Should you insulate interior ceilings and walls? – GreenBuildingAdvisor

You only insulate partitions (walls or ceilings) that divide spaces that will be kept at different temperatures. In a typical residential setting, this means you insulate the entirety of the “building envelope”, which is all of the walls/ceilings that have outdoor air on one side (i.e. all of the exterior walls, any attic floors or cathedral ceilings, etc.).

The only time you would insulate an interior wall or ceiling would be when you want to limit sound transmission through that wall or ceiling. I like to insulate around sleeping areas, for example, to help keep those spaces a little quieter. I use either fiberglass or mineral wool batts for this purpose, never rigid foam.

Bill