A cover sheet (100) comprising: an electrically-conductive spacer material (110) comprising an upper surface (115), a lower surface (116) and a thickness (117) measured between the upper surface and the lower surface； a vapor permeable material (110) proximal to the upper surface of the electrically-conductive spacer material； an electrical circuit (200) configured to measure an electrical parameter that is related to the thickness of the electrically-conductive spacer material； and an indicator configured to provide an indication when the electrical parameter is outside a predetermined range.
