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Green Building Bible, Fourth Edition
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General: insulation and ventilation in eaves

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  1.  
    • CommentAuthormattjcates
    • CommentTimeJun 11th 2024 edited
     
    I am converting my pitched roof loft with 50mm ventilated gap, mineral wool between rafters and pir below then avcl.
    I am installing an MVHR but recognise I will not be getting close to passive house standards on airtightness (retrofit, dot and dab plaster etc).
    The original plan was to run the mineral wool and pir down to the corner of the eaves and be continuous with cavity wall insulation.
    the problem is that trying to do the avcl detail in the corners is pretty much impossible because the space is so tight so I am planning to bring the avcl back to the ashlar walls.
    Some of the MVHR ducting will be running in the eaves, it is insulated ducting and can be covered in plenty of mineral wool and we can tape the ducting entry points through the ashlar walls.
    I am still interested in insulating the pitched roof down to the cavity walls which I think will help with temperature regulation in the eaves in both summer and winter but I'm now not sure about ventilation in the eaves! I'm a bit reluctant to use the pir in the eaves as this could trap moisture from living space below so I wonder about just using mineral wool between the rafters as a compromise.
    Any thoughts!
    thanks
  2.  
    By 'corner of the eaves' do you mean one side or (more likely?) the 'point of the triangle'?

    Do I understand you right that you plan to put the VCL down the sloping ceilings, down the wall and (???) but you plan to put the insulation all the way down the rafters, including in the void where there is no VCL, to the eaves?

    What depth are your rafters? I see your original plan was min wool and PIR but that you feel minded to dispense with the PIR in the eaves. You'd need 270mm of mineral wool to meet the 0.16 target. Look up 'Larsen Trusses' if you are not familiar. It's basically how to get 270mm insulation in a rafter which starts at 75-100mm.
    • CommentAuthormattjcates
    • CommentTimeJun 11th 2024
     
    thanks yes all of that is correct and I mean point of triangle. The plan would be to bring the VCL down the ashlar wall then tape around the joists to meet the ceiling below at that point.
    you are right that I would struggle to meet building regs for the pitched roof with mineral wool alone in the eaves. I wonder if this is a bit of a grey area though as I could get the ashlar walls up to building regs so any mineral wool between the rafters in the eaves would be an "added extra". I'm not sure how the BCO would feel about this..
  4.  
    Not sure if this will help, but there are pre-formed Eaves Baffle products (cardboard or plastic) that allow you to push mineral wool insulation up against a surface and still maintain a ventilation path above:

    https://www.greenbuildingadvisor.com/cad/detail/thin-profile-attic-eave-baffle
    • CommentAuthormattjcates
    • CommentTimeJun 12th 2024
     
    Thanks very much that does help, using these, nhbc seem happy with a continuous layer of insulation from flat ceiling to pitched roof so I'm assuming this principle can be extended towards the centre of the building.

    https://nhbc-standards.co.uk/7-roofs/7-2-pitched-roofs/7-2-15-ventilation-vapour-control-and-insulation/

    On balance I think I will avoid having insulation at flat ceiling and pitched roof level with a gap in between as I think that could be a condensation risk for the underside of the rafters.

    I should have enough space to cover my mvhr ducting in 400mm of mineral wool which will reach the rafters at some but not all points.

    I'll get the ashlar/dwarf wall up to building regs spec and save a lot of faffing in a very small space!

    thanks again

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