Q. We coated a house I designed in 1987 with a high-quality solid-body stain. Within two years, it was badly spotted with mildew. We power-washed the house with bleach, then recoated it using a product called "Mildew Check." Within a year we were called back to wash the house again. To make a long story short, in seven years, we have power-washed the house four times and recoated it three times, each time with a different high-quality solid stain containing a mildewcide. The same thing has occurred on other houses with wood siding I’ve designed, regardless of whether we used pine, redwood, or cypress. Please help.

A.Henri de Marne responds: All stained or painted wood siding will absorb water. Protective coatings help reduce absorption, but do not prevent it, especially after some weathering time. Plus, moisture is absorbed through shiplap joints, the tongues and grooves of solid-board siding, and the bottom edges of clapboards. Also, as any wood moves with the changes in the climate, thirsty, unpainted surfaces become exposed. Then there is capillary attraction between two meeting surfaces, which can drive moisture well beyond the exposed parts of any uncoated wood.

All these conditions make wood siding vulnerable to moisture. If it stays wet, it won’t be long before you have mildew problems. Once mildew sets in deeply, no surface power-washing, even with added chemicals, will reach deep enough to get rid of it entirely.

To avoid mildew growth, it is best to treat all surfaces of the siding with a water-repellent wood preservative. At the very least, back-prime the siding before installation. Consider using preprimed siding to speed the job along.

If you install uncoated wood siding, be sure to prime it within a couple days. Do not leave the siding to weather for weeks and months, as is too often the case. This practice only exposes the wood to contamination from airborne dirt and mildew spores, which must be removed chemically (bleach and TSP). If mildew becomes established in the wood, it will continue to grow through as many layers of paint or stain as you are willing to apply.

Also, keep in mind that mildew spores love linseed oil, so stay away from any coatings that are linseed-oil based.

One final, but very important point: Any wood siding must have a chance to dry out fast once it becomes wet. When the sun shines on a damp wood surface, it is not "sucked out" as one might think. Instead, it is pushed away from the sun, deeper into the wood, only to come back out later. If this moisture is blocked from migrating into the underlying sheathing, the siding will stay wetter for long periods of time, and mildew will grow. For this reason, mildew problems are most severe over foam sheathing, especially foil-faced foam that prevents any absorption of water.

One answer to this problem lies in installing a "rain screen." Apply the siding over furring strips installed over the foam sheathing (vertical strips for horizontal siding and horizontal strips for vertical siding). Be sure to apply furring strips around windows and doors that are sufficiently wide to provide a nailer for the ends of the siding and the trim. In retrofit applications, the existing trim should be removed, then reapplied after exterior jamb extensions have been installed. I suggest adding furring strips around the entire perimeter of each wall to seal the spaces between the furring strips from bats and insects. The air space between the sheathing and the siding will provide the proper climate for dissipating heat and moisture.

Henri de Marne, a former remodeling contractor and custom builder with almost 40 years experience, is now a home inspector and building consultant based in Waitsfield, Vt.