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General: external insulation

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    Im just starting to plan the renovation of a Welsh chapel,i can imagine il`l have lots of questions on here!

    The walls are all 700mm thick stone and one of these walls is covered in slate with a 50mm gap. I was thinking this would be a good place to add insulation.Is there a particular type which would be best and does a vertical slate wall need a breathable membrane?
    Thanks.
    • CommentAuthortony
    • CommentTimeMar 5th 2022
     
    Ideally you need a lot more than 50mm of insulation 300mm or 200mm, where would the breather membrane go? Behind the slates? But they are draughty and let water out by design
  3.  
    Posted By: chrisinbrightonIm just starting to plan the renovation of a Welsh chapel,i can imagine il`l have lots of questions on here!

    What do you mean by 'renovation'
    Or are we to assume renovation = conversion to a dwelling?
    Are you governed by planning restrictions concerning the external appearance (or anything other aspect)?
  4.  
    Yes conversion to dwelling but listed so slates have to stay(I would want to keep them anyway)

    Will 50mm of cellotex be hardly any benefit?
    • CommentAuthorJonti
    • CommentTimeMar 6th 2022
     
    Can you not remove the slates to put on a decent thickness and then replace the slates? Listed doesn't always mean you cannot touch it.
  6.  
    +1 to Jonti. Worth a discussion with your BCO/Conservation Officer. Also, although it has been done before with Celotex I would be tempted, even if using a plastic insulant, to use graphite EPS. Also since, as I understand, Celotex does not do a non-foil board (and you don't need foil for a VCL as it's the wrong place for a VCL), have you thought of phenolic? This is available non-foiled for EWI.

    Also, while 50mm is too little, as others state, if you use exg battens it's 50mm insulation with full-depth timber interruptions every however many mm. If you *had* to stick with 50 could you perhaps do 25 ins against the wall, battens, then 25 between, then slate.

    Agree with Tony re membrane. It wouldn't do any *harm* but you almost certainly don't need it.

    Edit to add Conservation Officer
  7.  
    Posted By: chrisinbrightonYes conversion to dwelling but listed so slates have to stay(I would want to keep them anyway)

    Will 50mm of cellotex be hardly any benefit?

    Putting 50mm rigid insulation between the wall and slates will (approximately) change the u value from 2.02 to 0.45 so it would have a reasonable effect. (I assumed a sand stone wall, other types of stone will have different values)

    However if that is the only wall you can do and especially if the slates are in a reasonable condition, I would leave that wall alone and concentrate in the internal surfaces.

    If you externally insulate on that wall only and internally insulate the rest you will get problems at the juncture of the internal and external insulation (corners) that would make it advisable to internally insulate a good bit of the externally insulated wall to mitigate the cold bridge at the corners so best to just treat the internal insulation as a whole and forget about insulating the outside.

    One approach that has been made by at least one other here is to build an insulated box inside the walls of the existing structure as opposed to internally insulating the existing walls. Each solution has its own problems and advantages.

    If you want to play the 'what ifs' with various types of insulation and needs for VCLs against what you have to start with then look at
    https://www.ubakus.de/u-wert-rechner/?
    Use the demo version
  8.  
    Thanks for the replies,very useful.
    The slates have got to come off anyway so it will be a good time to add insulation.I like the idea of doing a two layer unfoiled design(Nick Parsons) and will try and push for more than 50mm.
    At present I cant see that the other stone (slate possibly) walls can easily be internally insulated.
  9.  
    pic attached
      Screenshot 2022-03-06 18.29.22.png
    •  
      CommentAuthorfostertom
    • CommentTimeMar 6th 2022
     
    Oo - so would the EWI behind slates wrap around that return, so standing out from the side wall stonework? Might be hard for Listed Buildings to swallow; on the other hand it would go a long way to creating the overlap between EWI and IWI that Peter mentions.

    There's a quite modest limit to how insulative IWI can safely be, before it leaves the wall outboard of it so cold all winter through, like it's never before been, that so much winter condensation builds up within it, that it never quite evaporates in summer, leading to gradual accumulation of water in the masonry. EWI has no such problem, thicker the better, as that keeps the wall warmer than it's ever been!

    If wanting to IWI up beyond that danger level, that's where Peter's 'insulated box inside the walls' comes in, which must be coupled, strangely, with really free ventilation to outside air of the airspace between wall and box, so the wall becomes effectively a freestanding garden wall, at which it should have no problem. However I doubt you'll be trying to insulate by IWI up to that danger level.
  11.  
    Doesn`t external wall insulation need to be breathable.Im assuming warm dampness will work its way through the thick stone walls.
    ...and EPS or phenolic are not breathable I understand ?
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      CommentAuthordjh
    • CommentTimeMar 8th 2022
     
    Posted By: chrisinbrightonDoesn`t external wall insulation need to be breathable.Im assuming warm dampness will work its way through the thick stone walls.
    Not necessarily. As long as the EWI is thick enough that the temperature on its inner face is above the dewpoint for whatever humidity you have inside the house then there'll be no condensation on the inner surface so no problem. The problems can come with IWI, because it's cold outer face is against the stone (or other) wall surface, so condensation is much more likely.

    ...and EPS or phenolic are not breathable I understand ?

    EPS is breathable to some extent or at least not impermeable to moisture. µ is 20-40 for EPS 70. Phenolic is a lot more vapour resistant, as is XPS
    • CommentAuthorBeau
    • CommentTimeMar 12th 2022 edited
     
    I am no pro but I insulated two outside walls of my dad's farmhouse behind slate cladding. I used 3"x2" verticals (on edge) at 600mm spacing. Used injection resin studs to secure them to the 600mm thick stone walls. 60mm PU board between the studs and filled all gaps with PU foam from a gun. Then a layer of Tyvec, battens then slate. This was around 10 years ago and no problems we are aware of.

    In hindsight, it would have been nice to stop the thermal bridge of the uprights to the wall but it's way better than it was. The house used to get condensation on the inside of walls but this now never happens
  14.  
    Thanks for replies,all my questions have been answered..ill let you know how it goes.

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