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:

• Check the condition of the gutters, downspouts and drains and make sure that all storm water drains away from the house.
• Check crawl spaces and basements for leaks and standing water.
• Reduce the number of indoor plants and aquariums.

(3) To increase the Venting:

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.

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.