Might want to be carefull with insulation cause it may still sweat underneath it, then you are going to get mold and mildue. If you do insulate it, just might want to check it.
Professional opinion (Plumber) Just go to your local hardware store and buy foam insulation. tape the ends with poly tape or good electrical tape. This way the moisture can't get to the pipe and cause condensation. This will also solve any mold issues you would have
Since the pipe is secured to the rafter or truss, the easy way to do it is to staple some insulation with a moisture barrier around it. The moisture barrier side should go on the outside of the insulation.