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Housing - Renovation: Internal Wall Insulation - what about the returns?

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  1.  
    • CommentAuthorathomson
    • CommentTimeJun 26th 2013
     
    We are installing wood fibre internal wall insulation to all the outside walls of a late Victorian solid brick walled house. The build up will comprise 60mm of wood fibre and lime plaster. We have not gone overboard with the thickness of the woodfibre in order to avoid over-cooling the walls and the risk interstitial condensation.

    My question refers to the return of this insulation on the inside walls. The insulation manufacture (NBT - Paventro) recommends a return on to the inside walls for 600mm. A potential installer has suggested 40mm of woodfire on these returns in order to meet this reccomendation.

    This, obviously, would look a bit odd. Rooms with stepped walls, with additional insulation encroaching on the reduction on the room size.

    Has anyone else installed iwi. What did you do with the returns, if anything?

    Would insulated plaster be sufficient. Say, 20mm of Diathonite - http://newton-membranes.co.uk/diathonite-insulation-plaster/ - instead of the 40mm of woodfibre?

    Any thoughts/experience would be appreciated.Thanks.

    This
    • CommentAuthorSaint
    • CommentTimeJun 27th 2013 edited
     
    The advice from the EST and also from modelling on hygrothermal software suggests a return length of between 300mm and 500mm. Both agree that its the length rather than thickness that matters in this application as its the psi value we're looking at. Remember you will already have a 60mm start due to your wall insulation. A 10mm aerogel strip would suffice at around 300mm to 400mm. There are products now that have aerogel with mesh already applied so its simply a case of plastering over with a small stop bead. There are also tapered rigid insulation board options typically tapering from 30mm to 0mm over a length(width) of 300mm. This allows easier feathering of the joint and the finished taper is surprisingly not very noticeable. Other ways of disguising any intrusion would be to extend the length of the insulated returne to say take in, if applicable, the whole width of an alcove albeit with a thin solution or stop the insulation where it hits the architrave around an internal doorway should the opportunity arise.
    •  
      CommentAuthorfostertom
    • CommentTimeJun 27th 2013
     
    If practical, you could taper the thickness, down to nothing, so avoiding that step. Certainly in an old, wobbly-wall house that might pass unnoticed - consider it for your Victorian flat/straight one. Obviously no problem in the alcoves?

    Tapering it does recognise the diminishing temp difference that exists, as you come inboard. You can start it at the full 60mm.
    •  
      CommentAuthorfostertom
    • CommentTimeJun 27th 2013
     
    Posted By: Saintmodelling on hygrothermal software
    Was that WUFI 2D?
  5.  
    The end of the insulation return nearest the external wall should be at least as thick as the internal wall insulation.

    An alternative approach would be to put a thermal break into the internal/partition wall where it meets the external wall. However, before cutting a vertical slot into the internal wall, you would need to confirm whether any structural restraints would be required, e.g. wall ties.

    Will you be removing the existing plaster before starting work?

    David
    • CommentAuthorjamesingram
    • CommentTimeJun 27th 2013 edited
     
    Recently used 20mm(Pavadentro) on the solid wall returns on a project where 40mm was used on external walls.
    Found several walls to be stud, lath and plaster so just cut out and slotted the 40mm through.
    The rooms where quite higgle-piggly so in some instances covered the whole small return wall in 20mm to avoid a step.
    • CommentAuthorSaint
    • CommentTimeJun 27th 2013 edited
     
    Posted By: davidfreeboroughThe end of the insulation return nearest the external wall should be at least as thick as the internal wall insulation.

    David, strangely perhaps, that's not what the hygrothermal modelling showed. The difference is marginal using various thicknesses. Length along the wall has a far greater impact although as usual the Law of Diminishing Returns has a play in this
  8.  
    I get the length thing. If the bricks have 10 times the thermal conductivity of the insulation then the brick path needs to be 10 times longer than the insulation path to avoid it being a "short circuit". However, its a surprise that the insulation thickness isn't critical.

    David
    •  
      CommentAuthorfostertom
    • CommentTimeJun 27th 2013 edited
     
    Saint, we're talking thermal here, not moisture - just to check you were using the Therm-like thermal facility within the hygrothermal software?

    My recent experience with WUFI shows many surprises - both counter-intuitive ones, and surprises as in upsetting the rules-of-thumb that I've been relying on! Big re-think in progress - but generally liberating rather than constricting. WUFI (or Delphin maybe) looks to be indispensable for future work.
    • CommentAuthorSaint
    • CommentTimeJun 27th 2013
     
    Tom, yep it was thermal. Isothermal diagrams etc.
    • CommentAuthorathomson
    • CommentTimeJul 1st 2013
     
    Thanks for the input. Appreciated.

    Yes - the original plaster will be removed prior to starting.

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