Yes , you can insulate on top of the slab , 100mm PUR will pass regs , where will you put the DPM ?

I would put a DPM over the concrete unless you can expose an existing DPM under the slab to seal to.

Part L existing dwellings..
http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADL1B_2010.pdf

Appendix A, Table A1

Renovation of a solid or suspended floor involving the replacement of screed or timber floor deck.

The cost effectivness of floor insulation ic complicated by the impact of the size and shape of the floor (perimeter/area ratio). In many cases existing un-insulated floor u-values are relatively low when compared with wall and roof U-values. Where the existing floor U-Value is greater than 0.7 W/m^2.K then the addition of insulation is likely to be cost effective. Analysis shows that the cost-benifit curve for the tickness of added insulation is very flat, and so a target U-value of 0.25 W/m^2.K is appropriate subject to other technical constraints (adjoining floor levels, etc)

To get to 0.25 you need around 65mm of Celotex.

However Building Control do have some discretion. If you could show that the rest of the croft (walls, roof windows etc) would exceed Building Regs standard then they might allow 30mm.

Whats your proposed floor make up? Something like this?

21mm engineered wood
30mm Insulation
DPM lapped up walls
Sand blind
Concrete (old and new)

I believe the laminate floor would have to be at least 18mm thick.
Why screed under the DPM?

By screed, I meant a fine binding layer (possibly sand) to get a really flat, even surface. Talked to BCO and no movement on the amount required, it must meet the regs regardless of what other insulation goes into the building.

No money, no gel :-(

Jonti

Posted By: Jonti Talked to BCO and no movement on the amount required, it must meet the regs regardless of what other insulation goes into the building.

Well yes, obviously it has to meet the regs BUT the Building Regs allow some flexibility. See para 5.9 and 5.12 and the notes on Table 3 particularly note 5. It appears that not insulating at all meets the regs if there is a problem matching floor levels.

http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADL1B_2010.pdf

Note 5 A lesser provision may be appropriate where meeting such a standard would create a significant problems in relating to adjoining floor levels

5.9 If the achievement of the relevant U-Value set out in column (b) of Table 3 is not technically or functionally feasibly or would not achieve a simple payback of 15 years or less, the element should be upgraded to the best standard that is technically and functionally feasible and which can be achieved within a simple payback of no greater than 15 years. Guidance on this approach is given in Appendix A

If you would have to dig out and replace the concrete floor in order to fit in 80mm of insulation then that might make the payback more than 15 years.

5.12 Reasonable provision would be to upgrade those thermal elements whose U-Value is worse than the threshold value in column (b) of Table 3 to achieve the U-Values given in column (b) ..[same 15 year payback condion as 5.9]..

In other words for a floor if the U-Value is already 0.7 you don't need to upgrade it at all to meet the regs. If you do have to upgrade you should aim for 0.25 or whatever you can manage with a 15 year payback.

That's all from Part L for existing buildings. If necessary look at Part L for New Buildings (different document) that does allow you to trade off insulation from one part of a house to another subject to certain limiting values. A classic example of where this us helpful is a sun room with a lot of glass.

Normally if the BCO can see you are trying to do the best possible they are more helpful. If you just ring up and ask what the minimum is you can get away with they will quote the regs.

If it was mine I would go for a new floor all the same right through and no joins or potential future problems

Jonti, as you're in Scotland, I don't think we have to do mandatory upgrades to insulation. Right?

I didn't want to dig out 300mm+ on an already subsiding building with no foundations, so I didn't.

I believe he should show you Section 5 that I mentioned above. If you can't achieve the U-Value required of section 5 (even with the payback limitation) then I'd ask if section 4.4/4.5 or even 4.6 is a possibility. These sections allow you to put more insulation elsewhere to compensate for not having quite enough in the floor.