Cable (plenum) questions

Probably be best to check with your local building inspector or city planning department if you really want to be sure - different jurisdictions have different codes for how long unshielded non-plenum cable runs can be (in Vancouver proper, for example, I think you're allowed up to 9 meters before it either needs to be FT-6, or enclosed in conduit; different suburbs have different specs).

Whether or not the area is even considered plenum space depends on whether there are separate air-RETURN ducts; if there are, you're likely safe anyway. This is another area the city planning department could be useful, as they should have the building plans on file and can tell you what the requirements are.

Ultimately, you're probably fine with FT-4 cabling anyway, unless you're doing major construction requiring an electrical inspection. If there are no big renos and no permits involved, then nobody's going to care whether your cable is plenum-rated or not.

Some additional reading: