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Air Tightness & Scratch Coat

  • 12-10-2010 1:54pm
    #1
    Lemlin
    Registered Users, Registered Users 2 Posts: 22,255 ✭✭✭✭


    Folks,

    Just a quick question.
    First fixing by plumber and electrician are due to start on my house next week.
    Hope to also get the cavity wall beading completed either this weekend or next weekend.
    Just wondering which gets done first: are the membrane and tapes completed and then the wall is scratchcoated?
    Or is the wall scratch coated and then the membranes are put in place?

    I have a feeling the scratch coat is done first but just want to check.

    Also, I am getting the attic spray foamed. Should the airtightness tapes and membrane be put in before the roof is foamed? Or do they go on after?


Comments

  • #2
    tred
    Registered Users, Registered Users 2 Posts: 1,473 ✭✭✭tred


    Lemlin wrote: »
    Folks,

    Just a quick question.
    First fixing by plumber and electrician are due to start on my house next week.
    Hope to also get the cavity wall beading completed either this weekend or next weekend.
    Just wondering which gets done first: are the membrane and tapes completed and then the wall is scratchcoated?
    Or is the wall scratch coated and then the membranes are put in place?

    I have a feeling the scratch coat is done first but just want to check.

    Also, I am getting the attic spray foamed. Should the airtightness tapes and membrane be put in before the roof is foamed? Or do they go on after?


    Lemlin, just wondering, the fact your getting the attic foamed, does that not deliver the air tightness you achieve, so why u bothering then with roof membranes, window taping of course, but dont understand why ur bothering wiht the ceiling membrane now?.

    The scratch coat is basically to seal up any impurities apprently in the blocks, that might allow air to trasnfer through.....So its only really worthy behind the insulated board.


  • #3
    Lemlin
    Registered Users, Registered Users 2 Posts: 22,255 ✭✭✭✭Lemlin


    tred wrote: »
    Lemlin, just wondering, the fact your getting the attic foamed, does that not deliver the air tightness you achieve, so why u bothering then with roof membranes, window taping of course, but dont understand why ur bothering wiht the ceiling membrane now?.

    The scratch coat is basically to seal up any impurities apprently in the blocks, that might allow air to trasnfer through.....So its only really worthy behind the insulated board.

    Yes that's what I was thinking about the roof membrane. Is it not needed because the foam is airtight anyway?

    Does the tape not go on before the insulated plasterboard though?

    I was wondering does it go scratchcoat, tape and then board or do I have it wrong?


  • #4
    mickdw
    Registered Users, Registered Users 2 Posts: 23,688 ✭✭✭✭mickdw


    Also check that your roof construction is suitable for spray foam. THe installers should have an approved detail. For example some of thse systems are only allowable with a ventilation gap on top of thebreathable felt.


  • #5
    Lemlin
    Registered Users, Registered Users 2 Posts: 22,255 ✭✭✭✭Lemlin


    Anyone with a reply - is the tape and mebrane done and then the scratchoat or the scratchcoat and then the membrane and tape?


  • #6
    tred
    Registered Users, Registered Users 2 Posts: 1,473 ✭✭✭tred


    Lemlin wrote: »
    Anyone with a reply - is the tape and mebrane done and then the scratchoat or the scratchcoat and then the membrane and tape?

    Not sure if it matters really. Last few i saw, were taped up then scratch coated down. you cant cement over any of that tape, it doesnt hold mortar.
    So Tape up,then sand cement down from the finished tape line. . Let it dry out well then board it. .


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  • #7
    galwaytt
    Registered Users, Registered Users 2 Posts: 12,822 ✭✭✭✭galwaytt


    Lemlin wrote: »
    Folks,

    Just a quick question.
    First fixing by plumber and electrician are due to start on my house next week.
    Hope to also get the cavity wall beading completed either this weekend or next weekend.
    Just wondering which gets done first: are the membrane and tapes completed and then the wall is scratchcoated?
    Or is the wall scratch coated and then the membranes are put in place?

    I have a feeling the scratch coat is done first but just want to check.

    Also, I am getting the attic spray foamed. Should the airtightness tapes and membrane be put in before the roof is foamed? Or do they go on after?

    Are you only membran-ing the ceilings ?
    Oh, and +1 on the ventilation thing - you need to maintain an air gap inside the 'felt' - how are you proposing to do that ?

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  • #8
    Lemlin
    Registered Users, Registered Users 2 Posts: 22,255 ✭✭✭✭Lemlin


    galwaytt wrote: »
    Are you only membran-ing the ceilings ?
    Oh, and +1 on the ventilation thing - you need to maintain an air gap inside the 'felt' - how are you proposing to do that ?

    The felt is breathable felt.
    Yep, I'm planning to only membrane the ceilings.


  • #9
    mickdw
    Registered Users, Registered Users 2 Posts: 23,688 ✭✭✭✭mickdw


    Breathable felt still needs some way of getting moisture away from it. If you are doing full fill spray foam, a double batten would often be required outside the felt to give a free air gap under the slate finish. As said before, only install spray foam as per agrement board cert approved details.
    Ive seen suppliers trying to sell this material as retrofit to houses with non breathable felt and no way of ventilating the space as well as offering to spray over all vents. They will sell it to anyone.


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