I seal the edges of the foam board and exposed cardboard with strips of newspaper and PVA glue. I tear the paper into 3inch wide strips and apply over watered-down PVA glue, then more glue. Three separate layers of paper and four of glue - that's glue, paper, glue, paper, glue, paper and finally glue. The first layer of newspaper running from the top, down the sides and under the cardboard, the second across the joints from side-to-side and lastly top, side, bottom as the first layer. All sealed with another coat of watered-down PVA glue. Once the mix is fully dry I will apply some filler and sand smooth.
Photo One - the foam tile with edges sealed.