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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorMarkyP
    • CommentTimeJan 6th 2021
     
    Hi all,

    I'm renovating and extending my house and adding loft rooms. At the design stage I missed a pretty rotten cold bridge at the gable rafter line where the blockwork inner skin is a heat loss path to the outside. I have some marmox xps backer board which I am considering running a 300mm strip of along the roof line over the face of the inner skin blockwork fixed by tile adhesive and a few screws. The rest of the gable wall will be plastered with hardwall flush with the face of the marmox board, the joint taped and the whole lot then plaster skimmed. The marmox board is only 10mm of XPS but i have some spare and fitting will be easy enough, and it will at least reduce the cold bridge and extend the direct heat loss path to around 550mm - 300mm marmox plus rafter level insul depth so i think worth doing.

    Question is will the joint on the face of blocks between marmox and hardwall crack? I'll use wide scrim and I think it will be OK but plasterer is less sure. It will be all fixed to the same underlying blocks so ought not to be under much strain. I have done similar scrimmed joints between backing plaster and plaster board over same underlying blocks before without issue.

    Anyone ever tried anything like this? Am looking also at thicker marmox but too thick becomes a pain to match with the backing plaster so realistically max would be 20mm board or maybe 10mm aerogel/6mm mgo board but have just seen the prices!
    • CommentAuthortony
    • CommentTimeJan 6th 2021
     
    When I built my gable I omitted the inner skin of blocks all together tying the outer skin to the roof cavity insulation joins ceiling insulation

    Not quite sure what a gable rafter line is in your case
    • CommentAuthorMarkyP
    • CommentTimeJan 6th 2021
     
    A picture may help, the red line shows the position of the insulated board. Plaster joint with board on face of blocks at lower red line.
      wall2.jpg
    • CommentAuthorMarkyP
    • CommentTimeJan 6th 2021
     
    Sorry to be clearer the redline shows the proposed position of the marmox xps backer board at the area where the cold bridge is worst, fixed to face of inner skin following the rafter line on the gable. Below redline would be float and set wet plastered up to insulation board. Board and hardwall skimmed to flush finish, joint scrimmed
    • CommentAuthortony
    • CommentTimeJan 6th 2021
     
    Lintel looks bad too. Watch out for draughts entering anywhere they can.

    I see it cracking, I would do whole wall
    •  
      CommentAuthordjh
    • CommentTimeJan 6th 2021
     
    Where's the gable wall insulation?
    • CommentAuthorMarkyP
    • CommentTimeJan 6th 2021
     
    It's a cavity wall fully filled with mineral wool, subsequently EWI'd. The ewi carries over the face of door frame and lintel lip. The worst issue is the top face of the gable block inner skin which is uninsulated under the tiles. The wall cavity plus ewi amounts to 250mm of insulation, dritherm in cavity, graphite eps ewi.
    •  
      CommentAuthordjh
    • CommentTimeJan 6th 2021
     
    OK, thanks. I think your plasterer is right to be concerned but a wide scrim may well solve it. You're likely to get cracks at the junction with the roof anyway and the line along the edge of the Marmox should only be a hairline if it does occur. Decorator's caulk should fix if it does happen.
    • CommentAuthorMarkyP
    • CommentTimeFeb 11th 2021 edited
     
    so by way of an update i have decided to give the Diasen 037 insulating plaster a go here. the thermal conductivity is surprisingly good (part of me wonders if it is a bit too good!) but if datasheet is to be believed it sits in the same bracket as EPS . I'm going to stick about 20mm of it on the whole gable in lieu of what would otherwise been my original idea of 12mm XPS and backing plaster combo . The product is supplied by ecological building systems and have to say it would make a compelling proposition for IWI in a period house at stated thermal performance values. but like all these products it isnt cheap, though in my case I only need a couple of bags, not sure I'd want to do the whole house in it at £40 a bag!

