Foam insulation is a type of insulation product which, although typically more expensive than fiber insulation, is very effective in buildings with space limitations and where higher R-values are needed. Foam insulation R-values range from R-4 to R-6.5 per inch of thickness (2.54 cm), which is up to 2 times greater than most other insulating materials of the same thickness.
Foam insulation is often made with one of three materials: molded expanded polystyrene (MEPS), extruded expanded polystyrene (XEPS) or polyurethane, polyisocyanurate, or a related chemical mixture. Some are installed as a liquid while other types come as factory-made panels called rigid foam boards.
Although batts are typically used between studs or floor joists, rigid foam boards should be considered as an alternate approach. These boards are lightweight, and provide structural support and acoustical insulation. Rigid boards can also be added to basement walls, exposed foundations, cathedral ceilings, exterior walls, and attic access. Such boards may be faced with a reflective foil that reduces heat flow when next to an air space. Check your local fire codes, because often these boards must be covered with a fire barrier, such as gypsum wallboard.
Liquid foam insulation can be sprayed into building cavities as a liquid or in larger quantities as a pressure-sprayed product (foamed-in-place). It can completely conform to a building cavity, sealing it thoroughly. Therefore, it's good to use in areas where it would be difficult to fit rigid boards. If you spray this insulation into an enclosed space, be careful that you don't use too much, because that could cause other parts of the structure to bend or break.