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
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
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
www.woodsealants.com) or Ready
www.readyseal.com) to your
walls once they're clean. These products have fine
pigments that help overall durability.