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Hollowcore went in without airtight wrap

  • 07-01-2020 2:53pm
    #1
    Registered Users Posts: 76 ✭✭


    Hi all, our precast 1st floor went in without an airtight membrane around the edges. The 1st floor external walls have also been built up. We do need to add a structural screed which should help fill the gaps somewhat...

    Is this salvageable? Is there any other way i can make sure air doesn't travel out through the cracks between the slabs?


Comments

  • Registered Users Posts: 307 ✭✭tonytoc11


    A spray on liquid membrane might work to seal the slab to the wall. Have a look at blowerproof.ie


  • Registered Users Posts: 125 ✭✭selfbuild17


    The screed should seal the top face of the slabs. You still have potential air leakage paths at the junction between the top of the slabs and the external walls, the junction between the bottom of the slabs and the external walls, the the underside of the joints between the slabs, any exposed edges of the slabs (around the stairwell) and also through any cores in the slabs. Air will travel from the cavities in the wall through the cavities in the slabs. That’s why it’s so important to seal the slabs at those junctions as it’s very difficult to seal up all of those locations mentioned above afterwards. You’re going to need lots of rolls of airtight tape.


  • Registered Users Posts: 681 ✭✭✭caddy16


    Airtight paint on slab wall junction. I was advised to go with it instead of wrapping the slabs and worked well.


  • Registered Users Posts: 12,754 ✭✭✭✭mfceiling


    RoflHarris wrote: »
    Hi all, our precast 1st floor went in without an airtight membrane around the edges. The 1st floor external walls have also been built up. We do need to add a structural screed which should help fill the gaps somewhat...

    Is this salvageable? Is there any other way i can make sure air doesn't travel out through the cracks between the slabs?

    I've seen plasterers plaster down about 300mm from the hollowcore before. Usually a coat of bonding compound which would seal it up to some extent.


  • Registered Users Posts: 3,724 ✭✭✭Metric Tensor


    It should be salvagable. There's a few contractors I know who get good air-tightness results and refuse to wrap the slabs. Make sure the shear keys are well filled with the screed and seal both the downstairs wall and and upstairs wall to the slab with tape/membrane as well as plugging any open cores visible inside at stairwells, etc.

    P.S. Plaster the walls right up to the underside of the slabs before installing the suspended ceiling.


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  • Registered Users Posts: 12,754 ✭✭✭✭mfceiling



    P.S. Plaster the walls right up to the underside of the slabs before installing the suspended ceiling.

    I'm seeing this more often now.


  • Registered Users Posts: 3,724 ✭✭✭Metric Tensor


    Definitely a better job for air-tightness. I'm not sure does it make your life more difficult with the side rail for the ceiling?


  • Registered Users Posts: 12,754 ✭✭✭✭mfceiling


    Definitely a better job for air-tightness. I'm not sure does it make your life more difficult with the side rail for the ceiling?

    Doesn't affect it at all. Usually makes life easier to be honest. A nice smooth surface to put the perimeter channel on makes the job a good one!!


  • Registered Users Posts: 38 porrige


    Our dopey builders did the same. It’s a disaster for airtightness & I’m still struggling with it. Sealing the contact points between wall/floor/ceiling is no problem (think siga). But, think about all the penetrations through the slabs; electrical & plumbing, plus the fixing of suspended ceiling. To make matters worse, our “builders” (helped by a diabolical architect) used a hollow spring style anchor for fixing the timber suspended ceiling battens, thus peppering the hollow core with holes!


  • Registered Users Posts: 2,114 ✭✭✭DBK1


    It this was done with a solid slab instead of hollow core and taped top and bottom at wall to slab joints would it be sufficient?


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  • Moderators, Home & Garden Moderators Posts: 10,135 Mod ✭✭✭✭BryanF


    That would be a great construction challenge


  • Registered Users Posts: 2,114 ✭✭✭DBK1


    BryanF wrote: »
    That would be a great construction challenge
    Apologies, maybe I didn't make myself clear. I am in the same situation only it is solid slab used in my house and not hollow core.

    I wasn't suggesting the OP take out the hollow core and fit solid slab!


  • Moderators, Home & Garden Moderators Posts: 10,135 Mod ✭✭✭✭BryanF


    Ah ok. Have you internal walls up yet? Can you tape top and bottom continuously from perimeter in all sides? Is the building weather tight? Can you get water proof AT tape and adequately protect it until, for example, the floors are installed or from any coring that may be required?


  • Registered Users Posts: 2,114 ✭✭✭DBK1


    BryanF wrote: »
    Ah ok. Have you internal walls up yet? Can you tape top and bottom continuously from perimeter in all sides? Is the building weather tight? Can you get water proof AT tape and adequately protect it until, for example, the floors are installed or from any coring that may be required?
    Internal walls are up and roof finished just before the shutdown. There will be 50mm of insulation going on the slabs and then UFH in a 75mm screed.

    There is also some irregularities in slab thickness which now leaves them uneven in height where they join. I’m going to have to do something with this before insulation is laid as I don’t want air pockets in the insulation anywhere. Any ideas on that or is it of major importance when it is internal?

    Thanks.


  • Registered Users Posts: 701 ✭✭✭kilkenny31


    Hi Guys


    I have a hollowcore slab. The issue is the suspended celing is in and there is no airtighness around the slab.

    I was wondering would it work if I put a airtight membrane on the suspended celing on the ground floor. Then seal any electrical trunking with silicone and put airtight caps over any down lights?

    So finish the ground floor celing with a similar detail to the first floor celing.


