Kingspan Kooltherm® K5 External Wall BoardKooltherm K5 is a premium performance external wall insulation board for masonry walls. With a thermal conductivity as low as 0. 021 W mK, it allows for extremely thin wall constructions. The boards are suitable for use behind traditional and lightweight polymer modified renders and can be retrofitted externally to existing properties without disturbing the occupants as all work is done from the outside.