This tool visualizes the invisible battle of “Vapor Drive”—how moisture moves through your walls differently in Winter vs. Summer. It specifically demonstrates the dangerous “Plastic Bag Trap” (Double Vapor Barrier), showing how well-intentioned builders rot their tiny homes from the inside out by trapping moisture between two impermeable layers.
Vapor Barrier Guide
Avoid the “Plastic Bag Trap” & Mold.
1. Your Climate Zone
Class I = 6mil Poly Plastic or Foil.
Simulation ready. Select parameters.
The Rules of Rot
Vapor always moves from Warm (High Pressure) to Cold (Low Pressure). Your vapor barrier must stop moisture before it enters the wall cavity and hits a cold surface.
Winter: Warm Inside -> Barrier Inside.
Summer: Warm Outside -> Barrier Outside.
Never install a vapor barrier on both sides (e.g., Rigid Foam outside + Plastic Sheet inside). Walls must be able to dry in at least one direction. If you seal both sides, any tiny leak creates a “swimming pool” inside your wall that cannot dry, guaranteeing rot.