screed vs floating floor

[hog]

Hello Peeps

I'm not 100% sold on this screed idea ...

Nearing the end of my garage conv (hoorah) and for the floor base I've laid sand blinding+dpm+100mm PIR celotex over the existing garage slab, now ready for 75mm sand/cement screed. All as per architect plans.

Now that I can see the insulation in place I can't comprehend how this design allows a screed to sit happily on top of this spongy layer without bounding or cracking a bit when people will walk on it. OK the insulation is compact and level but surely the screed doesn't weigh that much? I've read threads that say a better alternative is to have the insulation under the concrete slab and the screed - not an option for me as the garage slab was around before I was.

Would I be doing myself a favour by abandoning this approach and opting for a floating timber floor? My brain suggests I won't have this problem with chipboard and timbers on the concrete slab (+ dpm) ...

Your thoughts and experience very welcome.

Cheers
hog

 

[theprinceofdarkness]

The weight of the screed tends to stabilise the foam. The thickness of the screed spreads any weight over a larger area. Its not a very good system as over a couple of years the two effects coupled with the fact that the foam contracts on its own, will result in the top surface dropping slightly, .5 -1 mm? leaving a crack around its edge. This is no worse then the shrinkage of wooden floors. My conservatory suffers from this , but as the screed is laid against the above DPC wall rendering, where it has sunk by 1mm, it has pulled a lump of the wall rendering down with it, giving a unsightly and long crack about 6" above the floor and running parallel to the floor.
Foam in general should not be supplied in 100mm thick sheets unless it can be supplied and kept absolutely flat. You should use 4 layers of 25mm foam with offset joints. This will automatically be better at sealing cracks as the foam contracts with age and is very much weaker to the effects of flattening out. i.e. the force required to flatten 4 off 25mm compared to one off 100mm is some thing like 1/64 !!!
A floating wooden floor is difficult to insulate with 100mm foam. It also needs either big joists or sleeper walls. But can accomadate large slopes or voids in the ground level.
Frank

 

[^woody^]

If it's proper floor insulation then it's good enough to support screed without compacting. Use chicken wire in the screed if you're worried about movement.