3F UL Gear Tyvek Sleeping Bag Cover Liner Bivy Sack, Type B

Ultralight Tyvek sleeping bag cover liner for Type B use

An ultralight Tyvek bivy-style liner for use with a sleeping bag, designed to help reduce condensation transfer and keep dirt off your bag.

This 3F UL Gear Tyvek cover liner is designed to be used with a sleeping bag, not on its own. The Type B version measures 230 cm x 90 cm and uses DuPont Tyvek, with a listed weight of 200 g.

The main job of this liner is practical rather than theatrical: it is made to help reduce condensation inside the tent from reaching your sleeping bag, while also adding a layer between your bag and everyday dirt. The listing also describes it as super light, ventilated, comfortable and moisture-proof.

What it is best suited to

• Use as an inner liner with a sleeping bag
• Helping limit condensation transfer inside a tent
• Adding a light barrier against dirt
• Trips where low pack weight matters

Important limitations

• It is not meant to be used alone
• The listing says it is not a replacement for rain protection
• It is recommended for use with curtains or tarpaulins
• Sleeping bag not included

There is also a smaller Type A version in the source data, but this product page refers to Type B, so the larger size and 200 g weight are the relevant details here.