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I've found data related to the deformation and shear performance of typical PIR (polyisocyanurate) foams such as Celotex/Kingspan, but I'm not sure how to use these figures in my own situation.

I am refitting a solid floor building in the UK. The existing oversite slab is intact and uncracked, but never had insulation. So the plan is to lay appropriate DPM/VCL, about 100-120mm Celotex (or equivalent), lay A242 mesh, and pour about 120 - 130mm of either dense concrete or screed. It won't be bearing any load other than usual UK floors (~ 2kN/m^2), and the area is <= 4 x 6 m.

Unfortunately due to the way the original slab was laid and the topology of the immediate area, there is a slight chance that moisture could seep under these, above the original slab. Realistically it's unlikely/no evidence, but it is possible, so I'd like to rest the Celotex/DPM/VCL layers on some kind of front-to-back orientedbattening, and provide airbricks, to create a shallow ventilated void between the original oversite slab and the new flooring.

If this is done, the batten widths and spacing would be quite crucial - if the battens are too narrow, or too widely spaced, the Celotex will deform excessively or even shear in the ~< 24 hours that the concrete/screed is fluid. (After it's set, of course, there will be no deformation beyond the equilibrium it was at when it set.)

I can imagine that narrow battens might have a limit on safe spacing different from wider battens, as shear and deformation are different phenomena. So there might be a range of "safe limit" combinations.

But I can't find anywhere, the appropriate formulae and safety factors that should be used, to calculate the limits of batten width/spacing beyond which shear and deformation would become issues.

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