Basement wall insulation can be placed on the exterior side of the wall, on the interior side of the wall, or on both sides of the wall. In this article, I’ll discuss the three most common ways to insulate a basement wall from the interior. (For a comprehensive discussion of basement wall insulation, including exterior basement insulation, see my 2012 article, “How to Insulate a Basement Wall.”)
Make sure that your basement is dry. Before installing any interior wall insulation, verify that your basement doesn’t have a water-entry problem. For more information on this topic, see “Fixing a Wet Basement.”
R-value minimums. In Climate Zones 3 and higher, basement insulation is required by the 2012, 2015, and 2018 International Residential Codes as follows: at least R-5 in Climate Zone 3, R-10 in Zone 4 (except Marine Zone 4), and R-15 in Marine Zone 4 and Zones 5, 6, 7, and 8.
That said, local codes may differ from these general guidelines, so it’s worth asking your local building department about minimum R-value requirements in your community.
Note that the IRC lists two different R-value requirements for basement walls: a lower number (for example, R-15 in Zone 5) for continuous foam, and a higher number (for example, R-19 in Zone 5) for “cavity insulation”—usually interpreted as fluffy insulation like fiberglass installed between studs. Since it is inadvisable to insulate a basement wall with fluffy insulation like fiberglass unless the wall has first be insulated with a layer of continuous rigid foam or spray foam, it’s generally best to focus on an approach that uses continuous insulation, and to ignore the “cavity insulation” approach.
Insulation choices. On the interior side of a basement wall, all three common types of rigid foam insulation—polyisocyanurate, expanded polystyrene (EPS), or extruded polystyrene (XPS)—perform well. That said, green builders usually avoid the use of XPS,…