Membrane thickness for biscuit/PUG mix

[Crowbar hero]

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..

[Mr Punter]

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.

[ProDave]

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.

[Crowbar hero]

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..

 

[ProDave]

You don't need any membrane here, lay the pug mix straight on the PIR once you have laid the pipes.

[Nickfromwales]

+1

[frslam]

On 24/08/2022 at 08:23, ProDave said:

You don't need any membrane here, lay the pug mix straight on the PIR once you have laid the pipes.

Hi, I'm also from the Highlands (Skye). Just converted a 70's house, turning half of it into a cathedral ceiling. The other half has a large 5m x 5m room/ensuite upstairs. The span of the house is 7.4m inside walls with a central load bearer running down the length, so approximately 3.7m each side of the load bearer. The first floor joists used back then were 6x2. Would you think the floor joists with a span of 3.7m either side of the (4x2) load bearer would support a 25mm biscuit mix without too much bother? Because the 6x2's kind of spoil the aesthetics when looking at the floor/ceiling from the open plan area, I also plan to brander the ceiling, also to make running services easier. Attached a couple of images of the cathedral ceiling which is 6m high. The open plan kitchen/lounge will be 7.4m x 6m. Many Thanks for any pointers

[Nickfromwales]

48 minutes ago, frslam said:

Thanks for any pointers

 

I’ve done a few projects with these cathedral type aspects, and have supported the clients with lighting design / electrical installation etc. 

 

Check out plaster-in up lights; I’ve used Tornado lighting and the results have been great. Lots of light but no obvious source or offending lightbulbs to stare directly at.

 

For this client I installed 4 small surface mounted uplights at the base of each ‘spine’ and then you’ll see the pockets along each wall which is where I fitted the plaster-in units. 

 

 

Some other examples for your information. 
 

 

 

 

 

[JohnMo]

As @Nickfromwales I did similar but surface mounted up/down lights. So there are no electrics in the roof section at all. I put a service cavity there but never used it. Didn't even put any MVHR up there.

 

Don't put lights high up, you need a scaffold to replace bulbs etc.