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BATTLING WINTER MOLD AND MILDEW
By Roger McAfee
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October 14, 2002. Editor’s note: This article appeared as part of a winterizing piece in the October, 2002 issue of Pacific Yachting, a Vancouver, B.C. publication that’s one of the best boating magazines in North America. Roger McAfee is author of the book, The Warm, Dry Boat. What follows here is one of the best explanations of mold and mildew on boats that I’ve seen. Pacific Yachting and Roger McAfee graciously granted permission for us to reprint the piece on this web site.
During a recent heating and ventilation seminar, I was asked what could be done to deal with mold and mildew problems on small boats laid up for the winter in the driveway. From the follow-up discussion, it emerged that most boaters believed they wouldn’t have problems if their boat was “on the hard” during the winter--clearly not the case. In fact, an acquaintance of mine who pulled his boat out of the water after every use and stored it under a tarp at his home, nevertheless found that mold still grew, both summer and winter.
Without getting into the biological details, molds grow from spores that are constantly wafting around in the air. When they settle on damp spots inside a boat they begin to expand. They survive on wood, paper, carpets and food, and cause substantial damage in a very short time. The most serious problems created by molds are the health risk they create--particularly for those suffering from asthma or other respiratory problems.
As a mold grows it digests and damages the underlying surface. If mold gets into carpet or upholstery those items will often have to be junked. A similar fate often awaits infected headliners, cloth, leather and vinyl.
Mold can take hold on hard surfaces such as Formica, arborite, fiberglass and even stainless steel, where it presumably survives on a thin film of dirt or other organic materials. Mold is stubborn, and requires a thorough wash with detergent, followed by rinsing and a thorough drying to kill it. Simply wiping it off with a damp rag won’t do the trick
Mold spores are literally everywhere. There’s no practical way to eliminate them completely. Therefore, the environment aboard must be altered to avoid conditions that support mold growth. Basically, this means every attempt must be made to eliminate moisture inside the boat.
Too often, when a boat is laid up for the winter the owner buttons it up tightly against the weather, perhaps leaving a porthole cracked open, then covers the whole vessel with a tarp. Unfortunately, this turns the boat into a big greenhouse. Especially on sunny days, the temperature builds up inside the vessel due to the greenhouse effect. The warm air absorbs moisture to the point where it starts to collect as condensation on any cool surface such as overheads and windows. It then drips down onto furniture, carpeting and fixtures. At night things get worse. Cool air holds less water vapor than warm air, so even more condensation is unleashed. If the damp, moist air inside the boat isn’t removed and replaced with fresh, drier air, the relative humidity inside will almost always hover near 100%-ideal for mold and mildew development.
Boaters often put in a small heater or a light bulb or two as a way to raise the inside temperature somewhat and move the air around. They may also use one of the various commercial products “guaranteed” to keep mold and mildew at bay during the winter lay up, but most of these units are nothing more than a low-output heating device. Some boaters also buy one or two desiccant units containing moisture absorbing crystals.
Low wattage heat sources and desiccants may work well in very small boats or enclosed onboard spaces, but on their own they are almost always doomed to fail aboard larger vessels. If the heater produces enough heat to get a column of air rising above it, and if that moving air is vented to the outside, fresh air may be drawn into the boat. That fresh air will help keep the mold and mildew at bay, but the heat itself is mainly encouraging ventilation.
As noted earlier, the key to battling mold and mildew is to remove moisture from the boat. The only practical way to do so is to make sure the boat is adequately ventilated. Just what is “adequate ventilation” and how do you make sure you’ve got it? Generally speaking a relative humidity in the 35%-60% range will keep mold and mildew at bay, provided lockers, drawers and doors are kept open.
There are two fundamentally different ways to achieve good ventilation. The first is to force air into the boat at one location, which, in effect, pressurizes the boat, pushing the moist air out of the boat at another location. This requires an intake fan, which should be positioned as far from the exhaust location as possible. This approach can help distribute the fresh air almost uniformly throughout the boat.
The second method is to use an exhaust fan to draw damp air out, while allowing one or more strategically positioned intake vents to admit fresh air. Again, the intake(s) should be as far as possible from the exhaust.
Either method usually requires a fair bit of work to rig the fan, and if the boat is to be used during the lay up, the whole system must be dismantled and re-rigged afterward.
On the other hand, any boater who is handy with basic tools can build a permanent installation that will suck damp air from the bilge and exhaust it directly outside. All that’s required is a bilge blower or, even better, an in-line fan, plus a few feet of ventilation hose and a humidistat. (A humidistat is a switch, which automatically closes in response to elevated humidity.) As with a heat-controlling thermostat, the boater can pre-set the humidity level as desired. The vent hose is fixed to the exhaust outlet of the blower and run to a vent fitting open to the outside. When the humidity builds up in the bilge the blower will kick in, exhausting the damp air--first from the bilge, then from the rest of the boat. As this happens, fresh outside air will be drawn into the boat until the humidity drops below the preset level and the blower shuts off.
If you’re not into “building it yourself” you can buy such a system--more sophisticated than the one I describe--from Nanaimo Humidity Control at 250-751-3003.v
Comment: This article points up the importance of getting of fresh outside air to flow steadily through the boat from one end to the other. While it describes fans to move the air, passive ventilators in the deck can do much to let new air in and get rid of old air. Combined with fans, enough air can be moved to make a dramatic difference in mold growth.
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