Heat reflective and/or absorbent material

A method of improving the properties of a glazing panel, said glazing panel comprising generally parallel passageways of box sections and having a plurality of inner walls of relatively transmissive material, the method comprising the first step of forming a longitudinally extending generally foldable sheet of material selected from the group consisting of a heat reflective material, a heat absorbent material and a heat absorbent and reflective material to have a major web portion and a longitudinal fold defining a side portion having a free edge remote from the fold, followed by the second step of folding the side portion to extend substantially at right angles to the web portion whereby to make relatively rigid the so folded longitudinal sheet to achieve a folded relatively rigid sheet, followed by the third step of inserting from one end of a passageway to be treated, an elongate portion of said folded relatively rigid sheet to cause the major web portion thereof to extend between opposed inner walls of the passageway and the free edge to contact another inner wall of the passageway.