A.Paul Fisette responds:
It's important to correctly identify the source of the problem
before you try to remove the mold contamination. Certainly
evaporation from the lake elevates the humidity of the air
surrounding your home and likely adds to your mold problem. But
there are probably other contributing factors.
In an "ideal" attic design, the air in the attic should be
completely isolated from the conditioned air of the home. As a
result, the temperature and humidity of the attic space would
be very close to ambient outdoor conditions. If you have mold
on the attic sheathing, you would then also typically find mold
on the outside of the home.
Most homes aren't perfect, however: They have air-leakage
pathways that allow indoor air to pass into the attic, bringing
household moisture along for the ride. If the attic air is
cooler than the air that eventually reaches the attic, the
relative humidity (RH) of the attic air will rise and you run
the risk that mold will grow in the attic. Attic ventilation is
installed with the goal of removing moisture from the attic.
But if the outdoor air is damp, ventilation is not as
You report that you have mold on only one side of the roof
sheathing — the north face. The underside of north-facing
roof sheathing is typically damper than south-facing roof
sheathing. This is because the daytime sun beats on the
south-facing roof, keeping it warmer and drier. The daytime sun
also works to warm the attic air. Thus, if the only source of
attic moisture were outdoor air, then the RH of the attic air
would be, on average, lower than the outdoor air due to the
drying effect of the hot sun. This supports the notion that you
may have moist indoor air leaking into the attic.
The remedy is to find and seal the air leaks. Go up into the
attic, lift up all the insulation, and seal any penetrations
wi0th expanding foam and caulking. Seal all pipes, wires,
junction boxes, fans, ducts, recessed lights, and chimney
penetrations. Also pay attention to the places where interior
wall plates intersect the home's ceiling. Indoor air enters the
wall cavities, rises up within the wall, and passes into the
attic through the gap that exists between the wall's drywall
and the top plate that it's nailed to.
Once the leaks are sealed, carefully replace the insulation and
work to remove the mold. Nisus Corporation (800/264-0870;
www.nisuscorp.com) sells a full line of
products designed to eliminate and control wood-destroying
organisms and mold.
Paul Fisette is director
of Building Materials and Wood Technology at the University of
Massachusetts Amherst and a