In this episode of If Walls Could Talk, Thomas Harris from PUR Consulting discusses the choice between the performance path and prescriptive path concerning spray foam insulation. Open cell foams allow vapor permeability, while closed cell foams act as a Class II vapor retarder, impacting the building's vapor control layer. Contractors often leverage performance path options to trade-off thermal insulation levels for other upgrades within the building envelope, permitted under Section R406 of the 2015 IECC, which allows for non-prescriptive code compliance, provided the overall energy performance surpasses the 2009 IECC reference standard.