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Humidity - the hidden cause of most indoor mold problems

Humidity and mold growth on a window.

What is relative humidity (and why should I care)?

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.

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


  • Full inspection will be necessary to determine the most appropriate method to provide the recommended ventilation.  This may include upgrades to existing exhaust fans, installation of a fresh-air intake system, or installation of a heat recovery ventilation system.

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

Hall Bathroom:


  • Surface mold growth noted on ceiling and door trim, due to condensation.
  • Heavy condensation noted on toilet tank due to inadequate ventilation.
  • Bathroom fan rattling, and likely to fail.
  • Increased moisture content noted in large areas of vinyl flooring.
  • Increased moisture content noted in baseboards between tub and door, as well as sheetrock immediately behind the toilet.
  • Significant visible mold growth noted on baseboards between tub and door.
  • RH = 70.2%   TEMP: 59.7F   CO2 = 846 ppm   CO = 0 ppm

Project Report > Inspection for Ventilation and Mold Issues

Inspection #: 201050

Reason for inspection:

Moisture + Mold at Window

Apartment that client lives in has been found to have visible mold growth in numerous areas.  Landlord claims to have re-mediated the areas but client feels that it is still a problem.  Requested a full inspection with indoor air quality samples.

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.
  • Second story home located in a 4-plex
  • Two bedroom, 650 sq ft,
  • Top floor of a two story home.


  • Adequate Ventilation Not Present
  • Recommended ventilation of residential homes should be between 0.35 and 0.7 air changes per hour (ACH).

Inadequate Ventilation


  • Recommend replacing 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


  • VOC Reading: 706 ppb
  • Recommend connecting exhaust ventilation fan to exterior of home.
  • Recommend regular cleaning of windows of any visible mold growth and/or condensation that may form.  Mold can be cleaned using a damp rag and a mild solution of soap and water or vinegar.

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.
  • RH:66 % Temp: 59°F CO2: 1092 ppm CO: 0 ppm
  • VOC Reading: 828 ppb


  • 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.

South Bedroom

  • Room was generally clean but cluttered.
  • Visible mold growth noted in upper left hand corner of closet.
  • Visible mold growth was noted on shoes in bottom left hand corner of closet.
  • Visible mold growth noted along bottom edge of window.
  • Visible condensation present on inside window pane.
  • RH:64 %  Temp: 61°F  CO2: 1117 ppm  CO: 0 ppm
  • VOC Reading: 630 ppb
  • Moldy contents should be removed from closet and cleaned with a damp rag in an open air environment.  Any contents with remaining staining may require disposal.

Hall Bathroom

  • Ventilation fan is present but is noisy and inefficient at removing relative humidity from inside the room.
  • RH:65 % Temp: 60°F CO2: 1100 ppm CO: 0 ppm
  • VOC Reading: 808 ppb


  • Insulation is present in attic space but appears to have a thin layer of mold spores covering the surface along the north side of the attic.
  • No air sealing was present in attic space
  • Ridge venting was present but was limited to 2 cut-in ridge vents.
  • Soffit venting was present and appeared adequate.
  • Visible mold growth was present on a large section of north facing roof sheathing.
  • Bottom and top rows of plywood had been replaced on the north side of the attic.  Little to no mold growth was present on this plywood indicating that the ventilation issues and mold growth may be from a previous roof.
  • Recommend conducting a two-stage chemical remediation to the attic sheathing.  This will kill all existing mold and cover any residual stains.

Project Report > Inspection for Heightened Humidity Levels

Reason for inspection:

  • The client has requested a detailed indoor air quality inspection to determine potential causes of an unexplained illness.
Summary of concerns:
  • Inadequate ventilation in the home has led to high levels of relative humidity, carbon dioxide, and VOC’s.
  • History of several leaks that had caused water damage, these leaks have since been resolved by the home owner.


  • 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.
  • Average tVOC level 323ppb
  • RH:44.9% Temp:63.4°F CO2:984ppm CO: 0ppm


  • This area appears to be adequately ventilated to the exterior.
  • Average levels of stored cleaners, chemicals, and pesticides were noted.
  • 2 large drip pans with automotive oil were noted on the floor of the garage.
  • Average tVOC level 8ppb
  • History of several leaks in this area, but water staining and mold growth is unlikely to be affecting indoor air quality.

Project Report > Inspection for Mold due to Water Intrusion


  • 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:  0.5 ACH (Air Changes per Hour)

Master Bedroom:


  • Sheetrock on ceiling is damaged at tape seams.
  • Increased moisture content was noted in heavily deformed portions of the ceiling.
  • Condensation noted on windows.
  • Inspection with FLIR revealed temperature differentials consistent with condensation in the ceiling cavity.
  • Wet carpeting was noted near the exterior corner of the bedroom.

Laundry Room:


  • Water intrusion on both sides of exterior door was noted.
  • Mold growth noted on sheetrock at either side of exterior door.
  • Increased moisture content noted in affected sections of sheetrock.



  • Bathroom exhaust fans not connected adequately to dedicated roof jacks.
  • HVAC ducting in the attic is not adequately sealed or insulated.
  • Many large open areas were noted that may allow warm humid air from the interior to enter into the attic.


  • Hire licensed roofing contractor to install appropriate dedicated roof jacks with 4” collars for all exhaust fans.

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