Screed over Hollowcore

[MortarThePoint]

On the ground floor I have a gas membrane over the top of concrete and on the first floor I have hollowcore concrete planks as the floor. On both levels I'm going to have 40-50mm liquid screed with UFH in. I've started putting down 125um polythene* and it feels pretty pointless. Downstairs there is already an impermeable membrane. I'll discuss this with the screed guys and see what they think. Upstairs is different, but still feels like there could be something better.

 

I don't have insulation on top of the concrete (GF it's below the 50mm concrete, FF don't want it as not worried about responsiveness).

 

On the first floor, the hollowcore (HCF) is largely 'waterproof' with a few scattered holes. What have others done here. It's impossible to not have some voids under the membrane and it feels unnecessary. It occurs to me I could do some of the following as an alternative to membrane:

1. Paint some form of sealant on the floor and add edge strip
2. Apply a thin layer of self levelling compound (1-3mm) to form a seal and add edge strip
3. Just fill any holes that would leak and add edge strip

All would give a more intimate connection between the HCF and screed as well as being much faster than membrane.

 

* https://www.toolstation.com/green-vapour-barrier-125mu/p86538

 

Edited May 21, 2021 by MortarThePoint

[MortarThePoint]

Interesting talking to one screed supplier who said liquid screeds can react with concrete, so definitely need the membrane. She also said that liquid DMs are not suitable.

 

I had read before that screeds can react with aluminium (e.g. on insulation) and know that wet cement does too. I recall reading about an unfortunate prototype aluminium cement mixer.

 

Looks like I have to carry on with the vapour barrier. I'd bought some spray adhesive in case I wanted to bond the membrane down against gusts of wind etc. I haven't used any of that yet though.

[MortarThePoint]

Rats, I think I should have done the perimeter strip before the membrane