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Humidity - Sources and Solutions

June 15, 2019 | Blog


High humidity in homes can cause discomfort, make you run your air conditioning at a lower temperature and promote mold growth. This is a complex problem that requires multiple considerations.

Sources of Humidity

Humidity Generated Within the Home. Humidity is generated inside a home every day by breathing, bathing, cooking, watering plants, pet water dishes, etc.

Negative Pressure. Humidity is present in homes due to negative pressure inside the home. Negative pressure is caused by running bathroom fans, drying clothes, indoor gas water heaters and gas furnaces, kitchen hood fans, attic fans, and leaking ducts in attics and crawl spaces.

Fans – any fan that removes air from inside the home, including clothes dryers, will cause negative pressure. In the case of bathroom fans, hood fans and clothes dryers the fans typically remove more humidity than they create but they still create negative pressure which will bring in additional humidity through leaks in doors, windows and wall penetrations for electrical and plumbing fixtures.

Gas Water Heaters and Furnaces – Gas water heaters and furnaces require combustion air to operate. This combustion air is drawn from inside the home and passively vented outside creating negative pressure. New homes have combustion air ducts in furnace closets to provide air to the furnace to offset the air being vented. Most older homes do not.

Leaky Ducts – The average duct systems in homes built before 2010 leak between 15 – 25%. If these ducts are in attics and crawl spaces the air that is leaked is not available to be returned to the air conditioning system inside. This creates severe negative pressure in the home every time the air conditioner or heater fan is running. New regulations require a maximum duct leakage of 14%. A truly “tight” duct system will still leak 5% or more.

Wind Load. Homes with leaky doors and windows will allow air to infiltrate the home on the windy side and “exhaust” it through other doors, windows and wall penetrations. This creates a natural flow of potentially humid air through the home.

Remedies for Humidity

There are a number of things a homeowner can do to solve humidity problems, the following solutions should be considered:

Offsetting Humidity Generated In The Home. In perfectly sealed home with neutral or positive pressure there will still be humidity to be removed. Consider the following:

Air Conditioning – An air conditioner removes some humidity from the air as a part of normal operation. Modern air conditioning has been engineered to remove additional humidity by running longer at lower capacity using two-stage and variable speed condensers. By itself, any air conditioner will not remove all excess humidity because it is not 100% effective and creates negative pressure in the home anytime it is running. It does not remove humidity when it is not running.

System Size – An oversized air conditioner will not remove as much humidity as a properly sized one. Next time you have to replace your system ask the contractor to perform a Manual J heat load to confirm proper system sizing.

Whole House Dehumidification – Unlike air conditioning systems, whole house dehumidifiers operate anytime the humidity level is above the desired level in the home. Dehumidifiers are used to supplement air conditioning systems in warm weather and to reduce humidity in spring and fall when humidity levels are high and outdoor temperatures are moderate.

Overcoming Negative Pressure. In order to neutralize or create a slight positive pressure in a home the following steps may be undertaken:

Seal Ductwork. Seal ductwork in unairconditioned spaces such as attics and crawl spaces. This will reduce a substantial source of negative pressure.

Add Fresh Air. Adding a fresh air duct to the air conditioning return air will replace air lost through ventilation fans, gas appliances and duct leakage. The air brought in is processed through the air conditioner’s evaporator coil and distributed throughout the home.

Limit Use of Fans. Kitchen hoods and bathroom fans should run no longer than 20 minutes after cooking or bathing. Use of a timer switch on these devices will help minimize the negative pressure they create.

Overcoming Wind Load. To keep wind from pushing humid air into the home take the following steps:

Doors and Windows. Seal doors and windows with weather stripping.


Electrical Fixtures. Seal electrical fixtures placing “socket sealers” behind faceplates on outside walls.

Plumbing Penetrations. Seal plumbing penetrations with expandable foam.

Southern Air Heating, Cooling & Plumbing can help you evaluate humidity problems in your home and assist you in formulating a plan to reduce it. Call us at (504)469-4232 or visit us on the web to schedule an evaluation.

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