Yikes! There’s Mold in My House

Mold Wall

These days, nothing strikes fear into a homeowner’s heart like the news that mold has been found in their house. visions of unknown illnesses and physical maladies flash before their eyes, fueled by incredible lawsuits and media hype. But what is the real threat? How much concern should you have? What should be done?

I’ve been perplexed by this issue and I’ve found that it is very difficult to find dependable information on this subject. Part of the reason for this lack of good information is that no one really knows answers with any scientific foundation. I’ve been doing a lot of reading and based on that research, I’ve come to the following conclusions. As you read these, please keep in mind that I am also among the legions of mold non-experts who have voiced their opinions on the subject. But this information was taken from what I thought were reliable sources. Hopefully this will put the mold issue into perspective.

  1. You can not eliminate mold in your house. It is always there. So the best recommendation is to control the moisture in the house by keeping the relative humidity between 30% and 60%. Mold needs moisture and it will grow when the relative humidity is above 60% to 65%. Surfaces that have condensation appear on them will be more prone to growing mold because condensation is 100% humidity, of course.
  2. There are over 60,000 known types of mold. Only a few are known toxins. The huge majority of them are benign or their effects are unknown. And they live everywhere around us all the time.
  3. Testing for mold has a limited value since nearly every test will show some mold. Even the spore count can be deceiving depending on the reproductive cycle of the mold. Some tests can show large releases only to be followed by extended periods of dormancy. Testing should be done on the outside of the house as a point of comparison. It is possible that similar levels of mold exist all around and the amount found in the crawlspace do not represent anything abnormal. Even the State of California Department of Health does not recommend testing for mold contamination because of the lack of standards for judging what is an acceptable quantity of mold! From what I’ve read, the only way to know if you have too much mold is if you can smell it or see it. Even then, the odds are highly in your favor that the mold you smell is not harmful since the vast majority of mold is not harmful.
  4. Most molds produce volatile organic compounds (VOCs) that become airborne and smell musty. These are offensive, but are not thought to cause illness. Some molds, at certain times do produce toxic chemicals called mycotoxins. If inhaled in sufficient quantities, some people can get sick from these. But this is not an established risk for healthy people. It is more likely in people with weakened immune systems.
  5. Molds can trigger asthma attacks or hay fever, but there is no proof that molds cause these ailments. The only cases of molds causing infections in humans are rare and only occur in people with a weakened immune system.
  6. Mold can grow on any surface. Metals and other non-porous surfaces are just easier to keep clean than surfaces like wood or paper. However some metals, like copper and zinc form a fungicide when they oxidize. That is why better asphalt shingles are made with zinc granules in with the stone granules to prevent the fungus streaks you often see on roofs in the South. Copper or zinc ridge strips were often used for this purpose, too. But metal duct systems can be easily cleaned and disinfected.
  7. Mold can germinate, or “bloom” in as little as twelve hours and start to grow in a day or two. So weekly monitoring is essentially useless. If the moisture is too high, mold will get ahead of you really fast.
  8. There are tons of alarmist stories and law suits out there right now so it’s about impossible to sort out the truth. That’s because no one seems to really know the truth.
  9. Lawsuits regarding mold almost always point to the builder unless there is some demonstrated neglect by the owners, such as allowing a leak to go unfixed. However, improper detailing can leave architects and engineers with a legal exposure, too. No matter the situation, it is very hard to determine fault with any accuracy since there are many sources of moisture entering a house and because of the lack of scientific data, the damage due to mold is hard to quantify. Still, many of the judgments awarded in mold lawsuits are based on sympathy for the homeowner and not actual facts like many fantastic lawsuits these days.
  10. The longer a house is under construction prior to the roof going on and getting the house “dried in”, the more susceptible it will be to mold. Work should proceed expeditiously to let the house get dry as quickly as possible.
  11. The greatest risk of mold growth actually occurs during the cooler months when the relative humidity remains above 90% for sustained periods. That would be a day like we had on Wednesday when everything seems to have condensation on it. I know it seems counter-intuitive, but during the high humidity, hot days in the summer, the relative humidity is actually lower than those damp days in the winter. This is because warm air can hold more grains of moisture and the surfaces are warmer and the dewpoint is not reached to cause condensation. Cooler surfaces cause condensation, not warmer ones.
  12. The New York City Department of Health and Mental Hygiene Guidelines on Assessment and Remediation of Fungi in Indoor Environments (2002) is the most widely recognized guide for remediation.
  13. One interesting thing I have learned is that mold will not grow on lumber with a moisture content below 20%. And even then, the wood must remain sufficiently wet for approximately seven days.
  14. Your yard (and mine) is loaded with mold. Everyone’s is except maybe in Arizona. Even there the spores probably exist, just waiting for a little moisture.

The bottom line is this. Your builder should warrant the health of the house he is turning over to you. However, I don’t know that there is any way he can really certify this since there is no “standard” in the industry. So a measure of reasonableness is required. The mere presence of mold is not a cause for alarm. But a large and visible outbreak of mold needs to be dealt with in a level-headed way. A well-educated builder is the first step in preventing mold. Controlling moisture is the primary mission. If you do that, you will control mold in your house.

If you want another perspective on the validity of the mold health issue, you might find this article, The Mold Scare: Medical facts versus dubious myths, by Gailen D. Marshall Jr., the director of the Allergy & Clinical Immunology Division at The University of Texas Medical School-Houston interesting and enlightening.

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I hope this information is helpful to you. You might want to get yourself a copy of my best-seller, Designing Your Perfect House. It is chockfull of valuable tips and advice that will save you many times the cost of the book on your house building or remodeling project. You might also like The Well-Centered Home: Simple Steps to Increase Mindfulness, Self-Awareness, and Happiness Where You Live. It will show you how to make your home a happy place.

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Bill Hirsch | Architect

Bill Hirsch


  1. Where To Buy Testing Kits For Mold on March 6, 2009 at 3:08 am

    […] Oh No! There’s Mold In My House It is possible that similar levels of mold exist all around and the amount found in the crawlspace do not represent anything abnormal. […]

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