Ask the Builder: Remove the mold from the vinyl siding

Question: F. My house is now 25 years old. It’s covered with vinyl siding. Every summer, mold and mildew develop on the north side of my house. I remove it every few years. Is there a coating I can apply to the siding to prevent mold and mildew from coming back? It’s aggravating. What about any color? I am open to all ideas. – Robert C., Sylvania, Ohio

A. I have the same problem in my home even though the siding is fiber cement. If you live north of the equator, black mold and powdery mildew or green algae may grow on the north side of your home. The north-facing position offers the perfect conditions for this organic growth.

Here’s the bad news. I understand that there is no such thing as a magical paint or coating that will keep your siding free of mold, mildew and algae. Here’s why: these three living organisms are almost everywhere. You only need two things to grow: water and food. The north orientation of the houses provides plenty of water.

Since direct sunlight does not wash the north-facing walls, the water there often evaporates more slowly. Morning dew can and does form on all surfaces of the house when the surface temperature drops below the dew point of the air.

Those surfaces on the north side of the house lose this dew last, so mold, mildew and algae can have a drink before the next morning. The ground directly under the siding remains far more humid than the ground on the other sides of the house, with all things being the same. Water evaporates from this soil, and some of this water vapor can also quench the thirst of the organisms.

Food for mold, mildew, and algae to grow is readily available in almost all regions of the United States. In Ohio, the numerous deciduous trees, especially maples, emit a fine aerosol. These microscopic drops of elixir contain sugars, which are delicious foods for powdery mildew, mold, and algae.

Dust and other pollutants can also feed some of these organisms. Realize that food is everywhere, even though it is invisible. Old oil-based paints were made from alkyd oil resins. These oils are delicious foods for mildew and mold.

Because of this, some modern deck sealers made with natural oils go black within months after you go to the trouble of making your deck look beautiful. They eat food so that mold and mildew can devour them. Some natural oil seals contain chemicals to slow organic growth.

Three years ago I wrote a book called Roofing Ripoff about why asphalt shingles fall apart faster than they should. The inspiration for the book came when I discovered that copper ions will add decades to the life of asphalt shingles.

Many have known for hundreds of years that copper is an amazing natural biocide too. That is why the hulls of clipper ships and “Old Ironsides”, the historic frigate of the US Navy, have a copper cladding. I used liquid copper solution to keep algae off decks. The only problem is that the copper is washed away with every rainfall and can create a bluish tinge. Some people are very opposed to using aqueous copper like this one, thinking that it is harmful to the environment.

If you’re having problems with mold and mildew on your vinyl siding, then you should try spraying a copper sulfate on the siding. However, if you have valuable landscapes or trees nearby, the copper won’t help them with life. The problem is that the copper on the vinyl siding is temporary and will wash off with repeated rainfall.

I think the easiest thing to do is to wash the siding once a year around Flag Day. Until then, it will be warm and the work will not be too difficult. If you’re washing the house when the siding is still free of mold or mildew, you may be able to achieve this with a garden spray on your garden hose.

Instead of putting dangerous chemicals in the sprayer, just use oxygen bleach powder and liquid dish soap. Spray this soapy solution on the house and let it sit for 15 or 20 minutes. This allows the oxygen bleach to attack and break down invisible food particles.

The liquid dish soap lubricates the surface so that the food comes off with minimal effort when the siding is washed up. This simple cleaning process will keep the siding looking great year round.

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