Huntsville real estate; crawl space moisture

Huntsville real estate; Is Crawl Space Encapsulation Right For You, Your Client or Your Situation?
August 10th, 2012 8:05 PM

Is Crawl Space Encapsulation Right For You, Your Client or Your Situation? Source: James A. "Buck" Durham, P.E. Now that summer is in full swing, you may find home inspectors telling you and your clients that they've found wet conditions in the crawl space area beneath a home. Since this is usually the dry time of the year, then more than likely, the wet conditions are related to high humidity rather than water seepage into the crawl space. The most common source of humidity inside a crawl space is related to a combination of moisture vapor rising out of the uncovered ground and warm, high-humidity air entering the crawl space through the perimeter foundation vents. The first problem has long been understood and easily combated by placing a vapor barrier over the ground. The second problem is the most misunderstood problem in our industry and something you must understand and be able to explain to your clients. Whenever warm (hot), humid air enters the crawl space through the perimeter foundation vents, its temperature drops about 20 degrees or more since the temperature in the crawl space is much cooler than outside air. This lower temperature is caused by a combination of factors including air conditioning duct leakage, cold radiation from the ducts/floor above and the fact that there is no direct sunlight or massive air flow into the space. When the warm/humid air enters the crawl space, its temperature immediately drops, so it must give up its moisture and this moisture condenses on cool objects such as air conditioning ducts, the wood floor (exposed to interior air conditioning) and the ground surface. As this moisture collects over time, the humidity in the crawl space increases dramatically. One of the main problems with wet/humid crawl spaces is the formation of mold/mildew and the deterioration of the wood floor system. Wood is a hygroscopic material, meaning that it tries to reach equilibrium with the surrounding atmosphere-either absorbing moisture from high humidity air or off-gassing moisture in low humidity air. (By the way, this is why joints in wood trim/moldings located inside a house open/close throughout the year.) When exposed to high humidity, wood takes on moisture and when its moisture content reaches a certain level, it becomes a perfect host for wood inhabiting fungi which can literally destroy it. See Photo 1. The end result is a condition we call decay, rot, dote or doze. Some fungi live largely on the wood's cellulose while others feed on both Photo 1 the lignin and the cellulose. The most common fungi we see feeding on our ever popular Southern Pine floor joists is Brown Rot. After prolonged fungi attack, the floor joists take on a brown color, crack across the grain, shrink, collapse and even crush into a fine powder. Brown, crumbly rot in the dry condition is often called "dry rot" but this is really a misnomer as it takes high moisture/humidity to decay wood. The wood may later dry but the rot/decay was caused by exposure to high moisture. Another common problem with wet/humid crawl spaces is cupped hardwood flooring inside the home. Cupping, also called "washboard" is a condition where the edges of a piece of flooring (across its width) are high and the center is lower. This generally develops gradually. Moisture imbalance through the thickness of the flooring is the only cause. Moisture is greater on the bottom of the board than on the top. The moisture from the crawlspace rises up into the home due to what is called the "stack effect" and this creates high moisture conditions on the bottom face of the hardwood flooring. This in turn causes the bottom face of the hardwood boards to swell (expand) dimensionally (wood expands when wet) and this creates the cupping condition. So How Do You Prevent High Humidity In A Crawl Space? The best way is to cover the ground with a seamless vapor barrier, extend the barrier up the perimeter foundation walls, seal it to the foundation walls and cover/close-off the foundation vents. This sounds like an easy proposition until you crawl under a home and see the network of ever present obstructions caused by A/C ductwork, plumbing pipes, electrical wires, foundation piers and floor supports -- let alone the low headroom, construction debris to be removed and muddy conditions which are often present. Another important factor when sealing off a crawl space from ground and air moisture is to use a high quality vapor barrier-typically 16- or 20-mil thick polyethylene as opposed to the long standard 6-mil plastic. This material is much more resistant to snags and tears caused by crawl space traffic and the storage of materials under the home. Plus, it is imperative that the vapor barrier be completely seamless and this is accomplished by sealing all the seams/laps with heavy duty vinyl tape. The thicker the vapor barrier, the better it is for tape-sealing. After the crawl space is encapsulated, there are a host of immediate benefits. Condensation stops, mildewy odors that once permeated the home go away, mold/fungus on the floor structure go away, cupped hardwood flooring may eventually lie back down and dust/allergens/mold spores no longer seep into the home through cracks/crevices in the floor. And if the floor system is damp or wet from long term exposure to high moisture, a central dehumidifier can be placed in the crawl space to dry it out. Also and alternatively, one can insulate the perimeter foundation walls and tap into the central heating/air conditioning system ductwork and heat or cool the space under the home-oftentimes reducing the home's annual heating/cooling costs. No matter how you view it, crawl space encapsulation is the best way to go in the humid, southeastern corner of the United States. Although the Building Codes still require crawl space ventilation in new home construction, we're seeing rapid changes in the thinking of Code Officials (writers) as they now allow sealed crawl spaces if continuously operated mechanical ventilation is provided or the crawl space is insulated and provided with conditioned air. So the next time you find a wet/humid crawl space during the dry time of the year, don't be misled by uninformed contractors that tell you that more ventilation is needed. This will only worsen the problem! For more information about this and other common "house problems" please visit my web site at www.dwengineers.com. And if you ever have a question or concern about a house problem, feel free to call me at 256-318-0982 256-318-0982 . I'll gladly discuss the problem free of charge. Cordially, James A. "Buck" Durham, P.E.
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About Durham & Wehrman Engineering Inc Durham & Wehrman Engineering was incorporated in the State of Alabama on February 12, 2007. We were formerly JADE Engineering and Inspection, Inc. which started business in 1987. We specialize in residential and commercial building inspections for major insurance companies, Huntsville real estate Realtors, mortgage companies, homeowners and contractors. We also perform foundation and structural design and specialize in the development of foundation and structural framing plans for complex building structures. Our owners, James A. "Buck" Durham, P.E. and Joel D. Wehrman, P.E., are graduates of the University of Missouri-Rolla (Missouri University of Science and Technology) and have been working together since the mid 80s. Collectively they have inspected more than 15,000 homes and commercial buildings. Our engineers pride themselves on an ability to inspect a home or commercial building and quickly diagnose any problems that might exist. In the event that we are inspecting a home or commercial building for a Real Estate transaction, we are able to explain the problems and put them into proper perspective for all interested parties. We typically supplement our written reports with photographs, CAD drawings and a list of authoritative references.