A building material comprising at least one electrically conductive low emissivity surface provided with at least one opening for boosting the transmission of an electromagnetic signal through the building material, the opening having a substantially lower electrical conductivity than the low emissivity surface , wherein - edges of said at least one opening provided in said low emissivity surface constitute at least one closed edge curve , wherein at least one edge of the opening comprises a section having a sinuous shape and the width of said opening is not uniform, and that - edges of said openings define a closed envelope curve so that said openings are within the closed envelope curve , and a surface defined by the closed envelope curve has an area substantially larger than an area of the openings within the closed envelope curve and a length of the closed envelope curve is substantially smaller than a length of the closed edge curve , whereby at least one electrically conductive area of the low emissivity surface is formed within the area defined by the closed envelope curve , at which the closed envelope curve is not congruent with the closed edge curve , and in which - said electrically conductive area of the low emissivity surface defines at least two edges of the opening , which are at least arranged at a distance from each other, and in which said edges are at least partly interlaced, and - said at least one opening comprises at least two sections of openings which are separated by an electrically conductive area of the low emissivity surface , wherein each of said sections of the openings comprise two or more narrow openings, and wherein the distance (234") between said sections is a multiple of the distance between said narrow openings. - edges of said at least one opening provided in said low emissivity surface constitute at least one closed edge curve , wherein at least one edge of the opening comprises a section having a sinuous shape and the width of said opening is not uniform, and that - edges of said openings define a closed envelope curve so that said openings are within the closed envelope curve , and a surface defined by the closed envelope curve has an area substantially larger than an area of the openings within the closed envelope curve and a length of the closed envelope curve is substantially smaller than a length of the closed edge curve , whereby at least one electrically conductive area of the low emissivity surface is formed within the area defined by the closed envelope curve , at which the closed envelope curve is not congruent with the closed edge curve , and in which - said electrically conductive area of the low emissivity surface defines at least two edges of the opening , which are at least arranged at a distance from each other, and in which said edges are at least partly interlaced, and - said at least one opening comprises at least two sections of openings which are separated by an electrically conductive area of the low emissivity surface , wherein each of said sections of the openings comprise two or more narrow openings, and wherein the distance (234") between said sections is a multiple of the distance between said narrow openings. <EOS>