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Humidity and Mold in Home

Humidity causes mold to grow on windows and other places in the home.

Humidity and mold growth on a window.

Is your mold growth due to humidity problems?

The first step in any mold investigation is to determine the source of the mold.  The most important distinction is whether it’s due to humidity or a liquid water intrusion such as a leak.  Let’s look at some examples of mold due to humidity.

Mold behind bedding or couches

This is caused by a combination of poorly insulated exterior walls and excess indoor humidity.  How to keep it from returning?  The short term solution is to run the bathroom exhaust fan 24/7 and pull back the bedding from the wall, leaving a 3-4″ air gap.  Long term, replace the bathroom fan with a constant flow unit such as the Panasonic WhisperGreen.

Mold on wall behind bed 2

Growth in the exterior corners

Air flow patterns in your home do not make 90 degree turns.  Instead, they make slow, broad curves.  This leaves a dead spot of cold air in the corners.  Over time, these cooler temperatures lead to condensation and mold growth.  Thankfully, this mold growth does not affect the inside of the wall cavity.  The insulation and framing will not require cleaning.

Mold in corner from humidity

A moisture meter will typically not pick up excessive moisture when the mold is caused by condensation.  The condensation occurs only on a very thin layer of the material.  While this is sufficient to cause mold growth, it typically will produce a normal reading on a moisture meter.

Mold in corner

Ceiling / exterior wall juncture

As you’ve probably noted by now, humidity causes cold spots on cool areas of the home.  The area where the ceiling meets the exterior walls is a prime candidate for condensation based mold.  This occurs because cold, outside air is drawn into your attic through the soffit vents.  These vents are very close to the ceiling and often cause the surface temperature to drop.  Also, in many cases, the insulation has been pulled back from the edge of the ceiling to allow the ensure the soffits vents are not blocked.  This is important for preventing attic mold, but can also lead to cold spots (and mold growth) on the ceiling.

Mold on ceiling near wall

Mold on furniture and contents

In extreme cases, mold will begin growing on furniture, clothing and other contents within the room.  Growth on the backside of furniture pressed up against an exterior wall is the first point of growth.  The limited airflow and cool exterior wall create a microclimate on the backside of the furniture.

Mold on dresser from humidity

Once mold begins growing on random contents in the rest of the room, you have a major humidity problem on your hands.  RH of 70%+ is necessary for this type of growth to occur.  This type of mold growth also requires the most labor intensive cleaning techniques.

Condensation on window frames and sill

Condensation on the windows is nearly always the first indication of a humidity problem.  This is especially true if you have modern, double pane vinyl or wood windows.  Condensation on single pane aluminum windows is fairly common, due to the thermal conductivity of the metal. If you’re seeing condensation on a vinyl window, the humidity in the home is far too high.

Condensation and mold growth on window

What is Relative Humidity?

Relative humidity (RH) is the actual amount of moisture in the air, relative to the maximum amount of moisture the air can hold.  For example, if the air in your home is 50% RH, it is half full.  You could double the amount of water vapor before condensation would occur.  Relative humidity is directly influenced by temperature.  If you take a given sample of air and heat it, the RH will go down.  The opposite is true as well.  If you cool the air, the RH will increase.  This is because warmer air can hold more moisture.

Picture a cup half full of water.  This represents a house at 50% relative humidity.  The size of the cup represents the air and the water level represents the humidity.  Let’s say you increase the temperature in your home by 10 degrees.  When you heat the air, it expands.   I.e. the size of the cup expands.  But heat does not increase the amount of moisture.  It stays the same, regardless of the temperature.  So as the size of the cup grows, the amount of water stays the same.  Now the water only fills 1/3 of the cup.  The relative humidity is now 33%.

What happens to humidity when you don’t sufficiently heat your home?

The graphs below represent the dramatic change that occurs when you lower the temperature.  The left side graph represents a home kept at 68F.  If the air is cooled down to 58F (i.e. an unheated basement), the RH sky rockets up to 77%.  At this level, mold growth will occur on clothing, furniture, etc.  If it’s an older home, you’ll almost certainly see mold growth on the exterior walls.

Humidity and mold growth chart.

The solution?  Bring in fresh outside air (even if it’s raining)

It’s counterintuitive, but humidity problems can often be solved by bringing in fresh outside air.   The graph on the left represents a typical winter day in the Pacific Northwest.  About 40F and 100% saturated air.  Whether it’s raining or merely cloudy, the RH in the winter is often very high.  At first glance, it doesn’t make sense to bring this cold, wet air into your home.  Won’t it make the humidity problem even worse?

After the 40F air enters your home, it is heated by our furnace (to 68F in this scenario).   As the temperature of the air rises, the relative humidity drops.  The drop is dramatic – from 100% down to 35%.  This is very dry air and mold growth is extremely unlikely to occur.

