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General: Target air tightness for retrofit

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  1.  
    • CommentAuthorgravelld
    • CommentTimeAug 24th 2017 edited
     
    Making progress on my PHPP. Only been caught out by copy and paste once or twice :devil: .

    For planning my retrofit I need to think about a target air tightness level. But I really have no idea what to aim for, or even what is achievable. The current AT is not measured, but I'm pretty sure permeability is high.

    The existing property is:

    - Masonry cavity, 1950s build, extended lots of times (plenty of steel!). Wool in some cavities, EPS bead in others.
    - Solid floor, uninsulated
    - Fully rendered (with plenty of cracks and popped sections)
    - Cold loft, lots of ceiling insulation

    The retrofit will involve:

    - AT layer to be the external render and (I think) the layer on top of the existing rafters. WT to be outbound of all insulation
    - EWI, including down to founds (currently thinking truss + cladding rather than sticky EPS blocks)
    - New joinery (high spec I hope)
    - Removing/narrowing some windows, enlarging others
    - Filling the cavities completely (those that aren't already)
    - Insulation over the roof to connect to the EWI
    - Nothing happening to the floor

    Anybody got any kind of stats for what retrofits can achieve?
  2.  
    Whats the building construction, cavity wall, solid brick, stone/rubble, TF or what have you?
    • CommentAuthorgravelld
    • CommentTimeAug 24th 2017 edited
     
    Posted By: Peter_in_HungaryWhats the building construction, cavity wall, solid brick, stone/rubble, TF or what have you?
    Thanks, edited...
  4.  
    ''The current AT is not measured, but I'm pretty sure it's high.''

    Highly air-tight? Or high permeability?

    MVHR?

    I'd look at aiming for 3 or less, and hopefully significantly less. I know 2 people who have got around 1 ACH on retrofit.
    • CommentAuthortony
    • CommentTimeAug 24th 2017
     
    That won't be easy due to the cavity wall construction
  6.  
    Sorry Tony. You are right. I had not properly read the piece re the a/t layer being outboard. The house I've been working on has the a/t layer inside, as membrane inside IWI, hard plaster on walls with EWI, OSB and Intello at FF ceiling level and floor VCL taped to membrane and plastered into walls where applicable.
    • CommentAuthorjfb
    • CommentTimeAug 24th 2017
     
    I managed to get an AT result of 1 ACH on a retrofit having never done anything like it before but was helped by solid wall construction (thick stone) and the airtight layer as internal render on stone (combined with OSB/membranes on roof/ new extensions).
    At the start of the project I wasn't even sure if I would need MVHR as I had no idea what AT result I would achieve so I am glad I did install mechanical ventilation. I was also the one doing most of the AT detailing which is maybe harder if you are trying to get a reluctant builder to do it.
    A lot comes down to detailing of wall/roof junctions and between different materials (what are you doing from the external render to new windows?)
    My point is I think if you are committed to achieving a good result (which it would appear you are) you might be pleasantly surprised by what you can achieve.

    What is the planned roof build up?
    • CommentAuthorgravelld
    • CommentTimeAug 24th 2017
     
    Posted By: tonyThat won't be easy due to the cavity wall construction
    But... cavities filled, and inbound of the AT layer anyway, because AT layer going over the roof. Still a problem?
    • CommentAuthorgravelld
    • CommentTimeAug 24th 2017
     
    Posted By: Nick Parsons''The current AT is not measured, but I'm pretty sure it's high.''
    Sorry, meant AT is low, permeability high.
    • CommentAuthorgravelld
    • CommentTimeAug 24th 2017 edited
     
    Posted By: jfbA lot comes down to detailing of wall/roof junctions and between different materials (what are you doing from the external render to new windows?)
    Tape I guess. Would be installed in the insulation layer, so not sure on the detail exactly.

    Posted By: jfbWhat is the planned roof build up?
    Again, not finalised detail, but probably:

    plasterboard
    Rafters+insulation
    AT layer
    Insulation
    WT layer
    Covering

    I realise I need to be careful about moisture regards the AT layer in the middle of insulation. But the reason it's there is so it can more easily meet the layer at the current external render.

    Thanks for the numbers people, it's good to know that 3 is at least something to aim at, and 1 is even achievable.

    What I haven't said yet is this is an incremental retrofit... just to add a bit of complexity.:tongue:
    • CommentAuthorjfb
    • CommentTimeAug 25th 2017
     
    OSB as the AT layer on top of the rafters? Or where you thinking a membrane?
    OSB handy for walking around on the roof while installing the insulation on top.
    • CommentAuthorgravelld
    • CommentTimeAug 25th 2017
     
    Either... again, not decided. But it was going to be one of those two. Something you are thinking of?
    • CommentAuthorjfb
    • CommentTimeAug 26th 2017
     
    well like I said I wouldn't want to install insulation on a roof without something to stop you going between the rafters. I guess it depends on choice of insulation/rafter spacing. EPS isn't going to support you without something underneath, maybe wood fibre sarking would, but then how are you going to install a membrane for airtightness with just rafters?
    There are quite a lot of previous posts on GBF on ways of making OSB airtight.
    • CommentAuthorgravelld
    • CommentTimeAug 26th 2017
     
    Thanks, I'll maybe start a thread about that soon. Although that said - the roof is likely to be the last stage of the incremental work.

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