Re: wanting to use radiant heat
hardwood flooring and radiant dont mix,, it will dry out the wood far too much and it will create gaps no if and or buts.
the other factors are, its much harder labour wise to install radiant between joists as opposed to laying it on the subfloor. tile or engineered hardwood can be installed though
if the radiant is on top of the subfloor you will need to do a 1.5" concrete overpour. adding this much concrete you will run into the question of can your floor system handle this additional weight or will it need to be beefed up.
for retrofits, using the wire coil infloor heat is much quicker and adds very little weight to the floor as is electric however still needs to be covered in a layer of thinset to smooth out the floor
fire up the saw and make some dust