Crowbar hero Posted August 22, 2022 Share Posted August 22, 2022 Hi all, I'm just wondering what thickness/gauge membrane to use for first floor between-the-joists biscuit screed? My understanding is it's mostly to stop the biscuit mix falling down any gaps in the supporting structure than as a serious damp-proof layer, though obviously it'll keep any residual moisture out of the exposed timber during the curing process. Would a 300ga be suitable? I'd prefer not to have something as cumbersome as 1000GA as it does need to dish down into the pockets between each joist in 2 dimension (I have noggins too!), and I need to get staples through it, but obviously needs to be tougher than clingfilm. Any thoughts or suggestions - past experiences? Cheers.. Link to comment Share on other sites More sharing options...
Mr Punter Posted August 22, 2022 Share Posted August 22, 2022 What is supporting the screed long term? I have seen biscuit mix used on top of the floor deck, between battens, but not between joists. 300ga will be OK as a separation layer. It is good to have a layer of insulation to stop the heat going to the floor below. Link to comment Share on other sites More sharing options...
ProDave Posted August 22, 2022 Share Posted August 22, 2022 You need a solid support for biscuit mix. There is a clue in the name, as it is laid dry, it is not strong like concrete and if you try and flex it, it will break, a bit like a biscuit....... Mine is supported on OSB sheet. Link to comment Share on other sites More sharing options...
Crowbar hero Posted August 23, 2022 Author Share Posted August 23, 2022 The biscuit mix is recommended by my supplier as the right thing (rather than heat spreader plates) for low temp supply and variable joist gaps when you don't want to raise the height of the floor. The biscuit mix will be on the membrane, supported by a sheet of PIR insulation, with 12mm plywood under, supported on all edges by battens nailed to the sides of the joists. I'm battening above the joists to avoid notching them for the pipes to cross - and the mix will be flush with the top of the batten, so in contact with the floor, but not really supporting it. See picture below.. The supplier states the mix is purely to provide a thermal mass, and doesn't have any structural qualities, cracking in the mix won't affect performance, etc.. Link to comment Share on other sites More sharing options...
ProDave Posted August 24, 2022 Share Posted August 24, 2022 You don't need any membrane here, lay the pug mix straight on the PIR once you have laid the pipes. Link to comment Share on other sites More sharing options...
Nickfromwales Posted August 24, 2022 Share Posted August 24, 2022 +1 Link to comment Share on other sites More sharing options...
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