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    Hi All,

    We have built a basement extension in Gloucestershire using QuadLock ICF which has a beam & block flat roof which will be covered in patio slabs on stilts, the parapet walls around the roof are also ICF. We will be insulating on top of the B&B roof with insulation board and then waterproofing with EDPM.

    We are trying to acheive good airtightness and have concerns about the B&B. I am assuming that the insulation and EDPM alone will not provide good airtightness, especially on the edges where the roof insulation will meet the ICF wall insulation?

    I was considering laying a airtight membrane over the top of the roof, cutting a slice out of the bottomg of ICF parapet wall, allowing the membrane to go up tight to concrete wall in the ICF walls and then covering in a liquid screed which could flow right up to the concrete. I am wondering if this is a excessive and simply bonding the membrane to the concrete ICF walls and then simply laying the insulation on top would be sufficient. No idea what kind of glue could be used?

    I attach a pic for understanding, appreciate any thoughts?

    Phil Chaddah-Duke
    • CommentAuthortony
    • CommentTimeJan 7th 2011
    Where/what is the air barrier in the walls? Is there a vapour barrier in the ceiling?
    The barrier in the wall is the poured concrete core of the ICF, I was not intendind to put any barrier on the inside i.e. behind plaster board ceilings. It seem to make more sense to seal it on the top where I can get a contineous seal between the inner ICF concrete walls.
    I can't open the JPG.

    Do you need a membrane directly on top of the beam & block? Couldn't you use a concrete screed & let it bond with the beam, block & ICF core? Membrane & insulation could then be fitted on top.

    • CommentAuthortony
    • CommentTimeJan 7th 2011
    I would do vapour membrane/air tightness barrier on the warm side of the insulation in the roof and seal it to the one in the walls
    This ICF warm roof detail may help.
    I don't have an ICF flat roof detail but you could marry this detail with the ICF wall above.
    • CommentAuthorTimber
    • CommentTimeJan 9th 2011
    A thin grout/screed on top of the beam and block will greatly enhance the air tightness of the roof, but as always, junctions are key.

    If the concrete core of the ICF is to be your air tightness barrier, could you leave blocks off the perimeter of the B&B roof, then flood the screed/grout down over the top of the block infill onto the head of the wall, and into the ICF spaces?

    Good idea Timber, but doesn't the block and beam detail reduce the insulation levels from 150mm to 75mm at the junction?
    • CommentAuthortony
    • CommentTimeJan 9th 2011
    The screed would tend to crack and become air leaky over time especially if it was thin but it would be better than nothing but not as good as a vb/air barrier.
    I think a picture would help, le me try and attach it again as pdf.
    What's the grey line where the beams meet the ICF? Is it concrete or Silver EPS?

    Put a 50mm screed on top of the block and beam with 6mm mesh to stop it cracking, try to get the screed to meet up with the concrete in the wall as per Timber's recommendation, put 150mm of EPS on top to make it a warm roof and 75mm on the inside to reduce heatloss at the wall/roof junction.
    Pic seems to have worked now, hopefully that helps.

    The screed company said that we'd need to lay somekind of membrane down anyway as the screed will pour through tiny cracks in beam and block. The membrane will be on warm side as the insulation will be on top of B&B. So if we cut channel around base of the ICF insulation back to the concrete, the screed could pour right up to the concrete core but as Tony pointed out over time it will probably crack especially on the edges anyway. I was thinking of laping the membrane up the concrete wall a little and using the weight of the screed to keep the membrane tight against the concrete but not sure this really a airtight seal and then the screed will not create a bond with the concrete wall for sure so maybe worse!

    If the screed is not sufficient anyway then I would rather avoid the additional cost, the membrane would be loaded with the insulation anyway. The key question then is how to get an airtight seal between the membrane and the conrete ICF walls?
    Actually the grey line is the metal tracking which is attached to the beams to keep the ICF in place during concrete pouring. It maybe be tricky but I am hoping I can cut it out or cut it off to get access to the concrete behind.
    Thanks for the comments all.