    And on the topic of insulating plaster, I read a few pieces online a while back about products in development with aerogel enriched plasters with target thermal conductivity values in the PIR range - could be game changing for IWI and EWI, looked really interesting but after searching found nothing commercially on offer yet for my situaiton other than a small supplier of aerogel fibres as an addmix for backing plaster but didnt come with much by way of data so gave it a miss. I think Vimark was one manufacturer prototyping aerogel enriched plaster but no news on a production ready product yet that I can see.
  1.  
    That is indeed a surprisingly low conductivity claimed for the plaster, 0.037 W/mK. I wonder how they do it?

    It's apparently a mixture of cork, lime and silica. Another cork/lime plaster, ecocork, claims a conductivity of 0.10 W/mK. Even unbound cork granules in air are 0.044 W/mK.
    https://www.lime.org.uk/ecocork-insulating-plaster.html
    https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
    • CommentAuthorMarkyP
    • CommentTimeFeb 12th 2021
     
    Yes, i am very doubtful it can be as good as .037 but figured that even if it is considerably higher then 20mm or so will be at least equivalent to 12mm xps i originally planned and no joint to worry about. I would guess that .05 would be more realistic for a product like this.
    • CommentAuthorRobL
    • CommentTimeFeb 12th 2021
     
    I used insulating plaster for newly exposed internal reveals after moving windows out into EWI. I used Diasen Diathonite Evolution - it seemed oddly hard to get hold of. It looked like it wouldn't mix with water for quite a while, but eventually comes good. They were the first ones I used - a couple of them have cracked in a couple of places - that probably means nothing except confirming I am rubbish at plastering. I think I will open up the crack a bit and put in some flexible acrylic sealant & repaint.
    I didn't quite have enough, but couldn't get it anymore, then found Proofshield Prooftherm, easier to mix and use but self reported only lambda 0.072.
    As you say, I saw reports of magic aerogel plaster, but I've no idea how you'd get it.
    Both of the plasters were a bit mechanically weak, I skimmed a bit of regular stuff over the top to give a robust surface finish.

    I found this GBF link that casts doubt on the lambda of the Evolution that I used - from username "Bauwer":

    "...As you could see we ranked Diathonite with estimated thermal conductivity of 0.07 vs declared value of 0.045....We thought Diathonite declared a thermal value of the filler, eg Cork, but not the material itself"

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14292&page=3
    • CommentAuthorphiledge
    • CommentTimeFeb 12th 2021
     
    Given that youve made such an effort to insulate to a high level, a work intesive option to get a decent level of insulation over the top of the gable would be to.....
    Run an angle grinder along the inner leaf of block in line with the underside of the rafters. Strip the edge of the roof covering and remove the cut top of the blocks. Refit steel straps to tie the gable to the roof. Fit as much insulation as you can over the top of the blockwork covering the cavity insulation. You might want to extend the insulation to butt up against the EWI but that could be even more rework to the gable end depending on how your gable is built. Reinstate and feel good despite the effort.

    Obviously theres quite a bit of work involved and whether its worth the effort/cost is a personal choice.

    I dont know if this would apply with insulated plaster but I would be concerned about condensation forming along a potentially cold strip at the gable/roof junction if the room was used as a bedroom or bathroom.
  2.  
    I am about to install Diathonite plaster as a levelling layer under woodfibre insulation boards. I too very much doubt the stated thermal conductivity value (how can a mixture of lime and cork have a better value than cork itself??). However it has plus points in that it does not contain any cement (the Bauwer plaster does) and it is very textured so useful for filling gaps and creating a seamless airtight finish for the insulation boards. I suspect you have to be careful with mixing it, the instructions state it must be 'foamy', so I think the stated thermal value is assuming you have trapped the maximum amount of air in the mixture and not troweled it down when applying. Something which I found difficult in my recent test patch.
    • CommentAuthorwookey
    • CommentTimeFeb 18th 2021 edited
     
    I chopped off the top 100mm of the inner leaf under the tiles in order to take the insulation (PUR in this case) from the skeiling insulation between the rafters over to the cavity insulation. Ii.e what philedge said. It is a bit of a faff, especially if there is not scaffolding, but should produce better results than just insulating the inside of the gable.

    I initially thought 'that's too much faff' but after thinking about it decided to do it properly. I've not done the therm sums to see how these compare. I guess I should...
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