WRAS Approved 5 Layer PEX-AL-PEX Pipe. WRAS Approved. We do endeavour to dispatch orders received later than this where possible. Underfloor Heating Pipe PEX AL PEX, made from a 5 layer composite material using PEX (Coextruded Cross-Linked Polyethylene) as the inner and outer layers coupled with a 0.2 mm thick aluminium core with butt welded construction.