The cotton in blue jeans makes an effective insulation to protect your house from cold and drafts. Bonded Logic, Inc. makes bat-type insulation for homes from shredded denim. They have a secret, patented process for making their insulation. Making a whole house-worth of blue jean insulation probably isn't practical for an individual, but if you have a small space that needs a little extra insulation, making your own out of old denim could be a fun recycling project.
Wash the blue jeans in the washing machine. Dirt and oil can reduce the insulating properties of the fabric. Dry the fabric thoroughly. Wet jeans won't insulate very well.
Cut away zippers, rivets, pockets, waist bands, belt loops and seams. Save these for other craft projects. You should be left with single layers of fabric.
Cut the fabric into 3-inch or 4-inch squares. Stack the fabric and cut two or three pieces at a time using the rotary cutter and mat. Don't worry if the squares aren't perfectly even. Your goal here is to reduce the fabric to small, manageable pieces.
Run the pieces, one at a time, through the paper shredder. A cross-cut shredder works best for reducing the fabric to confetti-like pieces. Stop every few pieces to allow the shredder to cool. You want to reduce the denim to the smallest pieces possible for the best results.
Gather all your shredded denim in a bag or basket. Dump or lightly stuff the shredded denim into the space you want to insulate. Fill the space full, but don't pack it too tightly. The air around each piece of denim helps insulate, too. That's why fluffy down insulates better than tightly woven wool. Air spaces between each piece of insulation slow the conduction of heat.