How smooth does sub floor need to be before insulation for UFH installed

[readiescards]

Do I need to be concerned about 3-5mm height difference between the blocks and beams in my sub floor?  

The plan is for a DP membrane (not sure of thickness), 200mm of PIR foam, ufh in 50-60mm concrete screed, Luxury vinyl tiles.  

 

Options I see are:

1. Do nothing 
2. 1200 gauge DPM
3. Dry sand 
4. Leveling compound
5. Other?

 

Thanks 

 

 

[CC45]

Hi,

 

We started off with what you've got.

 

Dpm then 2 layers of insulation - 1st at 90deg to the beams.  We used 75mm celotex - this 'crushes' slightly - joints taped.  Next 150mm celotex at 90deg to the first layer - if they rock (celotex is far from 'flat' - this surprised me) then use some sand to bed them onto.  Take your time and you will achieve a level floor that doesnt rock.  The screed will take out any imperfections.  We put another sheet down under the ufh pipe and don't skimp on the clips - you dont want them lifting while the screed is going down (screeders said this wasnt unheard of).  Remember to grout the B&B (ours wasnt done very well so I redid it).  Dont be tempted to turn the celotex upside down - the underside foil isnt bonded on very well.

 

Hope that helps.