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Insulating a sold concrete slab flat roof - advice needed

  • 26-11-2024 02:00PM
    #1
    championc
    Registered Users, Registered Users 2 Posts: 6,659 ✭✭✭


    Hi all,

    I have a solid concrete slab flat roof, about 1m wide, running the width of the house above a front window and creating a porch too.

    It is acting as an extremely cold bridge, so I want to insulate it. The plan would be to attach XPS (???) directly to the concrete surface and then put a waterproof membrane over the top.

    This is the type of roof

    Screenshot 2024-11-26 115607.png

    And end up with something like this

    image.png image.png


Comments

  • #2
    10-10-20
    Registered Users, Registered Users 2, Paid Member Posts: 10,141 ✭✭✭✭10-10-20


    The others in the estate who have done similar had to apply for PP?

    Plus you need to wrap the front facing section of the plinth with insulation and look at the rear section where it meets the wall, that needs to be waterproofed by embedding fresh leadwork into the wall.

    I think Steve Roofer covered segments of this in the past, I'll have a look.



  • #3
    championc
    Registered Users, Registered Users 2 Posts: 6,659 ✭✭✭championc


    Will be wrapping next year, but this is a pre-wrap quick fix. Yes, will wrap the front edge too.

    I think my main question is around what will be the top surface, on top of whatever board stuff I use for the insulation. If I use some form of waterproof / weatherproof membrane, would I then need a ventilation gap between it and the insulation layer ?



  • #4
    10-10-20
    Registered Users, Registered Users 2, Paid Member Posts: 10,141 ✭✭✭✭10-10-20


    Temporary? VCL → Insulation → ply → primer and then a liquid rubber. I think Owl Lava 20 is often recommended.

    I don't think it needs to be vented, you'd just need to provide a fall and proper detailing at the wall so that it doesn't leak.

    https://www.youtube.com/watch?v=YvZ0sU9OzZU

    I think I recall that the insulation needs to be specific for the job - I seem to recall it needing to be "fleece faced" but I might be wrong on that.

    Here's Mannok's suggested make-up:

    https://mannokbuild.com/insulation-boards/flat-roof/insulating-flat-roof-with-concrete-deck/

    Note the specific detailing at the wall provided in that image.



  • #5
    championc
    Registered Users, Registered Users 2 Posts: 6,659 ✭✭✭championc


    Many thanks. Will be 100% permanent. The wall will later be wrapped, which means that that joint will be even more protected then



  • #6
    monseiur
    Registered Users, Registered Users 2 Posts: 1,775 ✭✭✭monseiur


    If you plan to wrap the whole house next year I'd advice do the flat roof at the same time, it will make sealing the joint where the flat roof meets the house wall much easier/practical. Doing the flat roof only now would mean cutting a chase in the main wall for a new lead flashing something which is difficult, time consuming and expensive. All this will be obsolete once the new external insulation is added as a new flashing has to be fitted.



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  • #7
    10-10-20
    Registered Users, Registered Users 2, Paid Member Posts: 10,141 ✭✭✭✭10-10-20


    https://www.boards.ie/discussion/comment/122889170#Comment_122889170

    Yes, I'd share that concern. The last thing that you want is water intrusion under that insulation and into the plinth.



  • #8
    championc
    Registered Users, Registered Users 2 Posts: 6,659 ✭✭✭championc


    https://www.boards.ie/discussion/comment/122889170#Comment_122889170

    The only expense is the materials doing a DIY job.

    I'll get out my FLIR camera and look more closely as to how much cold is really coming in.



  • #9
    championc
    Registered Users, Registered Users 2 Posts: 6,659 ✭✭✭championc


    EDITED



  • #10
    10-10-20
    Registered Users, Registered Users 2, Paid Member Posts: 10,141 ✭✭✭✭10-10-20


    https://www.boards.ie/discussion/comment/122891215#Comment_122891215

    Pedantic, but screed initially cures then drys off. If it's a proprietary screed (from a bag) then there will be rules around temperature over the initial cure stages, but if it's just a cement mix then it's fine until 5C (especially as it will be a thin layer) and with no frost forecast.

    It probably won't dry off fully as there is significant dew at night, but you'd be safe enough to work on as long as there isn't standing water.

    Don't forget to either prime or prepare the surface beforehand anyway.

    Example below:

    https://irl.sika.com/en/diy-markets/waterproofing/sika-1-pre-bag-screedmortar.html



  • #11
    championc
    Registered Users, Registered Users 2 Posts: 6,659 ✭✭✭championc


    FLIR_20241127_074432_128.jpg

    Says it all !!!



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  • #12
    10-10-20
    Registered Users, Registered Users 2, Paid Member Posts: 10,141 ✭✭✭✭10-10-20


    https://www.boards.ie/discussion/comment/122893510#Comment_122893510

    That's nuts. I was imagining around 10c, but 2.1c is far lower than I had dreamed.



  • #13
    DC999
    Registered Users, Registered Users 2 Posts: 1,634 ✭✭✭DC999


    https://www.boards.ie/discussion/comment/122893510#Comment_122893510

    Wow. How could it be so low??? That's actual outside temps tonight. And I'm not doubting the camera.

    My parents in law have the exact same type of roof over the front of the house.

    There are tiles outside. So I guess the plasterboard inside is the only thing between the tiles and outside.



  • #14
    GreeBo
    Registered Users, Registered Users 2 Posts: 27,518 ✭✭✭✭GreeBo


    It might be a lot easier to just use warmboard on the inside, at least until you do the EWI.

    It would be a relatively easy area to board over and less detailing to worry about, if you dont do all the exterior edges I'm not sure you are going to see much improvment…



  • #15
    championc
    Registered Users, Registered Users 2 Posts: 6,659 ✭✭✭championc


    QQ - Can visqueen be used for the VCL, or are there better products to use ?

    I have some like this

    image.png


  • #16
    10-10-20
    Registered Users, Registered Users 2, Paid Member Posts: 10,141 ✭✭✭✭10-10-20


    https://www.boards.ie/discussion/comment/122895255#Comment_122895255

    The green one is a vapour control layer and the vapour permeability is listed.

    https://visqueen.com/datasheet/13871432/

    I can't see why not other than the thickness and it not being not specified in the doc.

    Others might have different views and I'd love to hear them.

    (Edited to add in about the thickness.)



  • #17
    championc
    Registered Users, Registered Users 2 Posts: 6,659 ✭✭✭championc


    Did it over the weekend. Still need to fix down the top felt layer. Was a great opportunity to catch the break of milder weather.

    Already, the sitting room is reaching temperature of 21 whereas it would barely scrape there by 22:00. And the hallway is showing as it's warmest ever. So the concrete slab is now a thermal mass 😃

    Overall cost was about €400



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