Vapor Retardant vs. Vapor Barrier In Insulated Walls

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Troy asks:
I’m going to install Applegate cellulose insulation in the exterior walls and attic of my new house. I live in Central Nebraska. Do I need a vapor retarder like Membrain with blown in cellulose for my region? I have a non-insulated panel Zip Wall system with the seems taped off with Zip tape.

Answer:
US Anual Average Annual Heating Degree Days Map I’m pleased to see the term “vapor retarder” rather than “vapor barrier” used in this question.  A vapor barrier is a wall or ceiling component that is essentially impervious to water vapor. The most commonly used one is polyethylene. It used to be conventional wisdom, standard practice, or even code mandated to put poly in every wall in Barrow, Alaska; Miami, Florida, and everywhere in between. Poly must be on the warm side of the assembly or it inevitably causes problems by trapping moisture that gets in the wall – and moisture is inevitably going to get in the wall. In most parts of North America a vapor barrier is going to be on the wrong side of the wall at least part of every year. Building scientists are pretty much in agreement that vapor barriers should not be in walls in climates less than 9,000 heating degree days. The coldest spot in Nebraska is Valentine at 7255 HDD. Class II and class III vapor retarders (polyethylene is a class I vapor retarder) are less problematic, since they slow the migration of water vapor into an insulated assembly, but are sufficiently vapor permeable to prevent trapping moisture in the wall. U.S. Dept. of Energy classifies cellulose insulation as a class III vapor retarder.  So the simple answer to the question is that a distinct vapor retarder isn’t necessary since you are using cellulose insulation. If a dedicated vapor retarder is installed please use Membrane or another “smart” material, never polyethylene – unless you are building near or north of the Canadian border.

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