One method to insulate your basement walls is to use styrofoam sheets. They come in various thicknesses from 1/2 inch up to several inches.
The sheets can be cut with a knife to fit any space. If you have stud walls, cut the sheets to fit between the studs and glue them to the concrete wall with construction adhesive.
Styrofoam sheets are also an effective sound barrier. You can use them to insulate the walls in bathrooms, laundry rooms, and furnace rooms, so you can enjoy your basement den in peace and quiet.