Hybrid roof - reducing build up

[SBMS]

In two minds… about to start on our flat roof. Originally had planned for 200mm PIR as a warm roof. Due to a number of design issues we are looking at reducing this down and potentially looking at a hybrid roof. 
 

have had an interstitial analysis done by Unilin. By their reckoning if we put 100mm between joists and 140mm on top that yields no risk of condensation..  

 

My thinking is whilst aesthetically we can reduce the parapet wall by a course (60mm) to improve aesthetics… is it worth it for the ‘risk’ perceived with a hybrid roof? Anyone else done this?

[Benpointer]

No, but I wish we had put additional insulation on top, instead of underneath our roof.  We did the latter because of concerns about roof thickness aesthetics but with hindsight and a bit of careful design we could have addressed that.  

[ADLIan]

Hybrid flat roof is never a good idea. Depending upon calculation inputs it’s possible to show the design is ok but look at the input data very carefully!

[SBMS]

16 minutes ago, Benpointer said:

No, but I wish we had put additional insulation on top, instead of underneath our roof.  We did the latter because of concerns about roof thickness aesthetics but with hindsight and a bit of careful design we could have addressed that.  

Why?

[SBMS]

12 minutes ago, ADLIan said:

Hybrid flat roof is never a good idea. Depending upon calculation inputs it’s possible to show the design is ok but look at the input data very carefully!

Agreed. Their analysis looks sound - and they don’t account for mvhr which probably helps even more. Even still.. I feel a niggle…

[Benpointer]

2 minutes ago, SBMS said:

Why?

Why do I wish we'd put the insulation on top?  It would have simplified the build (easier to lay insulation over the top than fix from below) and left us with the open web joists to run services (especially MVHR ducts) through.

 

Tbf ours our pitched roofs not flat so may not be relevant to you. 

[SBMS]

59 minutes ago, Benpointer said:

Why do I wish we'd put the insulation on top?  It would have simplified the build (easier to lay insulation over the top than fix from below) and left us with the open web joists to run services (especially MVHR ducts) through.

 

Tbf ours our pitched roofs not flat so may not be relevant to you. 

Thanks. We’ve got 254mm joists and would only put 100mm pir in between but still fiddly not as good as all on top etc…