it, so it must be pretty breathable.


    What makes you say it's 'clear' that it's breathable enough for EWI?

    I have, in fact, just this minute done the experiment suggested by Tom, on a bit of EPS I have here. I've sort of convinced myself that yes, it does seem to be possible to suck air through it. That's not really enough for me to say it's clear that it's "breathable enough" for any particular purpose. I assume that if I did the same experiment on a bit of woodfibre insulation of the same thickness, it would be quite a lot easier for me to suck air through. Likewise rockwool although I have no desire to try that. And the published figures I've found suggest that maybe it's more breathable than PIR/PUR, but almost certainly less breathable than woodfibre or rockwool.

    In any case this all seems a bit subjective though, and unlikely to capture all the information you need to know to predict performance in a real situation. There's just for example the "hydro something" characteristic mentioned by Kenny M in the post above. If EPS, or a portion of the thickness of EPS becomes waterlogged what then happens?

    The only thing that would make me fully confident would be to see the results of, say, an experiment where some timber framed, ply sheathed wall was constructed, deliberately made damp, and then covered up with various different combinations of materials. And then moisture content in the timber monitored over an extended time. Maybe such an experiment(s) has/have been done somewhere...if so I'd be interested to know about them.
  7.  
    Posted By: lineweightI've sort of convinced myself that yes, it does seem to be possible to suck air through it.

    For goodness sake - blow don't suck, what are you trying to put into your lungs:shocked:


    Posted By: lineweightIf EPS, or a portion of the thickness of EPS becomes waterlogged what then happens?

    It dries out - as tests of using EPS underground have shown. XPS however does not dry out easily (or at all) once water logged as a test comparing EPS and XPS used under ground showed.
  8.  
    Posted By: Peter_in_Hungary
    Posted By: lineweightI've sort of convinced myself that yes, it does seem to be possible to suck air through it.

    For goodness sake - blow don't suck, what are you trying to put into your lungshttp:///newforum/extensions/Vanillacons/smilies/standard/shocked.gif" alt=":shocked:" title=":shocked:" >


    Posted By: lineweightIf EPS, or a portion of the thickness of EPS becomes waterlogged what then happens?

    It dries out - as tests of using EPS underground have shown. XPS however does not dry out easily (or at all) once water logged as a test comparing EPS and XPS used under ground showed.


    Have you got any reference/link for those tests?
    •  
      CommentAuthorfostertom
    • CommentTime4 days ago edited
     
    Posted By: Peter_in_HungaryFor goodness sake - blow don't suck, what are you trying to put into your lungs
    Fair enough, but you can generate an impressive pressure (vacuum) differential by sucking, that you can't by blowing.

    Consider - how high a column of lemonade can you raise in a long drinking straw? I did work it out and it's way stronger than the 50kPa airtighness test pressure - a self-sealing (by suck, not blow) portable rig that can really test materials. But yes, at your own risk!
  9.  
    Posted By: lineweightHave you got any reference/link for those tests?

    The only ones I can find at the moment are
    http://www.epsindustry.org/building-construction/moisture-resistance
    and
    http://www.epsindustry.org/sites/default/files/EPS Below Grade103.pdf

    Fostertom might have more or other views.
    •  
      CommentAuthorfostertom
    • CommentTime3 days ago
     
    Probly - what about, this time?
    • CommentAuthorgoodevans
    • CommentTime3 days ago
     
    Is it important when talking about lime mixes to specify which type of lime is being talked about - I think it makes the world of difference as to whether you are using hydrated lime or hydraulic lime (and if Hydraulic how Hydraulic it is). I'm no expert here but just saying 'lime' may not be enough.
  10.  
    The performance of EPS when subject to water and its ability to dry out vs. XPS
    • CommentAuthorgravelld
    • CommentTime3 days ago
     
    Posted By: Peter_in_Hungary
    Posted By: lineweightHave you got any reference/link for those tests?

    The only ones I can find at the moment are
    http://www.epsindustry.org/building-construction/moisture-resistance
    and
    http://www.epsindustry.org/sites/default/files/EPS Below Grade103.pdf
    Yeah but...

    epsindustry.org
  11.  
    Posted By: Peter_in_Hungary
    Posted By: lineweightHave you got any reference/link for those tests?

    The only ones I can find at the moment are
    http://www.epsindustry.org/building-construction/moisture-resistance
    and
    http://www.epsindustry.org/sites/default/files/EPS Below Grade103.pdf

    Fostertom might have more or other views.


    Thanks.

    Still doesn't directly tell us what happens in the context of a thick layer of EPS against damp timber in a wall buildup though.
   
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