As gregzoll said, you don't want regular fiberglass batt there, it would send out little glass slivers to the kitchen while the motor is running. Hard on the lungs- people, dogs, cats, birds, iguanas, etc. to get in the return air of furnace and give all sorts of health problems (Fiberglass is a suspected carcinogen and carries a hazard warning label).
Clothes taking longer to dry?
Clean the dryer screen in HOT water if using fabric softener sheets.
They leave a residue that impedes air-flow, costing you money.
Clean the ducting in the last six months? 17,000 dryer fires annually!