    It seems a meshed screed is a good approach then, right up to the ICF concrete wall. Only question remaining, would be regarding the membrane which I will need to hold the screed. Do you think I should cut it short of the concrete wall and let the screed flow over and bond with the wall or would I get better seal by lapping the membrane up the wall and use the weight of the screed to push it against the wall?
    • CommentAuthorandy500
    • CommentTimeJan 10th 2011
    We're doing a similar-ish thing - beam and block with rooms below. Walls are partial fill blockwork cavity - but no parapets - so I've built the wall internal skin only up to beam and block level, laid the insulation on top of the beam/block but extended it over the top edge of the cavity insulation so it's all linked.
    Then DPM/screed/waterproofing/pavepads/pavers. The screed then goes right across the cavity to butt up against the slightly higher walls outer leaf. Ours is 75-100mm thick C35 to C40 mix concrete, so pretty strong. You could also add fibres, instead of meshing - engineer would need to confirm that. The waterproofing will be lapped up the outer leaf, forming a tray, with a fall to drain. Thinking of using RIW Flexiseal system - expensive but allegedly very good.
    Coping/ally capping and flashing down over the outer leaf to the 'tray', and it should be watertight and airtight. Ish.
    Big recommendation here - I've brushed very wet 4:1 silver sand/cement slurry across the beam/block before adding anything else - this is recommended by our beam manfr as it 'locks' the floor together - yes it does run through the gaps in the floor and make a mess, but theres an appreciable difference when you jump up and down on it afterwards. And it would help with air tightness to some extent.
    Could you seal the gaps in the block and beam with expanding foam and do without the membrane altogether?
    If you drilled some 12mm holes in the ICF concrete and put 6mm mesh into the holes, and then pour the concrete, that should minimise cracks forming between the roof and wall.
    • CommentAuthorandy500
    • CommentTimeJan 10th 2011
    That or drilling and resin-fixing decent lengths of rebar into the concrete wall and casting the slab around them would probably stop any cracks.
    • CommentAuthortony
    • CommentTimeJan 10th 2011
    what about vapour control?
    Tony, good point, we should include a Vapour Control/DPM underneath the screed anyway.

    Andy, yours is very similar detailing, whrere are you planning to terminate the DPM? Run up the wall above the screed like a tank but effecively stopping the screed bonding with the blockwork wall or underneath and let the screed flow over the edge to touch the wall?
    Posted By: tonywhat about vapour control?
    Not needed!
    • CommentAuthortony
    • CommentTimeJan 11th 2011
    I cant see why not! as far as I can see condensation will for or be pumped onto the underside of the screed and mimic a leaking roof in the extreme case.
    Posted By: tonyI cant see why not! as far as I can see condensation will for or be pumped onto the underside of the screed and mimic a leaking roof in the extreme case.

    That's like putting a vcl on the outside of a wall before you externally insulate it! The plaster is the vcl in the wall just like the screed in the roof. If you keep it warm you will get no condensation on the concrete.
    • CommentAuthortony
    • CommentTimeJan 11th 2011
    OK I agree
    • CommentAuthorandy500
    • CommentTimeJan 11th 2011
    <blockquote><cite>Posted By: Phil.Chaddah-Duke</cite>Tony, good point, we should include a Vapour Control/DPM underneath the screed anyway.

    Andy, yours is very similar detailing, whrere are you planning to terminate the DPM? Run up the wall above the screed like a tank but effecively stopping the screed bonding with the blockwork wall or underneath and let the screed flow over the edge to touch the wall?</blockquote>

    My DPM is between concrete and insulation - all I've done so far is lap it up the sides of the screed, hard against the outer leaf, and then it'll tuck behind the paint-on membrane (flexiseal) underneath the flashing and coping detail that makes any rain either fall off the edge of the building or run down into/onto the flexiseal and to drain. So yes - up, stopping the screed directly contacting the outer leaf.
    BUT the DPM, as far as I can see, is only there to stop wet concrete seeping into the insulation joints. The 'real' DPM is the flexiseal on the top. And the plasterboard has to be foil-backed - so that's the essential VCL. Without that, yes, you may as well have a leaky roof as condensation would be horrendous.
    I've queried our spec with Kingspan etc, as I want to add insulation/plasterboard laminate internally to improve the U values, and as long as the inner insulation isn't as thick as the outer or in-cavity-in-wall/next-to-roof-screed insulations, condensation won't be an issue, provided a VCL is used obviously.
    • CommentAuthorandy500
    • CommentTimeJan 11th 2011
    Grr. I can never do quotes right!
    • CommentTimeJan 12th 2011
    Posted By: andy500Grr. I can never do quotes right!

    You can always just edit the post and select Html instead of Text.
    • CommentAuthorvilarman
    • CommentTimeJan 13th 2011
    Interesting conversation, thanks. Aside: I will be using beam and block for my extension next year - out of interest, how much did yours cost per sqm?
    Sorry for slow response.

    We got very wide range of prices but in the end ACP (Concrete) Ltd worked out best deal. (http://www.thomasarmstrong.co.uk/product-catalogue/precast-prestressed-concrete/?catID=41)

    We ordered 450 meters (150mm deep beams) and paid £4.72 excl VAT per meter length (including delivery), depending on load the spacing changes so m2 price doesn't really mean much. This does not include blocks and ceiling ties.
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