So, we're having a wood burner in our new build, for better or for worse!

I'm looking at the detail for the constructional hearth; we have a massive (600mm or so) raft for ground stability reasons. Build up off that will be 150mm kingspan-type PU insulation with a thin, 50mm screed for UFH on top of that. The rest of the build is ICF; we'll obviously ensure continuity of insulation between the ICF and the PU.

Unfortunately, I can't quite decide if PU insulation is a non-combustible product in relation to the building regs around stoves / fireplaces / hearths which require 250mm thickness of cover. Obviously, we have plenty in the slab, but I want to maintain continuity of insulation where at all possible, including underneath the stove, even if we don't run any pipes there.

My plan B is to use a certified stove that will not raise the hearth above 100 C, but I'm not sure what this will do to our already limited by the need for an external air feed selection...

Any ideas or suggestions to insulating, non-combustible materials?

<blockquotee>My plan B is to use a certified stove that will not raise the hearth above 100 C.</blockquote>

In which case you don't need a 250mm thick "constructional hearth" at all. You only need a thin slab of slate, granite or glass. The top surface must be 12mm above any carpet or wood floor covering the rest of the room. This is to form a perimeter step to discourage people putting a rug any nearer. I used a piece of black granite worktop from a stone mason and got him to hone off the gloss top surface so it looks matt like slate but less prone to scratching. It needs to rest on something solid because stoves are heavy and people tend to step on the corners but no need for a 225- 250mm slab. We have wood under the granite.

If it will not raise the temperature over 100C see diagrams 26 and 27a. You shouldn't need to meet diagram 24, 25 or 27b.

http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_J_2010_V2.pdf

If possible avoid stoves designed for smokeless zones They typically have a stop to prevent the air being closed down during start up as that can cause a lot of smoke. Sadly this means they tend to burn too fast once the fire is burning nicely.