Floor system for UFH

[firekill]

hi guys , I'm currently doing a kitchen & utility complete renovation and laying a new floor ready for ufh , I've laid the mot , sand binding , DPM & 100mm of concrete for sub base . Got 90mm foil backed pir board for insulation layer and having a 65/75 sand & cement screed with fibres , now I was going to just lay the boards down straight ontop of the concrete but read somewhere about fixing them down with flexible tile adhesive or just laying them on top of another layer of dpm over the concrete , also as it's gonna be a dry screed do I need any membrane over the insulation/ufh before screening or not. Not done wet ufh before and want to get it right first time.

 

[Dumbo]

You need a membrane between your insulation and screed (slip coat ). Can't remember what guage but it's about half the thickness of what your dpm should off been . Then install ufh pipes and screed over the top . What size is the area that you are screeding and tiling .

 

[firekill]

Thanks , it's about 15 sq m with about 10 m of that having ufh because of units etc , thinking of using the speedfit single room pack

 

[Ajax123]

On top of concrete minimum 1200 gauge polythene then your insulation then minimum 500 gauge slip membrane then your screed. Obviously I recommend 50mm of flowing GYPSOL screed but if it as to be sand cement it should be minimum 65mm so the minimum being the bit over the top of the pipe it should technically be pipe depth plus 65mm so 80mm or so.

 

[firekill]

What is the purpose of the 1200 gauge polythene on top of the concrete? , I did put a 1200 g Dpm under the concrete slab .also my screed depth will be 70-75 mm as I have already got the insulation. Could I put some wire mesh and fibres in there too add strength maybe .

 

[Dumbo]

I stand to be corrected but if you have done that and lapped it up the walls you should be ok . As regards thickness I'm no expert on cement @Ajax123 is

 

[Ajax123]

Concrete gives up it' moisture slowly. If you put unheated screed on top the action of heating and thus reducing the vapour pressure in the screed can draw moisure up from the concrete causing issues with moisture in the screed. This is NOT a dpm for the floor. T does not need to lap up the walls. The 500g slip membrane divorces the screed from the insulation to allow it to move independently under thermal cycling. It also prevents the insulation from reacting with the hydroxyl ions in the screed which can cause isses in both cement based and anhydrite

 

[Ajax123]

What is the purpose of the 1200 gauge polythene on top of the concrete? , I did put a 1200 g Dpm under the concrete slab .also my screed depth will be 70-75 mm as I have already got the insulation. Could I put some wire mesh and fibres in there too add strength maybe .

Wire mesh is a waste of time. If it is sand cement it must have fibres in. If you go liquid it needs neither.