Royal Robbins brushed Hollow Core Flannel is engineered to keep you warm without weighing you down. It’s con-structed from lightweight ThermoTech hollow core polyester fibers, which trap air for greater insulation. This fabric is also wrinkle resistant and has UPF 50+ protection. Hollow Core Flannel. UPF 50+ . Dri X-treme quick drying. Traditional fit. Chest pocket with button closure. Shirt tail hem.