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Are Vapor Barriers Necessary?

  • Date: 04-2020

Battling moisture is part of every home owners life, and while everyone is aware of the moisture caused by leaky roofs and poorly constructed walls or the water that is standing in your yard after a heavy rain, but do you ever think about the moisture under your house?  Homes built over crawl spaces deal with moisture from another source: water vapor that rises naturally from the exposed soil and becomes trapped in the enclosed crawl space.  This type of moisture can cause serious problems for the home owner, however there is a simple solution.

Uncontrolled moisture can cause rot and decay in wood framing and other materials, and it fosters mold and mildew that can cause allergies and asthma. The amount of moisture that is present determines if you can finish a basement, for example. Moisture also determines what type of below-grade flooring you are able to install. It affects the kitchen and bathroom walls, floors, and especially ceilings. At its most extreme, moisture can even determine if the construction of a home or room addition is possible at all. Where moisture is a known problem, vapor barriers are integrated into wall, ceiling, and floor construction to combat the effects. But nowhere is moisture more damaging than in a crawl space, even though it is not seen. Because the crawl space has direct contact with the ground, it should be no surprise that moisture will develop here. It is difficult to overstate the problems that can occur because of crawl space moisture.

One of the most serious problems caused by moisture is mold, mildew, and other fungi. According to the CDC, mold can cause nasal stuffiness, throat irritation, coughing or wheezing, eye irritation, and skin irritation to people with sensitivities. And people with mold allergies can have much more severe reactions, including lung infections. Eliminating mold is difficult, and fully remediating mold-related problems is expensive. Mold’s effects go well beyond just harming people. As a direct byproduct of crawl space moisture, mold can blacken floor cavity insulation and structural elements. Because there is often no light and only minimal ventilation in a crawl space, the problem never improves unless preventive or remediation methods are enacted.  Even where moisture has not yet created obvious problems, its presence may turn away home buyers and erode the resale value of your home. When you try to sell your house, the property inspector will explore the crawl space and note the presence of water on the report. Prospective buyers will almost certainly want the problem fixed or demand a lower sale price because of it.

How Vapor Barriers Work

One method of  controlling moisture in a crawl space and it  is a very simple and inexpensive project: laying out rolls of ordinary sheet plastic. Traditionally, this plastic has been called a vapor barrier, but this strategy has been retitled by the U.S. Department of Energy, which now defines it as a vapor diffusion barrier. This is a more accurate term, since plastic sheeting, even with seams sealed, cannot seal off 100 percent of the moisture migration. Rather, the plastic slows down the process.

Sheet plastic barriers tend to help with the migration of gaseous water vapor up through the soil into the crawl space. They do not offer a remedy when water pools up in the crawl space. Pooling water comes from one of two sources:

A high water table or rainwater runoff may be causing water to pool in the crawl space. In this case, you will need to hire a water remediation company. In most cases, they will dig a trench around the perimeter, add drain pipe, cover the pipe with drain gravel, and add a sump pump. In other cases, adding roof gutters and downspouts, and reconfiguring the earth grading may be enough to eliminate pooling water.

Leaking plumbing pipes, either water supply pipes or drain pipes, may be allowing water to pool up in the crawlspace. Water leaking from damaged pipes above must be fixed before you put down a vapor barrier, or water will simply pool up on top of the plastic sheeting.

Improve Ventilation

Plastic vapor barriers will prevent most water vapor from seeping up from the soil into your crawlspace, but improving cross ventilation through the crawlspace is an important additional method that will prevent any residual moisture from being trapped in the space. Vapor barriers greatly slow the migration of moisture vapor up from the ground, but without cross ventilation through the space, some moisture can still collect.

Most crawl spaces have inadequate ventilation, and bringing the vents up to code can greatly help moisture issues. In more serious cases, an electric vent fan system can help keep a crawl space dry. 

Preventing mold growth and musty odors

One of the more common problems with crawl spaces that do not have a crawl space vapor barrier system is that they create high moisture levels, which may result in mold growth – as dampness creates the ideal environment for mold growth. By installing a crawl space vapor barrier system, you will greatly reduce the moisture levels, preventing mold and foul odors from growing and forming.