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To control mold growth, mold biology must be understood.  As it turns out, the most effective way to do it by controlling and managing moisture.

There are a lot of ways to control moisture.  Many are highlighted here. Specific actions to take by people living in houses and those who own them are separate articles following this one.

Mold Biology

The human argument with mold is the toxins (mcyotoxins) on the surface of mold spores.  The key then is to make sure there are only a few of them in our house and not give them what they need to grow.

Mold Triangle. Mold growth is activated by water or moisture vapor.  A good food source for mold is anything organic, such as paper, wood, cotton, wool, etc.  Mold spores are everywhere.  Remove anyone of these 3, and there is no mold issue!

Mold spores are and always will be in our environment.  It's unlikely will get rid of our clothes, home furnishings, and building materials.  The only practical option left is to control water.

Starting & Sustaining Mold Growth. The basics of starting mold growth is this: get relative humidity above 60% and limit air movement.  Once mold growth starts, relative humidity levels of 50% will sustain it.

Mold Spore Facts. The facts about mold spores is that they are so small they're invisible.  By the time we see mold colonies, there are thousands--maybe millions--of spores.  All it takes is a wisp of air or the lightest of touch to release a huge number of them.  Once airborne, mold spores fall through the air at about 1 foot/hour.  Turn on the fan and they won't land.

The best analogy I have for this is a flour sifter in the kitchen.  If the sifter were loaded up with flour and sifted, the air would be a dense white cloud in no time.  Eventually, the flour will fall out of the air and onto the counters, floors, walls, pots, pans, and other furnishings.  The air remains clear, until some walks through the area and stirs it all up again.

Mold spores behave like flour, but takes a lot longer to settle out of the air and is a lot easier to stir up.  Just imagine what what the house would look like if the air handler fan pulled flour laden air throughout the house.  It's a mess, is it?  It isn't easy to clean up flour.  It's even harder to remove mold spores.

Use Moisture Control. To avoid this scenario, the only real option is to not let mold spore density get large enough to cause us problems.  Hence the need for moisture control!

Ways to Control Water & Manage Moisture

There a many ways we can control water and manage moisture to avoid mold growth.  The obvious ones are avoiding water flow into the house and getting it out as quickly as possible when it does.  Less apparent is ventilation, humidity control, and avoiding condensation or vaporization as appropriate. 

The most important things we can do is understand moisture management principles and be aware of what we're doing.

Repair Leaks. Plumbing leaks.  Roof leaks.  Window leaks.  Air Leaks. These let "large" amounts of water or water vapor in.  These are obvious sources of moisture.  Except for the air leaks, methods for address them are well known.

Air leaks need to be stopped by air sealing the walls, floors, and ceilings.  Usually, this is done with foam and sealants by a weatherization contractor.

Divert Water.  Some people in the San Francisco Bay Area seem to think it's natural for water to flow through their crawlspaces.  We add insult to injury by draining water next to the foundation.  The worst case I saw was draining water from the roof directly into the crawlspace!  All that water has to drain or evaporate somewhere!

Clean Up Floods.  Floods come from some unexpected places, such as fire departments and broken plumbing fixtures.  Of course they also come form tsunamis, earthquakes, hurricanes, tornadoes, cyclones, broken dikes in the delta and Central Valley, and the like.  The trick here is to get it cleaned up and dried out within 48 hours.  A water damage restoration company is needed.

Don't wait until Monday to contact the insurance company about a water incident at 5 PM Friday afternoon!  The courts require you to take action immediately to protect your property.

Mop Up Water.  Water left standing will at least evaporate.  When the water is on a floor supported by a wood subfloor, water will find it's way through gaps less than1/32 of an inch.  When flooring is wood, it will swell. Laminate flooring is permanently ruined, while hardwood floor will resume it's shape when dried.

Dry Carpets. Spilling drinks, urine, and tipped over mop buckets routinely make carpets wet.  Try as we might with towels and anything else we can think of, we won't even get the carpet dry, let alone the carpet pad underneath.  The odd thing is that we can't tell if something is dry enough by touch alone to avoid mold growth.  A water damage restoration company uses a combination of surface blowers and dehumidifiers to dry them out.  Sometimes, the carpets are taken up for drying and put back.

Mechanical Ventilation. We routinely put lots of water vapor in a house just by living: cooking, cleaning, bathing, respiring, perspiring, etc.  Since the late 1970s, we've been tightening up our houses to lower our energy costs.  By doing so, our once leaky houses no longer replace the "polluted" air with "fresh" outdoor air.  To cope, we use bath area fans and kitchen fans.  For tighter houses, we put controlled fresh air intakes or energy recovery ventilators (ERVs) on our central heating and air systems.  We do this, in part, to lower relative humidity in the house.

Natural Ventilation. Opening windows and doors to air out the house is a time honored practice.  However, when was the last time you opened the doors and windows on a San Francisco bay winter, foggy day when it's 45 degrees outside?  I don't! 

Around here though, our summers are so temperate, many of us don't have air conditioners, so we use fresh, outdoor air to cool our homes.  (Don't try this in the Southeast!).  For those times of the the year where natural ventilation is not a practical option, mechanical ventilation is needed.

