An innovation in travel trailers is the hybrid travel trailer, combining the features of a pop-up and a hard-sided trailer. In its camping configuration, one or more bunks fold down from the side with canvas tent covers. When travelling, the bunks fold up, leaving four hard sides. Larger models allow the hybrid travel trailer to be used while "turtled", that is with the sides up. The primary advantage of a hybrid travel trailer is it offers a greater space-to-weight ratio. A disadvantage is the tent ends are not insulated and subject to heat loss and condensation.