Membrane systems exploit the double hull of the ship as the supporting structure on which a cryogenic liner – performing the two functions of gas tightness and thermal insulation – is applied. The gas/liquid retention role against methane leakage is effected by a thin (from 0.7mm to 1.2mm thick) sheet of Invar steel or waffled stainless steel, termed the primary membrane.
A fully redundant secondary membrane layer provided in case of failure of the primary barrier is constructed of either Invar steel or a glass fabric-aluminium composite material called Triplex. The insulation function for transportation LNG at -160°C is performed by plywood boxes filled with perlite or by reinforced polyurethane foam fabricated into panels. See also Mark III system.