Humidity and mold growth chart.

Relative humidity and seasonal changes

It’s important to know when you should monitor the relative humidity in your home.  In climates with mild, dry summers such as the Rockies and West Coast, summer RH readings should be taken with a grain of salt.  There simply isn’t enough humidity in the air to cause problems.  Unless the RH is extremely high, the readings should not be considered meaningful.  This is because it is extremely unlikely any surface in a home will reach the dew point without cool outside temperatures.  Additionally, people are far more likely to open windows during the warmer months, which greatly reduces the buildup of humidity.

The opposite is true in regions with humid summers, such as the east coast and south.  The warm, humid air can condense on surfaces cooled by the air conditioning unit.

Relationship between CO2 and RH

During our inspections we routinely measure the levels of CO2 in the air.  This might be a surprising, considering CO2 is completely harmless to humans.  Why bother with the measurements?  We collect CO2 readings to get a sense of how well a house breathes.   When a person exhales, the breathe out CO2.   Therefore, if the CO2 levels are high within a home, we can make a rough determination that the house has insufficient ventilation.  This is possible because CO2 levels are consistent in the outside air (though they are steadily increasing each year).

This type of measurement must be taken with a few caveats.  For example, if the house is unoccupied at the time of the inspection, the data is worthless. Carbon dioxide meters can also drift quite a bit and must be calibrated often.

Extreme examples of humidity based mold growth

Mold growing on ceiling from condensation.

Above is an example of extremely high levels of condensation and the subsequent mold growth.  Notice the mold growth is lighter in the areas where the framing sits above the sheetrock.  This is likely due to a lack of insulation in the attic above, causing cold spots in between the framing.

Humidity issues or a leak through the concrete slab?

High humidity in bedroom.

The picture below is from a bedroom in a slab on grade home (a concrete floor at the same level as the outside soil/grade).  Highly elevated levels of moisture were found in the carpet, pad and tack strip.  The client had recently received a bid for $16,000 to seal cracks in the foundation and eliminate the water leakage.  Environix was hired to inspect the related mold growth.

Mold growth on carpet from high humidity.

At first glance this appears like a straightforward case – it is certainly not uncommon for water to travel through cracks in the concrete slab.  However, something very unusual was found in the home.  Notice the readings on the RH meter (yellow).   The home had 100% humidity readings at 62F.  This is extremely humid for the local climate (Western Washington).  Notice also the readings on the CO2 meter (blue).  A typical home is around 500-700 and is considered problematic around 1100.   This home registered readings of nearly 1,900.

Humidity and CO2 readings in moldy room.

Conclusion?  The home suffered from extremely poor ventilation.  During a recent remodel the windows, doors and ceiling were sealed, but the ventilation was not addressed.  Over time this high humidity caused condensation on the carpet and pad.  While this is an unusual case, it highlights the importance of verifying the cause of the moisture damage before spending money on a solution.  The customer cancelled the concrete sealing and installed a ventilation fan, saving her around $15,500.

Reports from recent humidity & mold inspections

The following report is from a recent inspection we performed of a house suffering from humidity based mold growth.  This occurs when the moisture in the ambient air is sufficiently high to cause mold growth on interior surfaces.  No actual flooding or moisture intrusion occurred.  This type of mold growth often leads to elevated levels of airborne mold spores and indoor air quality problems.   While uncommon in hot/dry climates such as the southwest, it is a common problem in our neck of the woods (Seattle and Bellevue).



Mold growth on living room wall and contents next to wall


Normal moisture content in living room wall materials.


Humidity and CO2 levels are elevated indicating poor ventilation.


Closet full of contents experiencing mold issues.


Slightly elevated moisture in bedroom walls where condensation is severe.

Project Report > Inspection for a Mold caused by Humidity

Reason for inspection:

Mold growth from excess humidity

The client requested an inspection of mold growth in bathroom, entryway, and kitchen.

Summary of concerns:

Ventilation deficiencies in the home have led to extreme condensation and subsequent damage.


Adequate Ventilation Not Present

  • Recommended ventilation of residential homes should be between 0.35 and 0.7 air changes per hour (ACH).
  • Desired Ventilation Rate:  0.35 ACH (Air Changes per Hour)

Mold on wall due to humid conditions

Dining Room:

Humidity causing mold growth on door trim


  • Significant surface mold growth noted on exterior walls.
  • Severe condensation noted on windows.
  • Baseboards along exterior walls are wet, swollen, warped, and deformed due to excessive condensation.
  • Vinyl flooring below window shows increased moisture content due to pooling condensation.
  • HVAC boot is extremely soiled: active condensation, significant mold growth, heavy corrosion, and food debris was visible in ducting.
  • RH = 69%   TEMP: 60.2F   CO2 = 883 ppm   CO = 0 ppm


