How water condensation can lead to black mold growth

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condensation can cause mold growth

There’s a common misconception that mold growth in the home or office can only happen as a result of water damage, such as a broken pipe, flooding from weather, and so on.  This is simply not true.  While there is no doubt that water damage of this magnitude often leads to mold and other problems, it is usually fairly obvious when such an event has occurred.  Far more insidious and covert is the action of water condensation.  It can, very easily, lead to mold growth issues, often without the residents of a home or workers in an office being any the wiser.  It’s also much more common than flooding and broken plumbing.

Therefore, it’s worthwhile to understand how water condensation can lead to mold, how and why mold grows and spreads inside buildings, and the kinds of steps that you can take to reduce condensation and make your home or office less hospitable to mold.  Understanding the science behind condensation and mold growth, and how the environment varies geographically to make condensation more or less likely, is also critical.  We’ll cover all of that and more in this guide – and even offer up some advice on how to deal with a suspected mold problem.

The Three Ingredients Needed for Mold to Take Hold

First, it’s useful to understand the basic three ingredients that are necessary for mold to grow and spread.  Much like the famed fire triangle is required in order for a fire to start – requiring oxygen, a fuel, and an ignition source – a similar triangle of elements is necessary in order for mold to take hold.  Mold spores are all around us, in the air outdoors and indoors.  But they don’t grow into colonies everywhere and on everything.  Certain conditions must be met in order for these spores to grow into colonies.  Otherwise, they will continue to float around in the air, largely harmless and invisible.

The first element of the mold triangle is food.  Like any living thing, mold spores need food in order to reproduce and grow.  Most forms of mold will grow on cellulose-based surfaces indoors and use these surfaces for their food source.  Examples include anything made of plant materials, including plywood, framing, drywall, cabinetry, flooring, carpeting, underpadding, and similar.  Some molds can also get nutrients from other materials, including some synthetic materials, or even grow on the dust or dirt layers found on concrete flooring, for instance.  Since these sources are readily found in almost every home to some degree or another, the first element is usually easily satisfied.

Secondly, the temperature and airflow need to be just right for mold to grow.  Significant airflow makes it difficult for spores to “put down roots”, so to speak, and remain undisturbed for the 24 hours to 10 days needed in order to grow.  It also results in fluctuating temperatures much more readily than in static or stagnant air, making the environment less hospitable for mold growth.  Most species of mold like average to just above average room temperatures, on the order of 70 to 80 degrees Fahrenheit, in order to proliferate.  Areas of a building that are significantly warmer or colder can still allow growth, with many species capable of reproducing in temperatures between 40 and 100° F, but it is typically slower and somewhat inhibited as compared to this “butter zone” of temperatures.

Finally, and most relevant for this guide, the third prong of the mold triangle is moisture.  Like most living things, mold needs moisture to survive.  Moist environments are also most suitable for mold growth, as it helps promote the decay of plant matter, which is part of how molds digest nutrients and reproduce.  This moisture can be in the form of liquid water, such as from a leaky pipe or fixture, seeping into a basement from surrounding soil or from overflowing gutters, or many other sources.  It can also be found in the form of humidity in the air, which then condenses into liquid water when the conditions (primarily temperature) are right. 

How Condensation Forms

Since moisture is found readily in the air all around us, and most people don’t experience massive plumbing leaks or floods in their homes every day, condensation makes up the lion’s share of the moisture in a home that can lead to mold growth.  In order to understand where condensation comes from, and where it might accumulate, it’s useful to review some basic science about what condensation is, and how it forms.

Most people have experienced the phenomenon of condensation occurring on an ice cold glass of their favorite beverage.  The outside of the glass will get wet as it sits out in warm air, accumulating drops of water on the sides, and possibly making a little puddle underneath.  This perfectly illustrates the process and mechanisms behind condensation.  That water is coming out of the air, condensing on the chilled glass surface, as it moves from the gaseous phase of water vapor into liquid water.

