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Housing - Renovation: Window recess / alcove insulation help

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  1.  
    Hi all,

    Not sure if they are recess's or alcoves, but they are the internal recess for windows in circa 1850 rubble filled walls.

    I'm getting black mold on the plaster board which is a result of these areas being only 1 stone thick ? and not benefiting from the thermal mass effect that the rest of the building enjoys. (Walls are approx 24 inch away from windows, approx 8 inch under window sill)

    So, my question is, what is the best way to insulate these ?

    Any ideas very welcome.

    TIA
    • CommentAuthorDavycrocket
    • CommentTimeJan 19th 2021 edited
     
    Plaster boarded using the dreaded dot and dab method !
      20210119_115608.jpg
    • CommentAuthorrevor
    • CommentTimeJan 19th 2021
     
    A possible solution which would sort the issue is to externally insulate your walls avoiding cold bridging so is not a quick nor cheap solution but you would get the benefit of the insulation to all the wall. Others may have other ideas but I have externally insulated a 1750's stone wall and that has worked really well.
  4.  
    Hi, thanks for the reply.

    I'm not yet in a position to do the ewi, I'm looking for a relatively speaking cheaper option ?

    Thank you
  5.  
  6.  
    <blockquote><cite>Posted By: WillInAberdeen</cite><a href="https://readinguk.org/draughtbusters/basic-techniques/walls-and-windows/" rel="nofollow">https://readinguk.org/draughtbusters/basic-techniques/walls-and-windows/</a>Some good stuff here of Tony's.</blockquote>

    Thanks for the reply.

    The problem we have is that the dots of plaster board are where the mould is forming, so the polystyrene balls wouldn't fully alleviate the issue.

    I was thinking more along the lines of building outwards to form a boxed in area finishing flush with the thicker parts of the solid wall ?

    Is this advisable ?
    • CommentAuthortony
    • CommentTimeJan 28th 2021
     
    Good idea but i would remove the dot and dabbed bit first, if you add more inboard it will get even colder and more prone to collect condensation,

    could you post a pic of the dots with mould on them please :)
  8.  
    Posted By: DavycrocketI was thinking more along the lines of building outwards to form a boxed in area finishing flush with the thicker parts of the solid wall ?

    I'm in the process of building some window seats underneath a couple of windows to build the recess out to be as thick as the solid stone wall. There will be insulation stuck to the wall and concealed by the backrest of the seat. Like Tony said, I removed the existing plaster so that there is no gap left that will get condensation or draughts.
  9.  
    <blockquote><cite>Posted By: tony</cite>Good idea but i would remove the dot and dabbed bit first, if you add more inboard it will get even colder and more prone to collect condensation,

    could you post a pic of the dots with mould on them please :)</blockquote>

    Pic as requested
      20210202_131740.jpg
    • CommentAuthortony
    • CommentTimeFeb 2nd 2021 edited
     
    I am suspecting that the wall is very cold and is streaming with condensation, this water then soaks into the dabs and makes the plasterboard damp, when the heating goes off this makes the paint wet and it goes mouldy.

    Interesting to holesaw a bit out and put some Perspex over it then take photos every hour.

    Is there ant evidence of rotting skirting, puddles on the floor, efflorescence of brickwork low down outside or water running down the wall inside under the floor

    Bit surprised to see mould so close to a rad, what paint was used?

    Thanks for the pic
    • CommentAuthorDavycrocket
    • CommentTimeFeb 3rd 2021 edited
     
    Hi

    It was a dulux std emulsion used on the walls.

    There is no evidence of water running down walls or puddles forming and its red sandstone behind the board and as such it may soak any water up ?

    Looking at it the wall its approx 8 or 9 inch thick below the windows, and a good 20 to 24 inches elsewhere.

    So its likely to have zero value as an insulator ?

    Question is how do I remedy this ?

    TIA

    Dave
  12.  
    <blockquote><cite>Posted By: WillInAberdeen</cite><blockquote><cite>Posted By: Davycrocket</cite>I was thinking more along the lines of building outwards to form a boxed in area finishing flush with the thicker parts of the solid wall ?</blockquote>
    I'm in the process of building some window seats underneath a couple of windows to build the recess out to be as thick as the solid stone wall. There will be insulation stuck to the wall and concealed by the backrest of the seat. Like Tony said, I removed the existing plaster so that there is no gap left that will get condensation or draughts.</blockquote>

    Hi

    What type of insulation did you use ?

    Did you add a vapour barrier ?

    Thank you
    • CommentAuthorwookey
    • CommentTimeFeb 23rd 2021 edited
     
    That's pretty bad. Does that radiator rarely get used?

    Yes, your solid 8-9" wall with dot+dab plasterboard over it has negligible insulation value.

    I'd say the minimal fix would be pull that plasterboard off and put a sheet of insulated plasterboard up instead, or better (and cheaper, DIY) a sheet of PUR or woodfibre and plasterboard over the top. Woodfibre will remain breathable which might be a good idea if it's an old building with dubious DPC. PUR will be much better insluation per thickness.

    Almost any amount of insulation will significantly raise the temp of that wall, and remove the cold-spots which should be enough to stop the mould by removing the condensation. If it's single skin sandstore then I'd stick to 60 or max 80mm woodfibre to keep condensation risk within reasonable bounds. EWI is so much better if you can do it (no condensation risk, much better U-values possible).
  14.  
    Hi Wookey

    Thanks for the reply, sorry for my late one.

    The radiator does get used sparingly,but it's a pretty cold room.

    What is PUR please ? I know about wood fibre but not this abbreviation sorry.

    Dave
    • CommentAuthorEd Davies
    • CommentTimeJul 3rd 2021
     
    PUR = polyurethane. Typical brands are Celotex and Kingspan. PIR is similar (can't remember what it actually means, polyisocyanurate something?). Technically they're different things, I think, but the two terms tend to get used interchangeably to mean basically foamed plastic insulation.
  16.  
    PIR = polyisocyanurate resin

    PIR and PUR are made from similar kinds of isocyanurate molecules, just arranged differently, so they are very similar materials. There's another option called Phenolic which is slightly better.

    The brands Ed mentioned sell a variety of these in different products, so check which one it is you are getting.
    • CommentAuthorEd Davies
    • CommentTimeJul 4th 2021
     
    Posted By: WillInAberdeen…so check which one it is you are getting.
    Why? I.e., what's the difference? This is one of those questions I've been meaning to ask for a while.
    •  
      CommentAuthordjh
    • CommentTimeJul 4th 2021
     
    PIR is 'better' than PUR. i.e. lower lambda. I was going to point to a Kingspan page that provides a lot more detail but it seems to make my browser become unusable, so I won't :P

    But Kingspan are perhaps better known for their K range, which is phenolic, and has far less questions/black marks against it for fire safety. It wasn't involved in Grenfell for example.

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