These “air permeable” insulation types work by trapping air and must fully fill wall cavities (i.e. contact all six sides of a cavity and leave no air gaps) to be effective; otherwise convective air loops can move through the insulation and significantly degrade its performance. One exception could be TimberHPs' TimberBatt, which claims "flexible, semi-rigid, high density cavity insulation creates wind-tight performance in wood-frame assemblies that can almost eliminate cavity airflow".

For optimal performance, batt insulation must be properly cut and separated to fully fill odd-shaped cavities and fit around wiring and electrical boxes. This can be labor-intensive and so we strongly recommend blown-in or sprayed products, especially in walls.

We discourage low-density (open cell) or high-density (closed-cell) spray foam due to both installation costs and environmental concerns. Spray foam products are not included in these lists but if you are using closed-cell foam, look for products with HFO blowing agents (all open-cell products use water as the blowing agent).

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