Although most backpacks feature waterproof fabrics, they all have lots of seams and zippers where water has a tendency to work its way in. The Osprey UL Raincover (Medium) blocks this seepage with ultralight silicon impregnated ripstop nylon and fully taped seams. Fabric: 40 denier silnylon (silicon impregnated ripstop nylon). Fully seam taped. Drawstring closure with toggle tightener for a secure fit. Built in storage bag. Fits packs ranging from 2400-4300 cubic inches (40-70 l). Weight: 4.25 oz. (0.12 kg.).