I would like to make an existing bedroom larger by enclosing an adjacent  entrance porch that is 7′ x 9′.  The deck of the porch is an 8″ thick concrete slab with a basement below.  The joists that would be above this deck need to be 2×6’s.

My concern about doing this is that the deck would have two edges covered only with 1/2″ ply and vertical cedar siding.

Would it be sufficient to use closed cell spray foam in the joist bays to insulate this space?

Removing the deck would be a monstrous task which I would rather avoid.  Any thoughts would be appreciated.