sealing hollow core concrete slab

porrige

Hi,

Our masonry house is well under construction with hollow core concrete slab for ground and first floor. First fix electric is finished. Battens are already in fixed place for downstairs ceiling. House will have MVHR + Air to Water with UFH.

There's no air tightness applied at ends of hollow cores or at wall/slab junctions.

Has anyone experience of drilling small holes in slabs and filling with an insulation/airtight spray foam ?

Thanks.

Calahonda52

Bit late for that now.
The usual way to have done it is to have wrapped the ends of the slabs as they were laid with a tough airtightness layer.

Most foam is not airtight

porrige

Yes I realize it’s too late.
Would applying open cell spray foam be any benefit ? - would it age well ?
Are there any other options ?

MicktheMan

porrige wrote: »

Yes I realize it’s too late.
1. Would applying open cell spray foam be any benefit ? - would it age well ?
2. Are there any other options ?

1. No
2. Yes, but needs a site inspection and detailed specification and will cost a lot more now unfortunately than it should have.

Wartburg

porrige wrote: »

Hi,

Our masonry house is well under construction with hollow core concrete slab for ground and first floor. First fix electric is finished. Battens are already in fixed place for downstairs ceiling. House will have MVHR + Air to Water with UFH.

There's no air tightness applied at ends of hollow cores or at wall/slab junctions.

Has anyone experience of drilling small holes in slabs and filling with an insulation/airtight spray foam ?

Thanks.

Had two similar projects in the last two years. One is still in the construction stage without having an air tightness test done so far.
The other one ended with an air change rate of 2.77 @50Pa for the preliminary test (plaster was already completed).
Drilling holes into the slabs could work in theory, when it´s done from the top. The difficulty I see is the fact, that you can´t proof the performance of the foam inside, except you drill another hole beside and use an endoscope to check the quality of filling. Furthermore you can´t apply too much foam at one go, because it will lower the final quality of the foam.
I would rather recommend to seal everything underneath and on top of the slab, including all joints and penetrations. If you´re able to work this way, the air is trapped in the hollow cores without access to the rooms. Please see the attached photo of the prepared underside of a hollow core ducon slab.

porrige

Wartburg wrote: »

Had two similar projects in the last two years. One is still in the construction stage without having an air tightness test done so far.
The other one ended with an air change rate of 2.77 @50Pa for the preliminary test (plaster was already completed).
Drilling holes into the slabs could work in theory, when it´s done from the top. The difficulty I see is the fact, that you can´t proof the performance of the foam inside, except you drill another hole beside and use an endoscope to check the quality of filling. Furthermore you can´t apply too much foam at one go, because it will lower the final quality of the foam.
I would rather recommend to seal everything underneath and on top of the slab, including all joints and penetrations. If you´re able to work this way, the air is trapped in the hollow cores without access to the rooms. Please see the attached photo of the prepared underside of a hollow core ducon slab.

Thanks Warburg.
Yes I intend to tape up the joints & penetrations above & below the slab, but problem is that battens for suspended ceiling are already fixed & builder not keen to remove them - it won’t be such a perfect job. Can’t even plaster up to slab/wall junction (above suspended ceiling).


What do you mean by “you can´t apply too much foam at one go, because it will lower the final quality of the foam.” ?

I have access to some open cores at stairwell & was going to test applying foam via drill hole say 10cm back from the edge & see if it’s possible to fill approx 10cm length of the void.


2.77 is encouraging. I guess they did good job sealing everything else. What was used to seal penetration in picture ? For penetrations, I was planning on adhesive plus airtight tape.

Thanks again.

Calahonda52

2.77 is encouraging.
you mean 0.277 hopefully for a MHRV'd house

porrige

Calahonda52 wrote: »

2.77 is encouraging.
you mean 0.277 hopefully for a MHRV'd house

Oh!
So any ideas what I would get with just standard plastered walls and taped windows, no other airtight products ?
150mm platinum eco bead cavity insulation, 150mm kooltherm k7 between joists, 38mm kooltherm k118 ceiling plasterboard upstairs. 50mm thermafloor on first floor, 120mm on ground floor with 25mm insulation at floor level of perimeter walls.
Our RIAI architect told me 2 would be no problem.

Thank you.

Calahonda52

I was being, just a tad, tongue in cheek here with the 0.277
I have seen it down to 0.18


The key issue here is the continuity and integrity of the airtightness layer, so the list of insulation is irrelevant.

The test is , through a cross sectional drawing of your house, both ways, put a pencil down on the A/T layer and follow the line all the way around without lifting it, if you lift it, then u have a problem.
Same at windows and doors.

Re: The the builder is not keen: what does you architect think of this serious error?
You need to seal that with A/T layer, taped to plastered wall and underside of slab.
Plenty material around for that.
what brand are you using for tapes etc?

porrige

Calahonda52 wrote: »

I was being, just a tad, tongue in cheek here with the 0.277
I have seen it down to 0.18


The key issue here is the continuity and integrity of the airtightness layer, so the list of insulation is irrelevant.

The test is , through a cross sectional drawing of your house, both ways, put a pencil down on the A/T layer and follow the line all the way around without lifting it, if you lift it, then u have a problem.
Same at windows and doors.

Re: The the builder is not keen: what does you architect think of this serious error?
You need to seal that with A/T layer, taped to plastered wall and underside of slab.
Plenty material around for that.
what brand are you using for tapes etc?

Architect thinks no need for tape membranes and wet plaster will do the trick - which I know has some airtight properties, but problem is that it’s not applied at junctions - First fix electric is finished. 50x50 battens fixed to underside of slab with 50x50 counter battens attaches for downstairs ceilings.

We are considering to apply airtight tape as tight as possible over one of the battens from wall to slab. But I’m a bit concerned about condensation around the taped batten as I’d expect the batten to be a lower temperature than the room/suspended ceiling void.

Wartburg

Calahonda52 wrote: »

I was being, just a tad, tongue in cheek here with the 0.277
I have seen it down to 0.18

Do we compare apples and oranges?
If the house with 0.18 was built with a cavity wall structure, pumped with beads and hollow core slabs for the first floor, than I´d like to get access to the man who did the air tightness there.
The cavity wall structure is the biggest killer for air tightness. The best value I achieved so far was 0.56 ACH @50Pa for this type of house.
A timber frame house can brought down to 0.0... ACH, depending how much effort they put into the installation of tapes and membranes.

Wartburg

porrige wrote: »

What do you mean by “you can´t apply too much foam at one go, because it will lower the final quality of the foam.” ?

Standard PU foams do react on the principle of moisture to cure the foam. If you apply too much foam and the foam won´t get enough moisture during the curing process, it will create a terrible foam structure with plenty of air pockets. You should probably consider the use of a two-component foam, where the curing process is not dependend to moisture.

mfceiling

If anyone is following/reading this thread and is at the stage of getting ready to seal the hollowcore at the wall junction then do a search on here for a poster SAS. He did a great job sealing his build and put up loads of great photos showing the details. Considering this is a few years ago and he built to within a whisker of passive standards then he definitely did something right!!

Wartburg

mfceiling wrote: »

If anyone is following/reading this thread and is at the stage of getting ready to seal the hollowcore at the wall junction then do a search on here for a poster SAS. He did a great job sealing his build and put up loads of great photos showing the details. Considering this is a few years ago and he built to within a whisker of passive standards then he definitely did something right!!

I´d be interested in a link or pm with the link, if someone else had more luck than myself and found the suggested post & photos.

MicktheMan

https://www.boards.ie/vbulletin/showthread.php?p=71904248