A Gore-Tex bivy, or bivouac, sack is a lightweight weather protection sack for sleeping bags. The sack provides an external layer of protection for an outdoor sleeping system. According to the Gore-Tex Care Center, routine care and maintenance extends the lifespan of the sack.
Turn the washing machine on to a normal warm water cycle. Add 1/4 cup of liquid washing powder as the machine fills with warm water. Put the sack in the warm water wash. Don't add fabric softener or bleach in the wash. Use a prewash treatment product to remove any stains from the sack prior to inserting it in to the wash.
Put the sack in the dryer. Turn the dryer on to a normal cycle. Don't add a fabric softener to the dryer. The dryer heat reactivates the water repellent on the sack.
Put a towel between the sack and an iron to steam out the wrinkles. Ironing also reactivates the water repellent.
Restore the water repellent if water fails to bead on the sack's surface. Spray the damp sack with a durable water-repellent spray. Put the sack in the dryer to activate the water repellent.