FSK is an acronym for Foil / Scrim / Kraft (paper) and is also known as FRK which stands for Foil Reinforced Kraft.
FSK Tape is foil adhesive tape that is laminated foil with Scrim / Kraft paper. This product is reinforced to increase durability and ease of handling. It will be increased tear strength for taping exposed edges and board joints of the rock wool insulation.
FSK tape can act as radiant barriers seaming the thermal insulation's facing material.
They are both be used as building Radiant barriers. Radiant barriers are installed in homes -- usually in attics -- primarily to reduce summer heat gain and reduce cooling costs. FSK tape consist of a highly reflective material that reflects radiant heat rather than absorbing it. They don't, however, reduce heat conduction like thermal insulation materials.
Heat travels from a warm area to a cool area by a combination of conduction, convection, and radiation. Heat flows by conduction from a hotter location within a material or assembly to a colder location, like the way a spoon placed in a hot cup of coffee conducts heat through its handle to your hand. Heat transfer by convection occurs when a liquid or gas -- air, for example -- is heated, becomes less dense, and rises. As the liquid or gas cools, it becomes denser and falls. Radiant heat travels in a straight line away from any surface and heats anything solid that absorbs its energy.
Most common insulation materials work by slowing conductive heat flow and -- to a lesser extent -- convective heat flow. FSK tape with insulation systems work by reducing radiant heat gain. To be effective, the Aluminum foil reflective surface must face an air space. Dust accumulation on the reflective surface will reduce its reflective capability. The FSK tape should be installed in a manner to minimize dust accumulation on the reflective surface.
When the sun heats a roof, it's primarily the sun's radiant energy that makes the roof hot. Much of this heat travels by conduction through the roofing materials to the attic side of the roof. The hot roof material then radiates its gained heat energy onto the cooler attic surfaces, including the air ducts and the attic floor. The FSK tape reduces the radiant heat transfer from the underside of the roof to the other surfaces in the attic.
FSK tape works best when it is perpendicular to the radiant energy striking it.
FSK tapes are more effective in hot climates than in cool climates, especially when cooling air ducts are located in the attic. Some studies show that radiant barriers can reduce cooling costs 5% to 10% when used in a warm, sunny climate. The reduced heat gain may even allow for a smaller air conditioning system. In cool climates, however, it's usually more cost-effective to install more thermal insulation than to add FSK tape.