Putting the jacket over the foot of the sleeping bag is what many experienced hikers do. This bag can replace the jacket and is only 60 grams.
Prevents condensation from wetting the bottom of your sleeping bag and can also be used as an extra waterproof liner for sleeping bag compression sacks.
70D UHMWPE Grid: 60g
Tyvek :43g
Difference:
The UHMWPE version has taped on the inside and is waterproof but not breathable. So it can also be an extra waterproof bag for sleeping bag compression sacks.
The Tyvek version is breathable and blocks condensation in some extent.