The reason this process occurs is because of the cold temperature of the glass and liquid in it.  The air can only hold a certain amount of water vapor.  That amount goes up as temperature increases.  When more water vapor is held in the air than its temperature can currently support, it precipitates out as rain.  This won’t happen in your house, of course.  But air with a high humidity – high levels of water vapor in it – that is exposed to colder temperatures like those on the surface of your drinking glass will result in that water being released via condensation.  The air temperature decreases at the surface of the glass, and it can no longer hold as much water, allowing it to condense out as a liquid on the glass surface.

This same process can take place anywhere – not just on your chilled glass.  The temperature differences don’t need to be as extreme as an icy beverage either.  Cooler areas – the insides of cabinets, closets, inside drawers, windows/window frames and sills, basements, and much more – can vary by as little as a few degrees from the ambient air temperature and still generate condensation.  Anywhere there is a temperature gradient, and sufficiently moisture-saturated air/high enough humidity, condensation can form.

Humidity Levels Outdoors and Indoors

Naturally, it follows from the above logic that to have condensation, and therefore water to allow mold to grow, you need water vapor in the air.  The level of water vapor in the air is the humidity of the air, often reported on your favorite weather site or app.  This can vary wildly between outdoors and indoors, and geographically.  In southern California, we have many areas with unique microclimates due to mountains, sea breezes, and so forth.  Often, while the general humidity across the region may be only 20%, certain areas – especially coastal cities with a strong onshore flow – may see 60% humidity or higher outdoors.

Indoors tend to be a bit drier, even if the outdoor humidity is high.  Heating and/or air conditioning often change the humidity levels indoors.  Likewise, if your home or office HVAC system has a humidifier or dehumidifier, depending on the settings and other factors, the indoor humidity can be significantly different than that outdoors.  In general, though, if the humidity outdoors is high, all things being equal, the humidity indoors will be high, or at least higher than normal.  As this article is being written, the outdoor humidity is 68%, while indoor humidity with air conditioning active is 47%.  Your home or office may vary considerably from this example.

Higher levels of humidity, whether indoors or outdoors, mean a greater chance for condensation to develop.  Lower humidity levels mean the air is not carrying as much moisture, and therefore typically needs a greater temperature gradient to condense significant moisture.  Most molds really thrive in humidity levels of 50% or higher indoors, though, as with temperature, this is not an absolute hard and fast rule.  Even relatively dry air can still condense some moisture, and when you’re talking the nearly microscopic scale of mold, often “some” moisture is enough for at least some kind of growth and spread to take place.

High Humidity Areas in the Home

There are also sources of humidity in the home, which can create areas of isolated higher humidity with greater potential for condensation.  Some of the most common high humidity areas (where the higher humidity may be permanent or transient depending on the cause and human activity) include:

  • Bathrooms
    • Running a hot bath or shower allows a lot of steam – water vapor – to saturate the air.  That can then become stagnant in the bathroom, or circulate to nearby rooms and areas depending on the ventilation setup in your home.
  • Kitchens
    • Cooking a lot of boiling things, running a dishwasher, and any other applications with boiling or hot water will contribute to increases in humidity.
  • Laundry Rooms
    • While laundering clothes directly should contain any moisture within the washing machine, remaining moisture after washing is complete can still evaporate into the air.
    • Drying wet clothes – either air drying or via a dryer – can put more moisture into the air.  Proper venting helps reduce this problem.
  • Basements
    • Basements are notorious for having higher humidity than other areas of the house.  The combination of poor or limited air flow, typically unfinished surfaces, and groundwater seepage, among other factors, typically see basement areas at least 10% higher in humidity than other areas of a home or office.

