A downhole tool including a body having a hydraulic fluid chamber, and a flexible multi-layer barrier impermeable to gas and water mounted at the body separating the hydraulic fluid chamber from fluids external to the body. The flexible multi-layer barrier including a first elastomeric layer, a second elastomeric layer, and a gas impermeable layer arranged between the first elastomeric layer and the second elastomeric layer, the gas impermeable layer being formed from a metal layer.