If you have a skin in the new home building game right now, you must surely be acutely aware of the mold issues that are plaguing many new homes. There are many reasons why mold is growing where it should not be. In my opinion, the primary reason for mold growth is simply operator error. Many young builders, job superintendents, and sub-contractors do not have a crisp historical perspective of how homes used to be built. Many also do not treat their jobs as a vocation. Those craftsmen who have a true passion for building tend to avoid mold issues as well as other construction defects.
Before I started into the custom home building profession, I had the good fortune to work in the home remodeling field for nearly ten years. It was not apparent to me at the time, but I was getting educated each time I took apart a house to rehabilitate it or add a room addition. When you start to take the outside surface off a home, whether it be brick, stone, wood siding, stucco, etc., you soon discover how well the home was built. I have taken apart many 80 - 100 year-old homes that had no mold, mildew or wood rot.
When I did discover mold, mildew and wood rot, the cause of the problem was usually very obvious. Poor workmanship would allow water to saturate the wood used to build the structure of the home. Undoubtedly other builders before me had seen the same thing and figured out that if you keep wood dry, it simply does not promote the growth of fungi that we see as mold, mildew and wood rot.
You may wonder what sets those homes apart from the mold-stricken ones you see in the news. One of the significant differences between many of today's new homes and those your parent's grew up in is simply tar paper. Older homes that had exteriors made of wood siding, fiber cement, stucco, etc. had a weather-resistant layer of tar paper sandwiched between the wood framing and sheathing and the finished surface exposed to the weather. When water got behind the exterior surface, the waterproof tar paper would shield the wood from getting wet. By carefully overlapping the tar paper both vertically and horizontally, any water would be escorted back to the atmosphere.
In addition to the tar paper, metal flashings were common on top of windows, doors and other distinct horizontal breaks in the outside building materials. These simplistic flashings served one purpose. They would capture water that got behind the exterior surface and then redirect it back to the exterior of the home where it could continue its journey into the soil around the home. Flashings are simple to install and made from inexpensive pieces of aluminum, tin or galvanized metal.
The real trick to stop wood rot, mold and mildew is to create an air gap between the exterior skin of your new home and the structure behind the skin. The structure must have a waterproof membrane such as traditional asphalt saturated felt paper or one of the newer air and water infiltration membranes. Once installed, vertical furring strips made from ACQ treated lumber can be nailed to the solid studs behind the felt or membrane. The thickness of the strips can be as little as one quarter inch and up to one half inch. The wood or vinyl siding, stucco lath, etc. are then nailed to these wood strips. Long nails must be used so that the fasteners eventually travel into the wood studs in the wall.
Be sure the strips extend down past the top of the foundation at least one and one half inches. Galvanized hardware cloth that has one eighth by one eighth inch spacing needs to be fastened to the bottom of the vertical strips so that insects can't get up into the void space behind the outer skin of your home. This heavy screening will last as long as your home and will be hidden by the outer finish material that you and all of your neighbors will look at each day. The hardware cloth must lap up behind the furring strips, then span the gap and finally lap over the top of the furring strips to make an effective barrier.
The air gap your builder creates will allow your house to breathe. There are other products that help create this same space and your builder may find them to be more cost effective. It is my hope that the building code officials will eventually mandate this to be a required item on each new home built. Doing this is smart, easy and is a great thing for the health and well being of everyone.