Ideally, a wall may be declared adiabatic, diathermal, impermeable, permeable, or semi-permeable. As a durable elastomer, EPDM is conformable, impermeable and a good insulator. Each formation is divided into two different reservoir layers, separated by impermeable barriers. Lipid bilayers are generally impermeable to ions and polar molecules. The needle provides a smooth passage through those highly selective and almost impermeable membranes. In much of the aquifer's area, an impermeable layer of calcrete prevents precipitation from infiltrating. The casing has to reach down to the impermeable ice layers. Both impermeable and filter layers were added. Clay soils bind closely together and become impermeable to water. Aluminium foils thicker than are impermeable to oxygen and water. The secondary walls harden the cells and make them impermeable to water. Traditional oil paints require an oil that always hardens, forming a stable, impermeable film. The seal is an evaporitic package of rocks including impermeable anhydrite. Packaging materials include glass, aluminum cans, metal foils, and gas impermeable plastics. When permanent defenses fail, emergency measures such as sandbags or inflatable impermeable sacks are used.