basement interior walls insulation

The insulation of the interior basement walls may increase the risk of mold and mildew. If those walls aren’t perfectly dry, or if there is humidity coming from indoor or outdoor sources, or if that risk is reasonable, insulating the inside of a basement is a very wrong option.

Before proceeding with the insulation of the interior walls of your basement you should eliminate internal and external sources of moisture.

See:

Drainage
Exterior Basement Waterproofing and Insulation
Controlling Humidity

Interior Insulation

The insulation of the interior basement walls involves: 1) applying 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 the insulation coating to be covered by a building material like drywall.

Vapor barriers

If your climate is hot and humid, do not use vapor barriers. Vapor barriers are a common cause of mold growth and damages in basements. Walls should be dry and vented… If they aren't, vapor barriers are counter-productive.

Note: 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.

Image source: EERE.

The R-Value of the basement insulation

R-Values used in the basement insulation depend hugely on your particular climate.

Colder climates, with high humidity, demand insulation materials with higher R-values. Hot dry climates (zone 1, in the image) demand low or even zero R-Values...

Note: R-Value means Thermal Resistance (R-2 or R2, for instance, means a thermal resistance of 2). The R-value measures the resistance of a material to heat flow. See, for details: R-values and insulation