optimum spacing between the glazing layers is about 12 5 mm further it should be noted that the drawings illustrate only a small representative sample of some of the possible applications and design configurations of the foam spacer for multiple glazed sealed units referring to the drawings fig1 to 3 show the plastic foam spacer for double glazed units fig1 shows a cross section of a single seal double glazed unit the flexible or semi rigid foam spacer 40 can be manufactured from thermoplastic or thermosetting plastics suitable thermosetting plastics include silicone and polyurethane suitable thermoplastic materials include thermoplastic elastomers such as santoprene the preferred material is silicone foam the advantages of the silicone foam include good durability minimal outgassing low compression set good resilience high temperature stability and cold temperature flexibility a further major advantage of the silicone foam is that the material is moisture permeable and so moisture vapour can easily reach the desiccant material within the foam during the production of the foam desiccant is added as a fill the type of desiccant material used is typically 3a molecular sieve zeolites to remove moisture vapour and in addition smaller amounts of 13x molecular sieves silica gel or activated carbon are used to remove organic vapours.