Moisture Problems

Concerned about mold and mildew?

The real issue is moisture! Here is why, for mold and mildew organisms to grow and survive they require:

  • oxygen - and we don't want to reduce the oxygen in our homes!
  • cellulose (wood and wood fibers) - the most common building material most of our homes; and,
  • elevated moisture levels - and we can do something about that.

So, read on and find out how to reduce moisture problems in your home and prevent mold and mildew problems!


  • Interior moisture problems are a relatively new phenomena in temperate climates. Older homes tended to leak air and this air movement helped keep homes dry. The problem was that these older homes were also very expensive to heat and cool. As a result, we started to insulate and tighten homes. A very good idea, but the unintended consequences of such energy saving measures was that some homes experienced moisture buildup and damage.

    Sources of Interior Moisture

    Moisture in homes comes from a number of sources: roof leaks, wet lots, wet basements and crawl spaces, plumbing leaks and people. People? Yes, that's right, we are among the biggest contributors of interior moisture.

    The average individual puts off about 1 quart of moisture per day as a result of respiration and perspiration. We also produce interior moisture when we: cook, wash, clean, water house plants, and have pets. Most of this moisture is absorbed by the interior air of the house. The warmer the interior air, the more moisture it can absorb.

    Big homes with fewer inhabitants have lots of air volume to absorb the moisture produced by each individual. Smaller homes with many inhabitants have a much lower volume of air per inhabitant. Properly vented homes, homes with forced air heating systems, and homes in dryer and exposed locations have fewer moisture relatd issues.

    Moisture problems are also found in homes that were previously heated by a forced air furnace and those recently insulated and weather stripped. The problems in such homes is usually the result of inadequate or improper workmanship or modifications. The moisture problems may be caused by the following:

    • a decrease in the air movement through the house, e.g. new tight windows, elimination of a chimney.
    • a reduction of paths for the moist interior air to escape to the outside, or

  • a complete exclusion of heat from one part of the house, such as an unused back bedroom.

    These conditions result in relatively high interior moisture levels and some cold interior surfaces where water vapor can condense.

  • The results of such elevated moisture levels can be very serious and include the following:

    Damaged Sheathing This plywood roof sheathing had a moisture content of over 20% (normal would have been 8-12%). The black stain is a fungal wood rot. It will destroy the wood and may cause health problems.

    • Mold and mildew
    • Damage to framing, plaster, and finishing materials
    • Pest infestation
    • Interior and exterior paint damage (a common cause of blistering paint)
    • Roofing deterioration - yes! high interior moisture levels can damage roofing materials and reduce the life of the roofing.

  • Solving elevated interior moisture levels is often complex and involves several elements of the interior and exterior of the home. In addition, once mold and mildew problems exist, moisture levels will often require a set of solution that go well beyond standard construction practices. Here are some possible solutions to your moisture problems:

    (1) Look for homes and home sites with good natural drainage.

    (2) Reduce the source of interior and exterior moisture:

    (3) To increase the Venting:

    Bathroom Fans and Timers

    A key to moisture control is a good quality bathroom fan system. This system helps remove moisture from the bathrooms and from the rest of the house. Such a system consists of:

    • A quiet fan with a Sone (sound) rating of 1.0 or less (.5 is good, .3 is ideal).
    • The fan must be ducted to the outside through a set of sealed ducts.
    • The fan should be controlled by a timer/switch. The fan should operate automatically for at least 1.5 hours, 2 times per day. It should also have a manual switch and used whenever the bathroom is in use.
    • A similar fan should be installed in laundry rooms.
    • Tight homes also require air supply ports, e.g. window vents.
    • In units with past moisture and mold problems attention to moisture issues and the need for more venting is even more important. For example: venting and air circulation must be increased.

    • Install good quality fans in the bathrooms, kitchens, and laundry rooms. Verify that ducts lead outside. Never vent fans into the attic or crawl space.
    • Keep doors open between rooms as much as possible to increase air circulation.
    • Added insulation increases the need for venting. Improperly installed insulation can cause serious damage from condensation inside the walls of the house. If you add insulation, also add more venting.
    • Vent crawl spaces and attic spaces. The code requirements for venting may not be sufficient to properly control moisture in houses that use electric zone heating.

    Attic and Crawl Space Venting An Ongoing Debate

    The common wisdom has been that crawl spaces and attics should be vented. Some recent research suggests that such venting may not help and may even be part of the moisture problem.

    My experience in the Seattle area tells me that homes with moisture problems tend to have inadequate or blocked attic and crawl space venting. And I don't recall seeing a home with moisture problems and venting levels that meet the current code's venting requirements. My experience in this area tells be that good attic and crawl space venting is not the whole answer, but it is part of the answer.

    So, I keep on recommending attic and crawl space venting.

  • Denby Barnett of Custom Drywall is an expert in drywall installation and repair work. Here are some of his suggestions:

    1. Be Careful! Moisture damaged drywall may contain some harmful mold organisms. When in doubt, have stains tested by a qualified lab. And even if the drywall is dry and clean, wear dust masks. Dust is not good for you even if it doesn't contain Stachybotrys

    2. After removing the damage drywall it will be necessary to test and look for any damage to the insulation and framing inside the wall.

    3. Before installing new drywall, make sure that all surfaces are dry.

    4. Consider using paperless drywall in potentially moist areas. Its the paper on the drywall surfaces that provides the "food" for the mold organisms.