Things You'll Need
Mortar or cement
Styrofoam often has uses in home improvement applications. It is an ideal insulator because of the many small air pockets inside it that prevent outside temperatures from being conducted inside the house. After installing Styrofoam, you must use some cement or mortar to cover it up and harden the surface. If you want a highly reinforced structure, especially if you use a cheaper and "softer" polystyrene insulation, use some fiberglass mesh on the surface and apply a coat of mortar on the mesh.
Mix your cement according to the specifications from the manufacturer, written on the container you purchased.
Put some of your cement on a trowel and keep it balanced on your dominant hand. With the other hand, grab a corner of your fiberglass mesh and hold it up to the joint where the wall meets the ceiling.
Spread some mortar on the mesh somewhere below where you hold it and work up to the corner. Continue this process until you've covered the entire portion of fiberglass mesh. Cut the mesh at the bottom where you left off, using the utility knife and finish spreading the mortar to the bottom.
Repeat the entire process as many times as necessary until you've covered the entire wall.