Wall Vapor Barrier Placement: Interior vs Exterior Choosing the right wall vapor barrier(/glossary/vapor-barrier) placement can make or break a DIY eco-build. This article on wall vapor barrier placement compares interior (warm-side) barriers, exterior (cold-side) vapor control, variable "smart" retarders, and vapor-open assemblies so builders can reduce condensation risk, maintain drying potential, and protect sheathing and framing. Readers will learn how permeance values, climate (IECC zones), and insulation strategy (cavity vs continuous exterior insulation) change the safe choice for a given project. For a first-pass moisture screen, model the layers in the Wall Assembly Builder(/tools/wall-assembly-builder) and confirm the project region in the Building Climate Zone Lookup(/tools/climate-zone-lookup). TL;DR: - Interior warm-side barriers (6-mil polyethylene ≈ 0.06 perm) work well in cold climates if the sheathing stays warm; seal all penetrations and prioritize airtightness. - Sma...