In addition to the higher baseline humidity in certain areas of the home, many areas naturally promote condensation.  This is typically due to lower temperatures, materials that cool faster, and/or poor air circulation.  Some of the most common places that water condenses, and therefore common sites for mold to develop, include:

  • Bathrooms
    • Under flooring tiles or fixtures, in cabinets or closets, drawers, and behind walls/drywall.
  • Windows
    • Window frames and sills are often made from metal, which cools faster than drywall or wood, promoting condensation. 
    • Window panes themselves will often exhibit condensation if the vacuum between the panes has been compromised with age.  Losing this insulation, temperature gradients allow for direct condensation (for example, see how your car windows condense, as they are not insulated).
  • Closets, Cabinets, and Drawers
    • Any dark, cool place, especially where the air flow is poor, is ideal for mold to set up shop.  These locations have the additional advantage of naturally promoting condensation as they typically remain a lower temperature than outer walls or lived-in areas of the building.
  • Basements, Crawl Spaces, Attics
    • Attics are less of a concern in summer months, but may promote condensation and mold growth during wetter fall and winter months.
    • Basements and crawl spaces serve as natural chillers due to their location, typically underground, and the physics of air, where warmer air rises and colder air sinks.  Combined with higher humidity in these locations, they heavily promote condensation and allow for mold growth.

However, it is important to remember that almost any area of a building can develop condensation or a mold problem, and it is not isolated to just these higher-risk areas.  So much depends on the microclimates in each room or area of the home, temperature, humidity, airflow, and so on – as well as the level of mold spores naturally in the air at any given time or location, among other factors.  This is more of a “greatest hits” list of high humidity, high condensation areas that are likely to show the signs of a mold problem first in the average home or office.

mold on insulation in my home

Insulation and the Effect on Condensation and Mold Growth

Another factor that many people overlook as to areas where condensation and/or mold growth may occur is insulation.  Insulation is important for managing your energy bills, as well as the internal temperature, humidity level, and other climate-related factors of your home or office.  Newer buildings tend to be insulated, or better insulated, than older buildings.  These differences create different condensing points for moisture, and can have a direct impact on humidity differences (including how closely the indoor humidity aligns with the outdoor humidity). 

Lack of insulation, or poor insulation, tends to make condensation more likely, especially inside of walls, cabinets, closets, and drawers.  This is especially true in areas along the coast that already have a high ambient humidity, since more of that humidity tends to transfer inside the building than in one that is well-insulated. 

Conversely, newer windows and better insulation can also be a source of condensation.  Very well insulated areas of a home can form a kind of bottleneck for moisture and humidity, especially if ventilation is poor.  When humidity is higher inside than outside, and there is little airflow out of the home, this allows humidity to linger, condense into water droplets, and promotes mold growth.

Different types of insulation provide different levels of thermal and moisture protection, and the way in which they are distributed and applied in the home can play a big role in condensation, temperature, and air flow.  Blown insulation, roll insulation, and foam based products all provide different insulation levels and thermal gradients.  At the same time, voids in insulation, either intentional or due to poor installation can create cool and hot spots, which are ripe for moisture to condense and mold to develop.

All of this is by way of saying that there are many factors that come into play in determining humidity, condensation, and the potential for mold growth in one area of a home or office vs. the next, as well as in comparison between one building and another.  But, harkening back to the mold triangle we talked about in the beginning, removing one or more of the three primary factors needed for mold to grow – food, temperature/stagnant air, and moisture – can dramatically reduce the likelihood of developing a mold problem.  Understanding the interplay of these factors is key to any lifestyle or habit changes you may undertake to reduce your risk of mold growth, and at the core of any professional services you might recruit to renovate or remediate any mold growth that does occur.

The Importance of Ventilation

While it is unlikely that you can radically alter the temperature in your home or office and still be comfortable, it is possible to reduce the likelihood of condensation or moisture buildup, as well as mold growth, with proper ventilation.  As mentioned earlier, stagnant air is ideal for allowing mold to grow.  Tiny spores – nearly microscopic – take time to put down their root-like structures and start to grow.  Well-moving air makes it harder for them to do this, and thus decreases the likelihood of a mold problem developing.

