Plastic Underfloor ducting Q

[crispy_wafer]

Just planning the underfloor ducting for the log burner, I'm thinking of doing it in plastic however at some point as it rises through the b+b floor I guess it needs to change to something non combustable. 

Would plastic be ok to the top of the screed, then the air feed kit from there to the stove, or would it be best to use galv steel duct from the underfloor plastic duct to the top of the screed?

[farm boy]

I have simlar problem. I ve have a piled raft with vent pipe system in 110mm plastic drainage pipe in void underneath with vertical pipes with cowels on on each wall. I have then come though the slab with 100mm single skin stainless flue but now have the problem of thermal bridging if i continue with the stainless to above floor level. The 100mm stainless flue fits nicley inside the 110mm drain pipe so am considering doing thermal break just above the slab using 110mm drain pipe then carry on with the stainless to above floor level. Mine will come out just to the side of the stove which will be a max 100c floor model but I still concerned about heat tracking down the metal pipe which will be next to the pir insulation under the screed. still racking my brains on this one. I am considering wrapping the stainless pipe in mineral wool insulation so it cannot contact the pir. Not sure what the heatas manual says!