Originally Posted by Ara Pacis

To protect against the sun you only really need a shadow shield, basically a dense parasol. There are other radiation risks, such as that from cosmic rays, which would require different countermeasures, including a shield to surround the crew. However, neither of these systems necessarily have to be deadweight, as a proper design might use them to perform double duty at some point in the mission.

Eventually, we may have interplanetary cyclers that have heavy-duty purpose specific shielding, but since they merely cycle in convenient orbits instead of start and stop at specific destinations, the deadweight penalty for the initial acceleration isn't really important.