At the same time, air that is circulating sufficiently will help regulate humidity levels.  Even without a humidifier or dehumidifier, flowing air will help to even out or normalize humidity within a home or office building.  It will help reduce bottlenecks or areas where temperature gradients may occur, thus reducing the potential for condensation. 

Where high moisture content does exist, such as in those problem areas like kitchens and bathrooms, ventilating during periods of high water vapor production is key.  For example, simple actions like opening a window after a shower for a short period of time can immediately cut the humidity level and the potential for condensation or longer-term mold growth. 

A quality HVAC system, well-designed for the building, which is kept in good working order, is one of the easiest ways to improve humidity, condensation points, and air flow.  Coupled with a HEPA filter, humidifier, or dehumidifier associated with the unit, you can achieve good air circulation, filtering, and better balance and manage the moisture levels in your home or office automatically.  While it won’t stop all condensation or mold problems, it can go a long way towards cutting down on potential problems.

Preventative Steps and Behaviors

The best way to deal with water condensation or mold is to stop it before it starts.  While this is easier said than done, there are a number of preventative steps and basic behaviors that can be adopted.  For the average person and average building, these can go a long way towards reducing the likelihood of developing significant condensation or mold growth problems.  Some are entirely free, and simply require a little attention or change to your routine, whereas others are definitely a bit more of an investment.

  • Always use built-in exhaust fans in the bathroom and kitchen when producing large amounts of water vapor (such as during a shower, bath, or boiling water/liquids on the stove).
  • Open windows in high humidity areas, such as the bathroom after a hot shower, for at least 10-15 minutes to allow airflow and humidity to stabilize.
  • Periodically, shut off air conditioning or heating, and open the windows for an hour or so to allow fresh air into the home or office.  It’s best to do this when it’s relatively dry, rather than on an extremely humid day.
  • Use ceiling fans and/or portable fans to keep air moving regularly.
  • Ensure your HVAC system is sized appropriately for the home or office it supports, so that enough air is re-circulated and treated.
  • Get your HVAC system regularly checked and maintained to ensure it is in proper working order.
  • Consider a HEPA filter for your HVAC system to reduce airborne particulates.
  • Even without a HEPA filter, make sure the filter you do use in your HVAC system is cleaned or replaced regularly, consistent with manufacturer recommendations.
  • Consider getting a humidifier or dehumidifier, or both, depending on the humidity trends in your home or office.  Built-in units linked to the HVAC system are readily available, as are portable units that can be used in trouble areas like basements or bathrooms, closets, and so on.

What to Do If You See Condensation, Smell Mildew, or Suspect Mold

Even with good preventative behaviors, it is still possible that condensation may develop in one or more places within the home or office.  You may have mold start to grow, which you may be able to see visually – or not.  Many times, the same areas where condensation occurs and where mold will thrive is not an area you see frequently – or may even be hidden, such as behind walls or under floors.  Some mold and mildew have a sickly sweet or foul odor, whereas others are completely odorless.  You should not rely on any of these factors as the ultimate arbiter of whether or not you have a condensation or mold problem.

If you do happen to spot condensation – especially repeatedly in the same area – and all of the preventative steps listed above don’t help, it may be time to get a consultation from an expert.  Insulation issues should be addressed with a qualified insulation expert.  Ventilation issues should be addressed with a qualified HVAC technician.  If you can’t tell what the source of the problem is, or the exact nature of it – then consider a home inspection from a qualified mold testing and inspections firm.

If you do spot mold or smell mildew in the home, then it’s definitely time to get a mold inspection.  Experts can take swabs and tests to provide insight into the kinds of molds growing in the home or office.  They can take readings of humidity and airflow in various areas, and offer advice on how to prevent future growth.  They can also typically help provide guidance for cleaning or, in more serious cases, professional remediation efforts.

The worst thing to do is to ignore a condensation or mold problem, as it will tend to grow and spread the longer it is allowed to fester.  Therefore, taking prompt action when you first notice any signs of condensation, excessive moisture, mold, or mildew is the most cost-effective way to treat the problem and solve it for good. 

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