An easy and effective way to save time, and money by using less of the expensive materials, is to use a polyethylene foam for backing out gaps and joints prior to applying mastic joint sealants. Polyethylene is the most commonly used plastic in the world with it’s primary use in packaging such as plastic bags, films and containers including bottles. Many kinds of polyethylene are known, with most having the chemical formula (C2H4)n. Polyethylene is usually a mixture of similar polymers of ethylene with various values of n. Polyethylene is a thermoplastic, however, it can become a thermoset plastic when modified, such as cross-linked polyethylene, which, in the context of polyethylene foams are relevant to some of the filler foams we stock. Low density expansion and movement joint foam fillers, which we stock in convenient ‘fire hose’ type coils, circular rod coils or packs of ‘caulking strip with various dimensions, (perforated to tear off), are ideal for use in brickwork and blockwork due to their low compressive strengths. Also ideal for expansion and isolation joints in concrete slab construction. Low density foams are therefore specified in the construction of hardstandings, factories and warehouses, community and commercial buildings, airports and military bases.