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Mold and Mildew
Common Moisture Problem Areas | How to Avoid Moisture and Condensation | How to Raise the Temperature | Seal and Insulate the Attic and Crawlspace | Insulate Pipes | How to Lower the Humidity Level | Exhaust Fans | Central Ventilation | Vapor Barrier | Other Mold and Mildew Solutions | Air Sealing | Duct Sealing | Air CleaningQuick Facts
Mildew and mold both describe certain fungi which tend to grow in homes and buildings when conditions are damp or moist. As a living organism, mold needs three things to grow and spread:
- spores (which are everywhere)
- a food source (such as drywall paper, lumber, or other organic materials)
- a moisture source (plumbing leaks, condensation, etc.)
Even after visible mold and mildew is removed, it will always come back as long as moisture and a food source is present. The best way to get rid of mold and mildew at home is to make sure excess moisture is removed from the problem areas.
Common Moisture Problem Areas
| Problem Area | Where moisture collects | What can happen |
| windows | condensation often forms on cold window panes | moisture can seep into window frames and walls |
| under kitchen or bathroom sinks | leaky pipes or condensation on pipes | moisture can seep into the wall or collect under the sink |
| bathrooms | from showers which cause condensation on walls | moisture can seep into walls |
| attic | condensation on water pipes | moisture can drip onto insulation |
| crawlspace | outdoor or underground moisture can condense in the cool crawlspace | moisture can collect on wood framing and insulation |
| exterior walls | if temperature or humidity is very different inside and outdoors moisture will pass through the walls | moisture can accumulate in the insulation, wood, and drywall |
How to Avoid Moisture and Condensation
Condensation happens on colder surfaces (like cold window pane or a glass of cold water) because moisture in the air cools off and forms water droplets when it touches the cold surface.
Condensation tends to form in problem areas because of low air temperature and poor air circulation, causing the amount of relative moisture in the air to become critical. The temperature at which moisture will start to condense out of the air at a certain humidity level is called the dewpoint. The two ways of avoiding the dewpoint are:
- raise the temperature in the home or problem area
- lower the humidity level by removing moisture from the home
How to Raise the Temperature
There are places inside the home which do not get enough heat, such as closets and cabinets or rooms which stay shut off from the rest of the house. Leaving these spaces open to the rest of the home will make sure they are heated enough to avoid the musty smells which can mean that mildew is starting to grow.
It is commonly known that heating the home enough in general helps to avoid condensation and moisture problems. Although it saves energy, completely shutting off heat for several days or weeks to a home or to sections of a home can lead to mold and mildew problems. Below are other examples of avoiding mold and mildew by keeping surfaces warm enough to avoid condensation.
Seal and Insulate the Attic and Crawlspace
Attics and crawlspaces are special because they are usually not heated or considered part of the indoors. They are usually open to the outdoors through vents intended to avoid moisture build up, but many building codes are changing to allow for them to be sealed off from the outdoors. This is because sealing and insulating attics and crawlspaces can often better avoid moisture and condensation than venting can, since much of crawlspace and attic humidity actually comes from outside air *. Sealing avoids entry of moisture, while insulation keeps the attic and crawlspace warm enough to be above the dewpoint and to avoid condensation.
Insulate pipes
Cold water running through pipes lowers the surface temperature of the pipes which often leads to condensation, just like moisture in warm air condenses on a cold glass of water. By wrapping water pipes with insulation the cold pipe surface will not be exposed to the air and moisture won’t condense on the pipes.
How to Lower the Humidity Level
Common activities in the home, such as showering and boiling water, add moisture to the air. It is important to remove this excess moisture in order to lower the dewpoint and avoid condensation.
Exhaust Fans
Without proper ventilation moisture will collect and mold will grow. This is why bathrooms and kitchens have exhaust fans which remove the extra moisture which is added to the air.
Central Ventilation
Opening a window can help moisture dissipate if it is not humid outside, but windows tend not to be opened enough and opening windows can also let in pollution and allergens from outdoors. More houses with forced air heating are being built with an “outside air” option which works much like a central exhaust fan and replaces stale damp inside air with fresh outside air.
Vapor Barrier
Moisture from the air can actually go through walls where it can be trapped in the insulation and wood framing and end up causing mold growth in the walls which can’t be seen. This is mostly a problem in humid climates, or climates with very cold temperatures. In these climates excess humidity could be avoided by installing a vapor barrier such as a plastic sheet inside the wall, but it is crucial that any vapor barrier be be placed in the appropriate part part of the wall, if at all. Correct placement varies with climate and improper use of a vapor barrier can actually cause moisture problems.
Other Mold and Mildew Solutions
Air Sealing
Air travels through the many gaps and cracks which can be found in a lot of houses. If the air has a lot of moisture in it (from outdoor humidity or from moisture in the crawlspace or attic), the moisture can seep into walls and condense, encouraging mold and mildew growth on drywall or even on fiberglass insulation. Sealing those cracks can prevent this problem but it will also keep moisture in, which is why proper ventilation and wall assembly design is so important.
Duct Sealing
Return ducts with air leaks tend to suck air from the crawlspace and attic into the house. If there are mold problems in those places mold spores can actually get into the ducts and be distributed throughout the house. Air leakage in ducts is a common problem and sealing those leaks can make a big difference to indoor air quality.
Air Cleaning
Installing an air cleaner on the central ventilation system can remove mold spores from the air which would otherwise be distributed throughout the house. However, the filter must be rated to remove small enough particles to filter mold spores out of the air. It is also important to consider the energy consumption and the effectiveness of the cleaner at different particle sizes.