Knauf Earthwool wool cavity slabs are very light and malleable. You can simply use a sharp serrated knife to slice through Dritherm 32 and DriTherm 37. Always ensure you take proper safety precautions. Before you cut and fit into masonry cavity external walls, we recommend wearing goggles, aprons, and even protective glasses. If you are concerned about safety precautions, always consult with a professional.