  • Moderators, Home & Garden Moderators Posts: 10,135 Mod ✭✭✭✭BryanF


    Yes, there’ll be a cost and a good level control required to compete. Don’t forget the floor side as well.

    Just a general comment to all self-builders, construction drawings should not be issued without this info.

    And anyone with a contract with a main contractor should have an air-tightness clause/result included.


  • Registered Users Posts: 363 ✭✭Biker1


    kilkenny31 wrote: »
    Hi Guys


    I have a hollowcore slab. The issue is the suspended celing is in and there is no airtighness around the slab.

    I was wondering would it work if I put a airtight membrane on the suspended celing on the ground floor. Then seal any electrical trunking with silicone and put airtight caps over any down lights?

    So finish the ground floor celing with a similar detail to the first floor celing.

    You would be better stopping the air infiltration at source therefore the application of an airtight paint to all blockwork above the suspended ceilings is an option.
    Mystifies me as to why the airtightness details were not given to you.


  • Registered Users Posts: 2,566 ✭✭✭Yellow_Fern


    kilkenny31 wrote: »
    Hi Guys


    I have a hollowcore slab. The issue is the suspended celing is in and there is no airtighness around the slab.

    I was wondering would it work if I put a airtight membrane on the suspended celing on the ground floor. Then seal any electrical trunking with silicone and put airtight caps over any down lights?

    So finish the ground floor celing with a similar detail to the first floor celing.

    dont use silicone for that job


  • Moderators, Home & Garden Moderators Posts: 10,135 Mod ✭✭✭✭BryanF


    dont use silicone for that job

    ?


  • Registered Users Posts: 38 porrige


    I had same problem. Timber battens were used for our suspended ceiling and so we thought it better not to include airtight membrane there in case of condensation risk. Try to seal Wall/Slab junctions as best you can - you may end up using several products in combination - tape, airtight flexible adhesive, paint, foam. Its disgraceful these things are not specified correctly and signed off regardless.


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  • Registered Users Posts: 3,724 ✭✭✭Metric Tensor


    porrige wrote: »
    Timber battens were used for our suspended ceiling and so we thought it better not to include airtight membrane there in case of condensation risk.


    I can't follow this at all - I might be thinking of a completely different thing though. Are we talking about a suspended ceiling underneath hollowcore that separates the ground floor from first floor of a house?



    Where did you decide not to include a membrane?


    Did you anticipate condensation on the timber battens making a suspended ceiling in an intermediate floor?

    And how/where would a membrane have affected this?


  • Registered Users Posts: 38 porrige


    I can't follow this at all - I might be thinking of a completely different thing though. Are we talking about a suspended ceiling underneath hollowcore that separates the ground floor from first floor of a house?



    Where did you decide not to include a membrane?


    Did you anticipate condensation on the timber battens making a suspended ceiling in an intermediate floor?

    And how/where would a membrane have affected this?

    Yes, our suspended ceiling created by timber battens fixed to underside of hollowcore slabs which separate floors.
    We decided not to apply membrane to the under/room side of these timber battens (between timber & plasterboard) as air could circulate around the underside of slabs & battens which had potential for moisture/condensation.
    The timber battens had been fixed to the unsealed slabs before I realized our builder/architect/certifier ignored airtightness spec.


  • Registered Users Posts: 3,724 ✭✭✭Metric Tensor


    There wouldn't have been much point in putting an A/T membrane on the room side of your downstairs suspended ceiling unless you planned to wrap the envelope up the side of the stair ope and right the way around the floor of upstairs in order to somehow make it so that the entire floor slab was outside your building A/T envelope.

    I don't want to say this would have been impossible (because in theory it is possible) but it would have been extremely difficult so most likely putting the membrane in would have been a waste of money.

    Even with the cores left open and unsealed the insulation in your cavity will continue past the ends of the slabs and the other end of the core will be open to the warm environment so the chances of the hollowcore slabs being so cold as to cause condensation is very very unlikely. The only circumstances where I could see that happening is if the hollowcore was creating a flat roof and very little or insulation was installed - also in such cases moisture control membranes are deliberately installed in order to keep moisture away from the slab!

    I think you ended up doing the right thing by not putting in the membrane at ground floor ceiling level but in my opinion the thought process that led to that decision was not the most accurate!


    Edited to add: I fully agree with your point of the need to specify air-tightness measures from the start. Too many people half-ass them or don't consider them at all!


  • Registered Users Posts: 38 porrige


    I guess it depends on the construction detail. For ours, the slab/wall junction was a point of air entry to the gap between slab & plasterboard. Our slabs were considered cold(er) as many of them spanned without break across the house (from one cavity wall to another). We had access to slabs at stairwell so that wasn't a problem. It was highly recommended by airtightness manufacturer that insulation exist on both sides of membrane - we couldn't achieve this.


  • Subscribers Posts: 40,723 ✭✭✭✭sydthebeat


    porrige wrote: »
    I guess it depends on the construction detail. For ours, the slab/wall junction was a point of air entry to the gap between slab & plasterboard. Our slabs were considered cold(er) as many of them spanned without break across the house (from one cavity wall to another). We had access to slabs at stairwell so that wasn't a problem. It was highly recommended by airtightness manufacturer that insulation exist on both sides of membrane - we couldn't achieve this.

    To be honest porrige

    That build sounds like something with little or no professional input or forethought of detailing.


  • Registered Users Posts: 38 porrige


    sydthebeat wrote: »
    To be honest porrige

    That build sounds like something with little or no professional input or forethought of detailing.

    Exactly! Yet it was specified by a member of a supposedly reputable institute. Still dealing with the problems.


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