Excess moisture from humidity


  • Increased moisture content noted in sheetrock along exterior wall.
  • Visible mold staining was noted along exterior wall.
  • Apparent recent cleaning of mold growth on walls.
  • RH = 67.2%   TEMP: 59.7F   CO2 = 863 ppm   CO = 0 ppm

Project Report > Inspection for Ventilation and Mold Issues

Reason for inspection:

Moisture + Mold at Window

Summary of concerns:

  • Improper ventilation inside the apartment is allowing for excessive build-up of relative humidity and CO².
  • Excessive relative humidity levels inside the apartment are leading to condensation on exterior walls and windows.  This condensation provides conditions conducive to mold growth.
  • Mild to moderate amounts of mold growth found on attic sheathing.


  • Recommended ventilation of residential homes should be between 0.35 and 0.7 air changes per hour (ACH).

Inadequate Ventilation


  • Replace existing fans in both hall bathroom and laundry room areas.  This will assist in removing relative humidity as it is being generated as well as deal allow for additional air exchanges throughout the day when the home is unoccupied.

Crushed Vent - Inadequate Ventilation

  • Recommend connecting exhaust ventilation fan to exterior of home.

North Bedroom

  • Minor stains found on back wall of closet where visible mold growth was previously found growing.
  • Small amounts of mold growth were still found to be growing in the back corners of the closet.  This is due to high relative humidity levels and poorly insulated exterior walls.


  • Recommend keeping closet contents a minimum of 2 inches away from exterior corner and back walls when possible.  This will allow for additional ventilation
  • A small desiccant – type dehumidifier can be installed in the closet to help remove excessive moisture before it condenses on the walls.

Project Report > Inspection for Heightened Humidity Levels


  • The home owners have deactivated the electric forced-air heating system; all ducting has been sealed at the register.
  • The home does not possess an adequate level of ventilation to remove occupant generated moisture and miscellaneous building pollutants such as VOC’s from cleaners, fragrances, cosmetics, and building materials.
  • The home is approximately 1600 square feet, and requires approximately 84-168CFM constant ventilation to achieve the recommended air exchange rate of 0.35-0.7 air changes per hour.


  • Adequate air exchange rate must be established by one of the options below:
  • Enable furnace, clean ducting, and install a fresh-air intake or HRV sufficient to achieve the recommended rate of ventilation above.
  • If the furnace is not to be used, a separate ducted HRV may be installed to achieve the recommended rate of ventilation.
  • Exhaust fans may be installed in both bathrooms to run at a constant level to achieve the recommended ventilation rate.  The suggested model would be the Panasonic Whisper Green 80CFM units set to run at 60CFM constant-flow and for 60 minutes at 80CFM once the motion-sensor has been activated.

Hall Bathroom:

  • No bathroom exhaust fan currently installed in this area.
  • This bathroom not often used for bathing.
  • History of leakage from the roof in this area, but no obvious water damage or mold growth was noted.  Residual contamination may exist in the wall cavity, but it is unlikely to present a negative impact on the indoor air quality.

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First of all, thanks for this great site! I'd be calling you to come to my house if we didn't live on the opposite coast! I am feeling like a real dummy now. I was very proud of myself for not running our air conditioning all summer. But now I am seeing mold on our furniture. I was cleaning the furniture with a vinegar solution, but I think from what I read here I need to use something stronger. My main question for you is whether I need to be concerned with the interior of the walls, carpeting, drapes, and so forth. In addition to cleaning the furniture, do I need to arrange for some kind of mold remediation in my house? I am sure this was just from humidity. There was no water intrusion that would have affected the furniture. Just open windows and a very humid summer.

Mold growing on furniture from high humidity.

You’re on the right track in regards to running the air conditioner.  They serve a dual purpose of both lowering the temperature and reducing the humidity.

Regarding the furniture, any regular cleaning agent/solution will work.  You’re not trying to kill the mold per se, but rather physically remove the growth.   Porous surfaces such as upholstered furniture should be cleaned with a vacuum.  If you don’t own a true HEPA rated vacuum, bring the cushions/furniture outside to perform the vacuuming.

Professional remediation for systemic, whole house issues can get very costly.  It wouldn’t hurt to get a bid, but prepare yourself for a high price.  If the mold growth is minor/moderate, you can likely tackle the project yourself.


What does 'HRV" refer to in the above article?

HRV stands for Heat Recovery Ventilator.  These devices bring fresh air into the home while recovering the energy from the outgoing air.  The outbound and inbound air pass through a heat exchanger.  The heat from the stale, indoor air is transferred to the fresh, but cold outside air.

In cold climates they can make a large difference in your energy bill.  In more moderate climates, a bathroom exhaust fan can work well.