LLDPE/LDPE liner are available to prevent the incursion of moisture, these liners can be loose inserted, sewn in or glued to all kinds of outer bags.

Liners can be make in different shapes to be fixed inside of outer bags.

Full open top with sealed bottom
Fill spout with sealed bottom
Fill spout with cubic bottom
Fill spout / Discharging spout ( can be made cone shaped)

Sewn in methods including:
Sewn tab: Reinforcement tape tabs attached to the liner are sewn into the corner seams of outer bag. The tabs position the liner during filling and discharging of products.
Sewn flange: Extended flanges of the form-fit liner are sewn between the panels of top and side, bottom and side.