Q. I recently installed 20,000 square feet of 1/2x4-inch cedar siding, rough side out, on a project in Connecticut. The exterior wall construction was 6-inch metal steel studs sheathed with 5/8-inch exterior plywood and covered with building wrap. Inside, there is 6-inch batt insulation, a 6-mil poly vapor barrier, and 5/8-inch drywall. The siding was prefinished with a clear sealer at the factory.

We are now experiencing "extractive bleeding" in over 20% of the siding's surface area. The supplier and manufacturers of the sealer and siding claim that this is a natural occurrence due to excess moisture. The owner wants the stains removed. What should I have done to prevent this from happening? Would putting an air space behind the siding have helped?

A. Paul Fisette, a wood technologist at the University of Massachusetts, responds: The heartwood of western red cedar (and other species like redwood and cypress) contains dark- colored water-soluble chemical extractives. When the heartwood gets wet, these extractives dissolve, and the solution can run onto the surface of the siding. When the water evaporates, the face of the siding is left stained with brown streaks -- extractive bleed.

To avoid this, you have to prevent moisture from reaching the heartwood. The best way to minimize the effect is to pretreat all surfaces of the siding with a water repellent before installation, then treat the face with a semitransparent stain.

It's a common mistake to skip the back-priming step. Water gets driven behind siding by wind and is drawn behind siding by capillary suction. Joints, overlaps, and penetrations provide pathways for water to the back of the siding. When the unprotected back gets soaked, extractives can bleed onto the face of the siding below. If you've back-primed, the repellent will shed the water before it can soak the siding and bring the extractives to the surface.

Not all clear sealers are good water repellents. Check that the product you used is in fact working to repel water. Spray water onto the surface to see if it beads up. In general, I think using a clear product on siding is a bad idea. Even the best treatments lose potency through UV degradation in less than a year. Pigmented products are much more durable.

Condensation from excessive interior water vapor can also cause extractive bleed. You have installed a vapor barrier, but air leakage can transport high levels of water vapor into wall cavities. You might consider air-sealing improvements as an option.

Providing a vented space behind siding is a good overall rain management strategy and can minimize the amount of moisture that reaches the back of the siding. It also allows siding to dry more easily. It's a bit late for that in the project you've already built. You might try cleaning: Extractive stains often come off with a mild detergent if you clean the wood soon after the problem develops. Consider applying a high-quality semitransparent stain like TWP, made by Amteco Products (800/297-7325, www.woodsealants.com) or Ready Seal (972/434-2028, www.readyseal.com) to your walls once they're clean. These products have fine pigments that help overall durability.