Fill gaps larger than 1 4 in.
Rigid insulation between floor joists.
Seal and insulate using rigid foam board insulation.
A do it yourself friendly method for sealing and insulating the rim joist or band joist area is to use rigid foam up against the rim joist.
Run a bead of acrylic caulk around each section of foam to form an airtight barrier.
How to finish a basement.
This method can be hundreds of dollar less than the cost of using the commercially applied spray foam.
Cut the strips to fit between floor joists using a box cutter.
Unlike fiberglass rigid foam insulation does not absorb water support mold or allow air to pass through providing that joints between boards are sealed with tape.
Instead of installing the foam insulation between floor joists in the crawl space the foam board is installed against the crawl space foundation walls.
The fiberglass will also need to be secured in place using wire mesh twine wood laths or other securing methods which can be an intensive project that adds to the overall cost.
The insulation should press right up against the floor between joists.
Joist bays the open spaces between floor joists are prime hosts for insulation under specific circumstances.