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Housing - Renovation: Insulation fundamentals: airtightness and voids

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  1.  
    • CommentAuthorrsk1
    • CommentTimeAug 11th 2014
     
    When insulating with kingspan type PUR boards:

    Just how important is it to acheive airtightness / an intact vapour barrier?
    Why is it so important?
    How does moisture behind the insulating layer reduce the insulation effectiveness?
    If it proves impossible to have a complete vapour barrier on one part of a wall (eg unable to insulate below floor boards), does it still make sense to obsessively pursue it elsewhere?

    If a void behind the insulation can lead to heat loss through convection currents, is it an argument against mounting ins. boards onto battens (20mm battens means a 20mm void)?
    Is dot and dab a better method because you can get a smaller void?

    Thanks

    The context is insulating / converting an attic full of awkard bits and irregularities: I'm just trying to weigh up just how obsessive it's worth being..
    • CommentAuthortony
    • CommentTimeAug 11th 2014 edited
     
    Just how important is it to acheive airtightness / an intact vapour barrier? CRUCIALLY NO VB = structure decay
    Why is it so important? THERMAL BYPASS AND LOSS OF INSULATION VALUE
    How does moisture behind the insulating layer reduce the insulation effectiveness? MOULD SLIGHT LOSS OF VALUE
    If it proves impossible to have a complete vapour barrier on one part of a wall (eg unable to insulate below floor boards), does it still make sense to obsessively pursue it elsewhere? YES it is very draughty under and between the floor and ceiling eliminating condensation problems! better ait tight and insulated but difficult to find in the UK

    If a void behind the insulation can lead to heat loss through convection currents, is it an argument against mounting ins. boards onto battens (20mm battens means a 20mm void)? Draughts in the void from outside = disaster
    Is dot and dab a better method because you can get a smaller void? NO, see http://readinguk.org/draughtbusters/?page_id=45

    It is very easy to finish up with draughts from outside coming in to the room or under its floor, be diligent.
    • CommentAuthorrsk1
    • CommentTimeAug 11th 2014
     
    >Why is it so important? THERMAL BYPASS AND LOSS OF INSULATION VALUE

    could you explain that? Do you mean through warm air leaking out?

    >Draughts in the void from outside = disaster

    Why? what about insulating an attic ceiling, where by definition you have draughts in the void from outside?
    • CommentAuthortony
    • CommentTimeAug 11th 2014
     
    if you have gaps and cold spots you can get mould growth on the paint and if there is no vapour barrier moisture can then get into your structure possibly causing rot or decay. In Canada a pin hole in a vb can cause an ice build up in the wall in winter which can force plasterboard or cladding to bulge, hasnt quite been cold enough here yet for that but is more likely in a loft.

    thermal bypass renders the insulation ineffective or less effective by allowing air to move near or around it convecting heat to cooler places.

    if you allow a draught from outside to blow in under the attic floor, the floor will be cold and the ceiling of the room below too, this is typical in the vast majority of dormer bungalows and difficult to prevent both initially and through the life of the building due to alterations to services.

    it is OK to have draughts or ventilation outside your insulation not in it or through it.
  5.  
    ''The context is insulating / converting an attic full of awkward bits and irregularities: I'm just trying to weigh up just how obsessive it's worth being..''

    **very!!**

    A slightly different story depending on whether the Pu board is to go onto rafters or onto a masonry wall. In the latter case, very very important to reduce to an absolute minimum the risks of interstitial condensation - condensation on the old face of the wall behind the insulation. What can it do, you asked. Rot out timber lintels, joist ends, and hidden and vulnerable timbers.

    If it's into a properly vented roof, and the VCL is less than perfect, then the moisture should be scavenged away by the ventilation. With a wall it just hits a full stop. That is not an argument for taking VCLs less seriously on roofs, but you can 'get away with it' a little more than on walls, if there are deficiencies.

    You asked also:

    ''could you explain that? Do you mean through warm air leaking out?

    >Draughts in the void from outside = disaster

    Why? what about insulating an attic ceiling, where by definition you have draughts in the void from outside?''

    Yes, there is cold ventilating air behind the insulation, but you have also gone to town on sealing it, so that the cold air cannot get between, say, layers 1, 2 and 3 in a multiple-layer 'sandwich'.
  6.  
    Pic shows final layer. The under-layer, between rafters, was equally tightly taped.

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