Fiberglass or foam board insulation?

one good answer ..kingspan ..here in uk ..if we build an extension ..under building regs we place either 4"kingspan ..or 6" of polystyrene sheet ..under floor ..this is important ..have you insulated the floor?  i have built in many countries ..in france the walls are one block wide ..8" ..render exterior ..4" special polystyrene sheet with plasterboard on one side ..dot and dab it to walls ..very affective at retaining heat ..if your basement has stud walls insulate between the uprights ...or add stud walls and insulate ..and dont forget to add kingspan to ceiling ..the room above will draw any heat out if you dont

having experience with this in Vermont, I can tell you that anything you put inside the basement is unlikely to achieve what you want.  what we did was dig out the walls around the basement [one at a time and, obviously, in the summer], then apply 4 inches waterproof foam insulation to the outside -- after which we filled in the trench we'd created.  It's a huge amount of work [the digging part] though.  -- grampa

The best insulation board is Kingspan. It's not the cheapest but it is the most efficient. It's also fire resistant which other types may not be. So research online.

foam board is probably the least effective insulation there is unless its really thick .. better than nothing but not great .. the plus is its very easy to use .. if you do go that route i would try to at least use the 1" thick stuff ..