Mold requires water. No water, no mold. Mold is the result of a water problem. Fix the water problem. Cleanup the mold. And you have fixed the mold problem.
To avoid mold problems, avoid water problems. Design and build in a manner that reduces water problems.
- Building Science Corporation, Mold—Causes, Health Effects and Clean-up
A well-designed and constructed building enclosure helps prevent mold growth within buildings. This starts with the outside layer of the building enclosure where the rain-screen/cladding/siding layer stops most bulk-water/rain. Commonly, some wind-driven water will get past this layer, but it is then hopefully stopped by the exterior continuous insulation or the weather/vapor/air barrier that is attached to the sheathing on most residential buildings.
There are various types of weather/vapor/air barriers. Some weather barriers only stop bulk-water, while others are technically not considered vapor/air barriers, but instead vapor/air retarders, as they only reduce the water vapor or air movement through their surface. These products include building wrap, factory-applied surface layer on sheathing, fluid-applied/painted membrane, and peel-and-stick membranes applied to the building. Dampproofing or waterproofing also perform a similar function for foundations. A well designed and built continuous vapor/air barrier is the most important building enclosure layer for mold prevention.
The second most important building enclosure layer for mold prevention is exterior continuous insulation commonly installed between the vapor/air barrier and the rain-screen/cladding/siding layer. Exterior insulation prevents the sheathing from getting cold during the winter. Without exterior insulation, water vapor within the air in the building cavities could condense into water on the cold interior-side of the sheathing, leading to durability issues and mold growth.
Proper air-sealing of the interior wallboard, which is commonly drywall/gypsum board, is the third most important building enclosure layer for mold prevention. This air-sealing prevents moist air within the building from going into the building enclosure cavity and condensing on cold spots on the interior-side of the sheathing, which are called thermal bypasses. Thermal bypasses are spots around the building where exterior continuous insulation cannot be installed. As an additional benefit, the interior wallboard air-sealing also prevents air movement through the cavity insulation, which ensures the insulation is at full effectiveness.
One visible sign of a potential building enclosure moisture issue is an ice dam. The first sign is uneven snow melt on a roof, which can become a visible dam or mound of ice on the edge of a sloped roof, the roof eave. The ice dam can damage the roof and lead to a roof leak. The cause is poor insulation and/or warm indoor air leaking into the cavity below the roof. As that area of roof warms up, from heat escaping through the poor insulation or the warm air hitting the underside of the roof, the snow above begins to melt. The snow melt water then runs down the roof until it freezes into ice, usually at the cold roof eave. The cycle repeats, as more snow melts and builds up the ice into a visible ice dam. The snow on top of a roof will melt evenly, if the roof is properly insulated and air-sealed.