By Isaac Savage-  building-performance consulting.

To improve the comfort, efficiency, durability, and healthfulness of buildings, it's necessary to control the movement of heat, air, and moisture within a building envelope.  Attention to insulation and air sealing details throughout a house is critical.

Fiberglass Doesn't Stop Airflow

Stuffing fiberglass batts between floor joists is a common method of insulating the rim joist in many homes, but it's a severely flawed technique.  Batts don't fit snugly around wall penetrations or against uneven surfaces, and get stuffed behind floor registers.  Fiberglass cannot perform at its rated r-value because air and moisture can move freely in and out of the building through wall penetrations and wood joints.  Not only does air move freely around the batts, but because the insulation is not enclosed, it moves freely through the fiberglass as well.

Spray Foam Insulation Expands and Seals

Spray foam is insulation, moisture and air barrier combined.  The foam expands to fill holes made by wall and floor penetrations, and it seals wood joints, stopping air and water vapor from entering or escaping. Once the foam expands fully, it insulates the cavity as well.  Because the foam doesn't allow air to move through it (which diminishes r-value), it always performs at its full-rated r-value.

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