Vapor barrier and WWM

We have scheduled the concrete pour and now have to work toward that deadline. There are still three major items we have to complete:

  1. Installation of the vapor barrier
  2. Installation of the concrete reinforcement (welded wire mesh)
  3. Installation of the PEX tubing (radiant floor heat)

We are done with the insulation installation and bond breaks, which allows us to get started on the vapor barrier. The point of the vapor barrier is to prevent any of the ever present moisture in the ground from permeating through the concrete slab into the living unit.

The ¾ inch stone base is the first line of defense in the moisture management battle. It provides a capillary break and prevents liquid water from moving upward. The taped XPS insulation that is atop the stone base also acts as a vapor retarder. The polyethylene sheet vapor barrier atop the insulation is the last line of defense, preventing any remaining water vapor from permeating into the living space. In short: it’s all about moisture management and control.

Quick note: Vapor barriers are only as good as they are airtight! Moving air, such as through air leakage, can move impressive amounts of water vapor.

See also: Understanding Vapor Barriers (

I bought two 12-foot by 100-foot rolls of 6 mil polyethylene sheathing.

Cathy took the first roll and spread it out across one half of the basement. She carefully turned the sheathing up along the bond breaks and secured the 4 inch overlap to the top. Any patches around the footings were cut generously to allow a minimum of 18 inches overlap between pieces. She was also careful to allow for 18 inches overlap between the two rolls in the middle of the basement.

While Cathy was taking care of the vapor barrier, I began to wrestle with the welded wire mesh (WWM) for the concrete floor reinforcement. I had purchased two rolls of 10 gauge WWM, each measuring five feet by 150 feet.

I began to roll them out, cut them to the right length and put them to one side. This is where a bolt cutter becomes very handy!

Once Cathy was done with the vapor barrier in one half, I rolled the WWM pieces into their final position and began to flatten them out. The middle pieces required some fitting around the footings.

Last but not least I attached the flattened welded wire mesh sheets to each other with four inch zip ties.

About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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