A pressboard for high-voltage devices is disclosed. The pressboard has a density of 0.6-1.3 g/cm3 according to IEC60641-2, and includes 1-15% microfibrillated cellulose (MFC) based on the total dry weight of the pressboard. The pressboard also includes an insulation element. For example, the pressboard is impregnated with an electrically insulating compound. The insulation element may be used in a high-voltage device, such as a power transformer.