our outer hearth looks like poured concrete from below and its tiled on its working surface above. the subfloor? or is it just the form-work from pouring the hearth? has one board that has fallen out. the remaining boards are resting on a framing member at the front edge and seem to just be stuck to the concrete and terminate close to the brick of the chimney base. should we re-affix this board? should we add framing crossing the unsupported ends of these boards? the outer hearth is freezing cold in winter, should we insulate it from below? this is all accessible from our unconditioned/unsealed crawlspace.
that wood should have been removed after construction and yes you can insulate it, I am just wondering if it is still structurally intact, if so then you are all set, I would have a local Certified Chimney Sweep www.csia.org, (there you can look up by zip code to find one near you) take a first hand look at what you have they should be able to help.