Cork sheets are usually glued onto the inside walls. It is advised to cover the cork layer with wallpaper. Cork sheets are usually 1/8 inch thick. They are not as effective as the traditional insulation materials and are mostly used where other methods are not feasible. Still cork sheets can have a decent impact on a house’s energy balance. It’s K-value is 0.040, which is better than mineral wool and worse than recycled newspapers and sheep wool. If you use cork for wall cavity insulation, it will be one heck of an expensive endeavour. Recycled bottle-cork is cheaper but hard to get.