Vapor Barrier

VAPOR BARRIERS VS. VAPOR RETARDERS

With the use of vapor barriers on the rise in residential construction, contractors should learn how these products can be used to protect the home.

Vapor barriers are commonly installed in commercial buildings with interior on-grade floor slabs, especially when impermeable floor coverings are specified.  Without them, water vapor traveling from the ground through the concrete can loosen bond to the floor covering or destroy the covering.  But until recently, vapor barriers have not been used much for residential construction.  The primary reason for the move to vapor barriers for residential construction is the “tightening up” of residential buildings to improve their thermal efficiency.  Mold and fungus litigation has also promoted increased use of vapor barriers because the ground moisture enters the house envelope primarily through floor slabs.

Vapor Barriers or Vapor Retarders

The permeance of a membrane is defined as the rate of water vapor transmission through it.  By definition, vapor retarders must pass less than 0.3 perms; the generally accepted definition of a vapor barrier is that it passes less than 0.01 perms.  ASTM E 1745 defines three classes of membranes (A, B, and C) by three physical characteristics.  The class of material you select depends on the amount of abuse you expect during construction.

The decision of whether to install a vapor barrier or a vapor retarder rests with the degree of protection needed.  Lee Quigley, assistant general manager for the Riverside, Calif., office of SelectBuild (formerly Campbell Concrete California), says that in Southern California, moisture problems and mold issues are common, so owners regularly specify vapor retarders ranging from polyethylene sheeting to more sophisticated membrane systems.

Membrane Placement 

Membranes can be placed either directly beneath the concrete or under a sand or stone blotter layer.  What’s at stake here is the amount of curling that can occur when mix waater leaves the bottom of a slab more slowly than it does the top surface.  Protection of the membrane during construction is also a factor.  Bret Houck, director of business development for Stego Industries, San Clemente, Calif., suggests that the sub-base stone should be covered with a layer of fine, compactable material to protect the vapor barrier from punctures when concrete is placed directly on top of it.  “For slabs in basements,” Houch adds, “it’s also best to wrap the membrane under the footings and up the outside wall surface enough to achieve a good seal connection to the vertical waterproofing.” 

Installation Basics

Our focus has been on water vapor transmission more so than liquid water.  Most regional codes now require that drainage systems be installed on both sides of footings.  And the practice of installing sump pumps has long been mandated.  The first consideration is always to remove standing water before it rises to the level of the membrane.  The membrane, therefore, is only to prevent water vapor transmission, “brown gas” emissions, and radon gas, which is a leading cause of lung cancer in the United States.

The installation standards of ASTM E 1643 require 6-inch seam overlaps between pieces of membrane for flat-slab installations.  According to Houch, the sames should be sealed with manufacturer’s tape that has a perm rating less than the 0.1 perms maximum for a vapor retarder.  “You should be able to look down at the finished membrane application and not see dirt anywhere,” Houck says.  In his book, Concrete Floors and Moisture, author Howard Kanare says that seams should be arranged either perpendicular to the direction of concrete placement or so that concrete flows in the same direction as the overlap.

It is more difficult to install membranes on slab-on-grade foundations with interior footings (also referred to as waffle slabs).  The best option is to provide overlap seams going through each depression without taping them.  During concrete placement, the material can freely move into voids under the membrane without stressing it.

Sealing around penetrations through membranes is also very important.  Form stakes are the leading penetration.  Concrete contractors can aboid this by setting bulkheads and forms without stakes—plumbing penetrations then become the primary focus.  Kanare provides several diagrams in his book showing how these penetrations can be sealed against sub-slab moisture.  His book can be purchased through Portland Cement  Association at www.cement.org.

Houck says the cost for a flat-slab application using a premium product adds about 25 cents per square foot to the cost of the job, including labor.  But this is one place where a penny’s worth of prevention is worth a pound of cure.

This article originally appeared in the January White Cap Contractor Trader and was excerpted from Concrete Construction with permission. It was written by Joe Nasvik.

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