EPS polystyrene beads are one of the most effective and thermally efficient cavity wall insulation materials on the market, however, like any material, they come with their cons worth considering. Cavity wall insulation polystyrene beads problems include the fact that they are synthetic polymers that are non-biodegradable and not environmentally friendly to produce. They can also dislodge from their bonding in the cavity space to create gaps that can accrue moisture.