Recycled “rags” come in different shapes and kinds. The blow-in materials are usually pure cotton while the batts come mixed. One of the most advanced mixes is a blend of hemp and cotton fibres. The downside of many mixed fibre materials is that they need a filler to hold them together. A filler is usually polyester or any other synthetic fibre, which somehow reduces the material’s eco credentials. The K-value of recycled denim can be as low as 0.039.