Airtightness at hollow core level

stef22

building an airtight house, blocks with hollow core.

I am unsure what to with regards to airtightness below the hollow core where the slabs sit on the inside block, any advice would be greatly appreciated.

Biker1

Here's one way of getting it right. This is an area that is generally overlooked and persistently increases air leakage in all the two story houses I test.

General Details 1.0.pdf

JustJoe7240

https://www.boards.ie/discussion/comment/121598323#Comment_121598323

Is there a public source for these details or is this info related to your work? Looking for detailing for deadwork for a house.

Le shovelle

Would filling the cores in the slab with an air tightness foam also solve this issue? Or perhaps the foam would leak over time.

Biker1

https://www.boards.ie/discussion/comment/121598389#Comment_121598389

Filling the cores makes no difference. All the leakage is under the hollowcore and unsealed blocks above the suspended ceiling. These are just some of the details I provide to self-builders when they don't get them from the engineer. Also provide details to engineers and architects who should know better !!!

Le shovelle

Ah, I understand now. Great info, thanks

stef22

thank you for the info Biker1.

the hollow core is in place already with the block work done all the way up to the roof so cannot put the membrane (it wasn't fitted when the hollow core went in !)

Understand with the plaster all the way to the underneath of the hollow core, that would mean we would need to plaster the inside walls first and then do the suspended celling , not sure how that will work with the plasterer (would need to do it in 2 stages) and the electrician as first fix would have to be done without the suspended ceiling.

Yellow_Fern

https://www.boards.ie/discussion/comment/121598997#Comment_121598997

As you say, it makes no difference to this leakage at the ends but it should reduce thermal looping that occurs inside the core. Not a big issue though by any means.

keno-daytrader

https://www.boards.ie/discussion/comment/121601209#Comment_121601209

We used airtight paint/membrane under the hollowcore to avoid the plastering timing issue(brought down to just under suspended ceiling level). Many brands out there, I think we used "blowerproof"

Membrane was installed as hollowcore went in also. I would query this with your builder and engineer as to why this wasnt done. Basic detail these days that can make a huge difference in airtightness.

Biker1

https://www.boards.ie/discussion/comment/121601209#Comment_121601209

Don't forget to paint all the chasings with airtight paint before electrical first fix.

stef22

yep the chasing are pained, so is the first row of blocks and the last one underneath the hollow core.

Jus need to seal the connection point between the last block and the underneath of the hollow core, maybe foam ?

keno-daytrader

We did this, I'd say our electrian thought I was bonkers, also sealed the conduits in the attic with airtight mastic. We wanted to get as close to passive standards as we could, so all the little things added up in the end.

SodiumCooled

https://www.boards.ie/discussion/comment/121601209#Comment_121601209

Our plasterer just went around and did the top row of blocks with sand and cement up to the hollowcore (on external walls) before he moved on after finishing plastering outside. It has two jobs, it airtightens the blocks themselves and seals between the wall and hollowcore. It only took him a few hours and he had allowed for it in his price without even asking as it’s standard now he said.

The suspended ceiling went on then and it sits just above where the plaster is so when the plasters is back again he can plaster up to meet the top bit of plastering.

I also went around after and filled any gaps where there is a V where slabs meet with airtight silicone or for a big gap expanding foam and painted over with airtight paint.

Engineer also specified solid ends on the hollowcore where any slab sits on an external wall to prevent air getting into the cores which would in turn get into the house where the slab sits on an internal wall which would not be airtight.