Insulating under 2nd floor, overhanging open air

Like someone else who posted here recently, I have a bit of second floor which slightly overhangs the rest of the building (essentially in front of the front door, making a covered but open porch area). I have seriously considered a boiling oil hatch, but think I might leave that for another day.
Currently I just have the joists there. I need to insulate it, and cover it up, so what I was planning to do was: - Screw some kingspan to the underside of the joists, to ensure the joists don't act as a cold bridge. - Screw some ply under the kingspan to cover it. - Add some additional insulation between the joists, above the kingspan.
In fact I have a bunch of plasterboard backed kingspan offcuts that I might use here, so would attach it plasterboard side up (so plasterboard side faces upwards).
Does this sound like a sensible approach?
Piers
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It needs insulating. Important too that there are no gaps. Leave a space and fill with canned foam.
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I lifted the floorboards above my porch, and laid fibreglass loft insulation on top of the porch ceiling, but it sounds like you don't have a porch ceiling? If so, do you want the timbers to still be visible (are they part of the building style/design)? How deep are they?
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[Default] On Sat, 03 Aug 2013 10:48:18 +0100, a certain chimpanzee,

Insulation and soffits with a low vapour permeability (e.g., rockwool) are recommended for exposed floors. The use of impermeable materials can hold water, especially from leaks or spills. Thermal bridging through the joists is likely to be low; it's more likely to occur in the walls if they carry up above the bottom of the joists - the outer leaf suddenly becomes an inner leaf.
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On 04/08/2013 20:35, Hugo Nebula wrote:

Useful comments - thanks. I'd already planned on treating the external->internal wall as an external wall for its entire height. But yes, it seems like i should use something permable for insulation and then to cover.
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