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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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      CommentAuthorfostertom
    • CommentTimeJan 29th 2020 edited
     
    I am faced with EWIing using wood fibre or cork boards, having only used EPS to date. The joints will prob be relatively gappy or tapered, as boards follow the wall face, which is uneven.

    Q1: Part of an EPS job is to fill the joints with squirty foam, especially if uneven, to kill heat-bypass inboard-to-outboard convenction currents. Is this possible using 'organic' insulation boards - does squirty foam 'take' or bond to these?

    Q2: With EPS, any upstanding corners/edges can be rasped to a smooth, or even a curved face, before rendering. What's the technique with fibrous 'organic' boards? In this case, it'll be lime render, not patent/acrylic
    • CommentAuthorjfb
    • CommentTimeJan 29th 2020
     
    I used foam behind boards if the wall was a bit off. Did it as the boards were fixed so can't say how well foam bonds to wood fibre but pretty sure it does.

    I used a lot of curved corners for reveals and walls. I made a 45 degree cut on the boards before fixing and then rasped after to get a bit more like a rounded corner. The render can make up the slack so the wood fibre doesn't need to be perfectly round at all.

    Wood fibre board is very messy/unpleasant to cut. I was doing bigger cuts with a normal plunge saw/rail. You can get wavy blades for reciprocating saw which makes less mess. And maybe with a thinner board you could use a wavy bladed hand saw.

    I used lime/sand render not pre bagged or patented and it worked well. Make sure you use a mesh - I think mine was 4mm x 4mm. The lighter face of the board is the rendering face (I think that's right - anyway there is meant to be a face that you render on so check which it is).

    Why woodfibre and not EPS? Fire concerns? I would much rather use EPS externally for anything 2 story and below.
    • CommentAuthorKenny_M
    • CommentTimeJan 29th 2020
     
    If the reason for using wood fibre is that it is more vapour permeable than EPS, would using squirty foam partially defeat that purpose?

    I remember checking this out a while back and the brands of sticky foam I was looking at were closed cell, while most regular expanding foams were open cell.

    I suppose that means sticky foam would reduce the vapour permeability of EPS too when used to stick it to the wall, but if its only being used to fill joins then I suppose the effect would be minimal.


    As always, I don't know much about this, just thinking out loud, so if this is not helpful just ignore!
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      CommentAuthorfostertom
    • CommentTimeJan 29th 2020
     
    Thanks jfb - v practical info.

    It's the client's firm desire to use 'eco' woodfibre or cork and patent lime/cork render - but I'm hoping to talk him into EPS and acrylic render, which ain't too bad toxin-wise and tho it has quite high embodied energy and CO2e (but much less than all other plastic insulations), that pales to insignificant compared with the lifetime of energy and CO2e saved by its use. And it's far cheaper, both materials and consequent labour, and better lambda.

    Also EPS is adequately vapour permeable, unlike all the other plastics. So, Kenny_M it's not about permeability.

    Yes, foam makes a %age of the area impermeable, but until that %age gets really high, like 75%, I don't think it makes much difference to the overall permeability - vapour paths just route around the blockages. That's my intuitive feeling - anyone got data or better intuition?!
  1.  
    ''Q1: Part of an EPS job is to fill the joints with squirty foam, especially if uneven, to kill heat-bypass inboard-to-outboard convenction currents. Is this possible using 'organic' insulation boards - does squirty foam 'take' or bond to these?

    Q2: With EPS, any upstanding corners/edges can be rasped to a smooth, or even a curved face, before rendering. What's the technique with fibrous 'organic' boards? In this case, it'll be lime render, not patent/acrylic''

    For wood-fibre IWI I always use 'cutting-fluff' mixed with water as a filler. It's a pain, but I cannot bring myself to mix WF and foam.

    Q2- You *can* rasp, but it's much harder than with EPS. If you have a wobbly wall to follow you can always build up several thicknesses of WF. We did this with a very wobbly rubble-stone wall.
    • CommentAuthorKenny_M
    • CommentTimeJan 29th 2020
     
    Posted By: fostertomYes, foam makes a %age of the area impermeable, but until that %age gets really high, like 75%, I don't think it makes much difference to the overall permeability - vapour paths just route around the blockages.


    I accept that I am no expert, but that doesn't make sense to me. If you take something that has a vapour resistance of x, then you increase the distance that the vapour has to travel then surely you increase its vapour resistance, almost by definition. If you cover even 50% of its area that has to have a significant effect on how much vapour can pass through in a given period of time, I would have thought by 50% or thereabouts.
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