Dehumidification.  During the rainy season, the moisture levels in the house are frequently much higher than they are outdoors.  This is particularly true for older houses over crawlspaces.  Relative humidity is a function of temperature and the amount of water vapor in the air.  Reducing the amount of water vapor also lowers relative humidity.

Before using a dehumidifier though, because they are expensive to run, control all the other moisture sources the best you can first.  The good news is that a dehumidifier is also a heater!

Temperature Control.  The other factor in determining relative humidity is temperature.  I've gone into several houses where folks have kept the temperature low to avoid high utility bills.  When mold appears, any savings is eaten up by doctor and cleaning bills. 

The trick is to control moisture the best you can with all other means, including a dehumidifier and then find the temperature needed to keep relative humidity below 50%.  The Federal suggestion of 68 degrees, oddly enough, should do it in most cases.

I'd rather control moisture, seal up my house, use mechanical ventilation, and run my house at 72 degrees, thank you!  Not only do I use less energy, but me and my family are healthier and far more comfortable.

Avoid Condensation.  Single-pane windows, missing vapor retarders, lack of insulation, flat paint and the like contribute to condensation, especially when warm, moisture-laden indoor air meets exterior walls.

Two Window Panes. Adding storm windows or replacing single-pane windows with dual or triple windows does wonders for both your energy bill and moisture control.  The second pane of glass helps keep moisture vapor in the air, rather than allow it to collect on window frames, curtains, etc.

Wall Insulation. Insulation is either missing or inadequate in the exterior walls of many older homes.  Unfortunately, this moves the "cold plate" from the siding to the interior wall.  Warm, moisture-laden air will easily condense on it.  Ideally, the walls should be insulated.  In the mean time, keep your personal items away from the walls to allow air to circulate, as calm air encourages mold growth.

Vapor Barrier/Retarder Location. Water vapor goes through plaster and gypsum board drywall covered walls.  To moisture vapor, the wall looks like a sponge.  The water molecules just pass through the holes. 

The trick is knowing where you want the water condensation to occur: on the interior wall, behind the interior wall, in the middle of the wall cavity, on the exterior wall sheathing, or somewhere else on the wall.  Moisture will condense.  We have have to determine where with our choices of vapor retarders.  Unfortunately, in our environment, any decision we make will be wrong about half the year.

My current preferred method in the Metro Atlanta Area is to pack the walls with insulation without any vapor barriers or vapor retarders.  I choose to use enamel paint on the interior walls to avoid let moisture in the house penetrate into the exterior wall cavities.  Since moisture is blocked on the interior side by the paint, the insulation needs to dry from the outside in.  In the summer, our walls are "dry".  In the winter, let's not make the exterior wall cavities wet from both the fog and our living activities.  The debate rages about this subject, so do your research!  (Do not try this approach along our nation's Northern border!)

Avoid Vaporization. Showers, cooking, cleaning, toilets, and crawlspaces put large amounts of moisture vapor in the air.  Showers put a cup of moisture in the air after each use.  Who knows how much is put there by cooking, cleaning, and toilets.  We have mechanical ventilation for these sources.

Crawlspace Vapor Barriers. Crawlspaces are a different matter all together.  Even if we control water at the foundation, a lot of moisture evaporates from the soil.  Houses close to bodies of water are especially vulnerable because water will wick up through soil 16 feet. 

When soil moisture gets to the surface, it evaporates.  Then, from the crawlspace's perspective, the house is a giant chimney.  When heated attic air is ventilated through the roof, it draws air through house in the ceiling and walls from the living space.  In-turn, the living space draws air through holes in the floor.  Isn't that the crawlspace?

The first step is avoid evaporation in the crawlspace in the first place.  This is done with a sealed vapor barrier across the ground and to the walls and pillars.

Control Crawlspace Moisture.  Evaporation from the ground is one of four sources of water vapor in a crawlspace.  Moist air drawn through the vents is another.  Moisture coming through foundation walls is a third.  Finally, moisture coming in from the living space above. 

Recent studies show the best thing to do with a crawlspace is seal it and provide conditioned air to it.  To do that, moisture must first be controlled the best you can by all other means.  People who have them so them off to their friends.  They're that good!

Be Aware of What You're Doing. So often, we're going about our busy lives without paying much attention to moisture control.  That's why building scientists (should be your architect) design houses to manage it unconsciously for you.  However, most houses haven't been completely thought about in terms of moisture control and management.

We have to think about what we're doing to take appropriate precautions.  You must pick up where the builder left off.

Ignore Faulty Advice.  When I signed my apartment lease,  I laughed when I read the contract addendum about my responsibilities for managing mold.  The best advice the property manager could give me was to air out my unit frequently and keep my stuff away from the walls. 

I hope you know by now, based on the discussion above, how inadequate this advice is!  There's a lot more out there.  My hope is that you have a much much better understanding of the science of moisture to make informed decisions for moisture control and management.

As a tenant, there are better things you can do, but you must work with management to take care of the the things you can't control.  The constructive eviction you avoid may be your own!

If you're a homeowner, property manager, or landlord, you can do better too.  Most of them are under your control anyway.  This website and our consulting services are here to help you make sane, cost-effective decisions for controlling moisture in your homes and properties.

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