Abstract: A double wall vent pipe is provided which comprises a first tubular wall that is helically corrugated to form radially outermost ridges spiraled around the outside of the first tubular wall and corresponding first recesses spiraled between the outermost ridges. The vent pipe also comprises a second tubular wall slidably disposed in a concentric relationship with the first tubular wall. The second tubular wall is helically corrugated to form radially innermost ridges which spiral around the inside of the second tubular wall and corresponding second recesses spiral between the innermost ridges. The radially innermost ridges are oriented in a direction oblique to the orientation of the radially outermost ridges such that a space between the first wall and the second wall will be maintained within the first recesses and the second recesses regardless of any bending of the vent pipe.