All FasNSeal (single-wall) vent lengths and components feature patented built-in mechanical locking band and triple-lipped directional gaskets. All pipes and components are engineered with smooth clean welds located 90 deg from mechanical locking bands. This construction feature allows for ease of installation. The 1/2 in air space between the inner and outer wall on FasNSeal W2 helps to reduce the clearance to combustibles in addition to creating a heat shield for additional safety. All FasNSeal components which come in contact with exhaust gases and condensates are now silicone free. This important design and manufacturing feature greatly reduces the chances of system breakdown and failure. The main problems with silicone are there are too many adhesion factors which are very difficult to control and the process of binding similar materials with an epoxy, which can weaken the bond over time. Welded seams ensure a leak-free product that is meant to last. DuraVent® FasNSeal® Special Gas Vent Pipe, Single Wall, 4 in Dia, 12 in Length, Stainless Steel