Though common and sometimes advantageous, interior insulation of basements remains a controversial issue.

Before proceeding with the insulation of the interior walls of your basement you should...

1) eliminate internal sources of humidity and
2) fix eventual gutters and downspouts problems and eventual limitations of the surface drainage system around the house (for details, see: Drainage, gutters, downspouts).

Mold and mildew risks

There is a strong risk of mold and mildew when insulating the interior walls of basements. If the walls aren’t perfectly dry, or if the risk of humidity coming from indoors or outdoors sources is high, insulating the inside of the basement is a bad option: if the insulation ever becomes wet, the whole system will be in cause.

Vapor barriers

The application of vapor barriers is a common cause of mold growth and damages in basements walls. If your climate is hot and humid, do not use vapor barriers, since they prevent walls from drying. In cold climates, vapor barriers are important in the basement insulation process, but only if the walls are perfectly dry and vented…

Definition:
A vapor barrier (or vapor retarder) is typically a plastic or foil sheet able to resist to the passage of moisture, often installed with the insulation material. In the case of basements, a vapor barrier helps prevent moisture from penetrating.

The r-value of the basement insulation

R-values used in the basement insulation depend mostly or your particular climate. Colder climates, with high humidity, demand insulation materials with higher R-values. Hot dry climates (zone 1, on the image below) demand low or even zero r-values... See, the image below (source: EERE), showing the R-values recommended for basements insulation in North-American climate zones. 

Definition:
R-Value means Thermal Resistance (R-2 or R2, for instance, means a thermal resistance of 2). A material’s R-value is the measure of its resistance to heat flow. Insulation materials with the same R-value have the same insulation efficiency. See, for details: R-values and insulation.

Insulation materials for interior insulation

Rigid extruded polystyrene insulation is a very good option (though batt insulation is more common).

For new homes, concrete block insulation and insulating concrete forms are also excellent options. Loose-fill insulation and sprayed foam insulation are common choices for finished basements.

Insulation steps

The insulation of the interior basement walls involves four major steps: 1) applying of a vapor barrier, 2) installing the insulation material, 3) framing the insulation and 4) drywall application.

1- Applying a vapor barrier

In non-hot and non-humid climates, if the walls are perfectly dry, it’s common to install a vapor barrier before the insulation, to best accommodate moisture pressures. The vapor barrier should be perfectly sealed and installed continuously, from the bottom to the top of the wall.

2 - Installing the insulation material

The insulation material (rigid extruded polystyrene…) should also be installed continuously from the bottom to the top of the wall. The joints should be sealed by a durable tape (metal foil tape, for instance).

3 – Framing the insulation

The insulation material should be supported by a 2x3 or a 2x4 framing (wood or metal channel…). Besides supporting the insulation cover, framing also provides a space for plumbing, wiring and for the drywall application…

4 - Drywall application

Fire codes and safety demand that the insulation coating should be covered by a building material as drywall (also known as gypsum board, plasterboard, gibraltar board, gib or gyprock). As expected, the drywall is installed on the cavity wall framing.

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