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Vapor barrier over parapet and plastic thermal fixings

Discussion in 'Roofing and Guttering' started by Buildme, 17 Jul 2021.

  1. Buildme

    Buildme

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    Hello, I have two questions about my warm roof concerning VCL and thermal fixings.

    The VCL I have chosen is the Alutrix 600. This was the most expensive brand but its quality outweighs the extra pounds compared to the cheaper brands IMO. My question about this is do I continue the VCL over my cavity on my parapet wall or is it OK finished at the top of the inner faced block on my parapet as shown in pic. Its difficult to find any information about this on the Internet, but my main concern is if any vapor from below where to get through the barrier up the parapet, it could get trapped under the roofing covering materiel(either epdm or fiberglass)I yet have decided to use.

    I will be mechanically fixing the tissue faced insulation boards onto the osb&joists with plastic thermal caps. Below is the only thermal caps I can source. Would these be acceptable for either fiberglass or EPDM roof.

    Screenshot_20210717-094920_Photos~2.jpg 20210717_095015~2.jpg
     
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  3. Buildme

    Buildme

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    I hope yous don't mind me tagging you on here. It's just browsingthe forum you seem to be 3 main posters on here with a lot of valid information.

    @noseall
    @catlad
    @^woody^
     
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  4. catlad

    catlad

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    What's the parapets purpose and can't you make the warm roof meet the cavity insulation?
     
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  5. Buildme

    Buildme

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    What do you mean it's purpose. We choose a parapet for ascetics.

    I have the joists built on top of the solid concrete blocks. In between them and above is thermal blocks on the inner course. Osb ontop of fittings onto of 9x2s. Alutrix 600 onto of the OSB and will have 150mm insulation mechanically fixed onto of this and 25mm insulation up the parapets. I have the cavity closed with 50mm insulation boards and onto of this, I will have osb fixed to both courses of clocks for the EPDM to bond to for a neat finish. Is there any need for the VB to continue over the cavity?
     
  6. catlad

    catlad

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  7. Notch7

    Notch7

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    It’s a bit difficult to visualise without a construction detail.

    given you seem to want robust construction details, I am surprised you aren’t bonding the insulation - mechanical fixings will pierce your VCL. Also bonding avoids thermal bridging at mechanical fixings.

    I can’t see what point there is carrying VCL over top of blockwork.


    What sort of cavity tray are you using?
     
    Last edited: 20 Jul 2021 at 8:41 AM
  8. ^woody^

    ^woody^

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    Yes, a diagram is needed as it's hard to determine what is going on and what is intended.

    Fundamentally, the issue is "what is the purpose of the vcl?" rather than does it need to go up and over a parapet.
     
  9. Buildme

    Buildme

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    150mm cavity built in concrete block. Inner faced top two blocks are thermal blocks.

    Cavity closed off above with 50mm Kingsland insulation . Amy gaps foamed insulation. Cavity pumped with beads.

    9x2 built into the blocks either side.

    Firrings on top of 9x2.

    18mm osb ontop of fittings.

    Alutrix laid on osb and up the outside of the inner thermal blocks.

    150mm tissue faced insulation ontop of alutrix. Mechanically fixed with plastic tube fixings to stop cold bridging.

    Top of Cavity and top of blocks closed off with 11mm osb.

    EPDM all in 1 over the tissue faced insulation, up the parapet and over the cavity. Bonded with contact adhesive sprayed on.

    My question is, do I continue the alutrux over the cavity. You can see in my pic in my first post, I have it to the top of the thermal block! What I have done, would this be efficient enough to achieve full cover to stop any moisture/vapor from entering sections of the roof where it could potentially cause damage later on.

    20210720_211928.jpg 20210720_211928.jpg 20210720_211928.jpg 20210720_211928.jpg
     
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  11. Buildme

    Buildme

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    I did consider this but choose to go with mechanically fixings rather than chemical. I do understand it does pierce the VB however this is a bitumen VB which when you pierce it, it self seals from the information I've gatherer on it. I am using plastic thermal tube fixings for securing the insulation to the roof to stop cold bridging.

    Closed the cavity with Kingspan insulation.
     
  12. Buildme

    Buildme

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    My understanding is the vcl stops any moisture/vapor from entering the roof and destroying the osb. A very important detail on a warm roof.
     
  13. ^woody^

    ^woody^

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    The purpose is to prevent moisture from the warm room below entering the roof structure and becoming trapped there.

    So it does not need to go over a cavity or up and over a parapet.
     
  14. ^woody^

    ^woody^

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    Cavity tray?
    DPC?
    Drip?
     
  15. Buildme

    Buildme

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    Yes, with the information I have gained on it I do understand this. One thing woody, I disagree with the vapor barrier not going up the parapet. If the VB stops at the osb where it meets the block wall, surely one would get some seeping at this detail. Going up the parapet makes sense although going over the cavity not so much. . But I do thank you for your input. Much appreciated.
     
  16. Buildme

    Buildme

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  17. ^woody^

    ^woody^

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    A vapour barrier is intended to stop water vapour (from air) and is not like a DPM which is intended to stop water moving through a material.

    So its a waste of time putting a vapour barrier over a material such as a wall
     
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