Crawl Space Horror Stories: The Summer of Mold

It all started with the awful, musty smell that seemed to be present in every room, and wouldn’t go away, no matter how often or thoroughly Mrs. Smith cleaned the house.

Next, Mr. Smith began to notice that Junior’s allergy symptoms seemed to get much worse indoors. Though he felt fine at school and outdoors, at home, the child was constantly congested, complaining of relentless headaches, and had frequent asthma attacks.

Their energy bills were sky high and yet, the home was always cold and drafty during the winter. Some rooms were humid and uncomfortable during the summer, even when they ran the air conditioner at full blast.

As time progressed, the smell got so bad, especially during the warmer days, that the family began to wonder about what might be living (or dying) under their home. The floor began to sag and buckle and every time the children ran past the dining room to the kitchen, Mrs. Smith was afraid that her precious Heirloom China would break, because the bouncy, creaking floors made it rattle inside the curio cabinet.

For many years, the Smiths lived under such conditions, not knowing what was wrong with their home. While they suspected that the crawl space had something to do with it, they had done everything they were instructed to do. Their crawl space was vented up to code, the floor was covered with a 6mil poly sheet to act as a vapor barrier, and the vents were kept open during the summer and closed during the winter. At some point, Mr. Smith had mechanical fans installed in the vents to help dry out the crawl space, but the problem was never solved.

Then, one summer, the floor under a particularly heavy bookshelf in the den began to cave in. Alarmed, Mr. Smith called a crawl space repair company, and had a specialist come to inspect the place. The Smiths’ crawl space was smelly, humid, filthy, and mold infested. Pieces of moldy, melted fiberglass insulation lined the floor, and dry rot was already compromising the structure. Some floor joists were already so rotten that the specialist could poke a hole in them with a pencil! Water from condensation dripped down from the air ducts that ran through the crawls space, pooling on top of the vapor barrier.

The Smiths, while not surprised by the findings, couldn’t help but wonder:

What went wrong with that crawl space?
The problem with the crawl space in the Smiths’ home is happening in millions of homes across the U.S. and Canada. It’s a problem so prevalent that building scientists call it a real “housing epidemic”, and it happens due to two main factors. The first is a poor understanding of building science by those who first wrote the venting recommendations for crawl spaces in building codes. The other is the reluctance of many officials in changing these codes, based on new studies that prove these recommendations were only based in assumptions and fallacies, without a shred of scientific evidence, according to this article by Dr. Craig deWitt, PHD.

It was previously believed that the only source of crawl space moisture was ground water evaporation. To address this problem, for many years, building codes required only adequate grading and yard drainage and a vapor barrier, usually a 6mil poly sheet, lining the floor to keep ground water from evaporating into the crawl.

Once the floor was lined, they assumed that an adequate number of vents would allow the air to circulate through the crawl, thus drying any leftover moisture. At first glance, it seemed to make perfect sense.

It wasn’t until years later, and after millions of reports about mold and moisture related problems in code-compliant crawl spaces across America, that building scientists began to take a second look at this issue, challenging the assumptions that served as the base for building code recommendations.

Reputable organizations such as Advanced Energy, Building Science Corp., Habitat for Humanity and even the U.S. Department of Energy began to conduct numerous studies in search of a solution to put a stop on this epidemic. Their findings and solutions might surprise you.

Finding #1 – Ground water is just one source of moisture!
Crawl spaces actually get moisture from two main sources. The ground and the outside air!

This diagram illustrates what happens in a crawl space on a warm sunny day. The air that enters the crawl through the vents brings some moisture as well. Because of the differences in temperature between the crawl space and the outside, when the air enters the space and cools down, the relative humidity in that air will increase 2.2% per each degree it’s cooled. So if your crawl space is 10 degrees cooler than the outside, the Rh levels will increase by more than 20%!

Mold loves moisture, and any environment with relative humidity levels above 60% is mold-friendly by definition. Mold also needs warmth and organic matter to feed on, and crawl spaces just happen to provide plenty of both.

It gets worse. Every time RH levels rise above 100% — which is quite common during hot and humid summer days — water begins to condense all over the cold crawl space surfaces. The wood that supports the crawl will soak up all that moisture and becomes an even more attractive food source for mold and dry rot. You know what else loves soft, warm and moist wood? Termites and carpenter ants!

Finding #2 – Air doesn’t circulate in the crawl space; it moves up, into the house.
There is a physical phenomenon that building scientists only recently began to understand and pay attention to: it’s called the “stack effect” and it relates to the way the air moves inside a building — any building, regardless of foundation type, size or architectural features.

As we all know, hot air rises. When it rises inside an enclosed structure, it tends to escape through the uppermost levels – roof, attic, upper floor windows and any such openings.

As the air escapes, a negative pressure area forms, causing the outside, unconditioned air to be sucked in through any openings in the lowest levels of the building. In homes with vented crawl spaces, that air is being sucked straight through the vents, being cooled causing all the moisture problems under the house, and then carried straight upstairs with all the moisture, mold spores and whatever pollutants are threw in the crawl space.

This is why the house smells so bad and Junior feels so sick inside his own home. This is also why the energy bills at the Smith household are so high. Their air conditioner is working harder than it should to keep up with all that hot, humid air coming from the crawl in the summer. In the winter, the furnace is practically running in place, while all that cold air is being sucked in, making the house cold and drafty. To make matters worse, the air that they pay to cool and heat has to travel through ducts housed in a hostile environment and winds up losing or gaining heat before it reaches the living space!

According to Advanced Energy, energy penalties from vented crawl spaces are so significant that you might as well forfeit any other attempts to improve a home’s energy efficiency until you get your crawl space fixed.

A New Four-Step Solution for an Old Problem
To permanently solve every single problem in a crawl space like the Smiths’, modern building science recommends a four-step approach:

  1. Correct Drainage. Make sure the terrain is graded to divert water away from the crawl space, keep gutters clean, and extend downspouts as far into the yard as possible. If necessary, install foundation drains and a sump pump.
  2. Seal Vents. Close the crawl space vents using air tight caps developed for this purpose.
  3. Encapsulate the Crawl Space. Line the entire space (not just the ground) with a thick 20mil vapor barrier and seal it to completely isolate the area from both ground and outside air – the two main moisture sources.
  4. Condition the Space. Use a dehumidifier to monitor and keep relative moisture levels under control.

This technique is now recommended as best practice by green builders everywhere, and will transform the crawl space in a healthy, clean and mold free environment, protecting your home’s value, preserving its structural integrity, improving indoor air quality and – are you ready for this? – make homes an average of 18% more energy efficient! Crawl space encapsulation is a one-time investment that pays for itself in energy savings but also adds comfort, improves indoor air quality, and preserves your home’s value and structural integrity.

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