Expandable polymer compositions are used to reinforce or provide various types of insulation to cavities in structural elements. The polymer compositions include at least two different segments, which may or may not be joined. At least one of the segments will expand before another segment when heated. The first-to-expand segment forms a physical barrier to the expansion of later-expanding segments, thereby restricting the expansion of the later-expanding segment in at least one direction. In this manner, the location of the expanded polymer within the cavity can be easily and inexpensively controlled.