What Causes High Humidity in a House?
Your house is humid because moisture is entering faster than your air conditioner can remove it. Every home takes on moisture through windows and doors, crawl spaces, leaky ductwork, and everyday activities like showering and cooking. Your AC removes moisture as a side effect of cooling. When the amount coming in outpaces what the system can pull out, indoor humidity climbs above the 55% comfort threshold and your home starts to feel damp, sticky, and warmer than the thermostat says.
That’s why indoor humidity can read higher than the humidity outside. If your AC is oversized, it cools the air quickly and shuts off before it has run long enough to dehumidify, leaving the moisture behind. Short run times, a clogged condensate drain, or duct leaks pulling in humid crawl-space air will all do the same thing.
Where Does the Humidity in My House Come From?
An easy way to understand why humidity is in your home and how to get it out is to compare it to a boat. If you have a boat with some leaks, then you may consider installing a water pump to push out some of the water, right? Well, if your boat only has a few leaks, the water pump should be able to handle getting all of the water out. However, if your boat has some big leaks, it may be more than the water pump can handle, and you will probably need to repair the leaks or get a bigger water pump. Now imagine your house is this boat and your heating and air system is that water pump.
It is important for any homeowner to understand that all houses leak moisture into them and that all air conditioning systems can remove moisture.
The biggest reason for high humidity in your home is moisture intrusion from outside. This could be through leaky windows, leaky floors, wet crawlspaces, leaky HVAC ductwork, or very small leaks in all of these areas that form a large amount of leakage when combined. Even if your home is sealed up, moisture can still travel through walls, wood floors, and even concrete. However, the rate moisture travels through some products (such as concrete) may be slow, but it all adds up.
Another way moisture comes into your home is from within the house. Boiling water, taking a hot shower, steaming clothes, or using your dryer are all examples of everyday items that add moisture to a home.
Why Is the Humidity Higher Inside Than Outside?
Your indoor humidity can read higher than the outdoor humidity because relative humidity changes with temperature. “Relative” is the key word. It measures how much moisture the air is holding compared to the maximum it could hold at that temperature. Warm air holds a lot. Cool air holds much less.
So when your AC pulls in 90-degree Charleston air at 60% humidity and cools it to 75 degrees without removing enough of the water, that same amount of moisture is now packed into air that can’t hold as much. The number on your hygrometer climbs even though nothing was added.
Then your household adds more on top of it. A single shower, a pot of pasta, a load of laundry, a few houseplants, and the simple act of breathing all release moisture into a home that’s sealed up tight against the heat. Outside, that moisture disperses into open air. Inside, it has nowhere to go.
The third piece is run time. Your AC only dehumidifies while it’s actually running, and moisture removal takes sustained run cycles. An HVAC unit that’s oversized for your home cools the space fast, satisfies the thermostat, and shuts off in short bursts, a pattern called short cycling. It hits the temperature you asked for and leaves the humidity behind. That’s why a house can feel humid at 72 degrees while the reading outside is lower than the one in your hallway.
What Should My Indoor Humidity Level Be?
The EPA recommends keeping indoor relative humidity between 30% and 50%, and below 60% at all times. Here in the Lowcountry, hitting the bottom of that range in August isn’t always realistic — so the golden rule our Trane comfort specialists use is 55% or lower with your thermostat set at 75 degrees. If you can hold that through a Charleston summer, your coastal home’s HVAC is doing its job.
Here’s what each range actually feels like:
How to Lower Humidity in Your House
Usually, your Charleston, SC heating and air system can maintain proper humidity levels in your home. When your air system is operating in cooling mode, it is also dehumidifying the air. As the air travels through the heating and air system, it passes through the cold coil and causes condensation on the aluminum fins of this coil. Similar to a cold soda can with condensation that forms on the house. The soda can is turning water vapor in the air into liquid water. Your Charleston, SC, HVAC system does the same thing but it has a drain line that takes the liquid water out of the house. However, your heating and air system can only remove so much water from the air. If there is more moisture coming into your home than the HVAC system can remove, your humidity level is going to go up.
Remember the boat analogy? If there are more leaks in the boat than the pump can handle, the boat will sink. If there are more leaks in your home than your Charleston, SC, HVAC system can handle, the humidity will rise.
With this being said, you should first make sure your heating and air system is operating properly and is the proper size to remove as much water as it can. It is also important to never leave your AC fan in the ON position and to always leave it in the AUTO position during the summer. Additionally, make sure the ductwork is not leaking air out of your house or pulling outdoor air into your house.
Another way to combat a humid house is by installing a dehumidifier. A dehumidifier will work with your heating and air conditioning system to remove moisture from the air. A typical air conditioner usually isn’t running all day, therefore it is not dehumidifying all day, but installing a dehumidifier will run constantly to ensure you achieve the desired humidity setting. If you have known leaks, however, then you should consider fixing the problems before adding a dehumidifier.
Here is a quick list of problems that can contribute to a humid house:
- Improperly sized or improperly functioning AC System
- Leaking AC System Fan in the ON position
- Leaky Windows, Doors, and Floors
- Very Humid Crawlspace
- Poorly Vented Attic
- Dryer Vent Clogged
- Bath Fans Do Not Have Backdraft Dampers
- Stove Vent Does Not Have a Backdraft Damper
When to Call an HVAC Pro About High Humidity
If you think you may have high humidity in your home, the best thing to do is to consult with local HVAC contractors. The team at AGL Heating and Air would be more than happy to help and check that your air conditioning equipment is working properly. We also work with great moisture consultants and comfort specialists who can take an in-depth look at your house to discover any problems you may have. For more information about humidity or your heating and air conditioning system in Charleston, SC, please contact us today.
Frequently Asked Questions About Humid Homes
Why is my house so humid all of a sudden?
A sudden jump usually points to a change in your system rather than the weather. The most common causes are a clogged condensate drain line backing water up in the unit, a frozen or dirty evaporator coil, a failing blower, or a duct leak newly pulling humid crawl-space air into your supply. If the shift happened over days rather than weeks, it’s worth a service call.
Why is my house so humid with the AC on?
Your AC removes moisture only while it’s running, and only after it has run long enough to condense water on the coil. An oversized unit cools the air quickly and shuts off before that happens, so it hits your temperature while leaving the humidity in place. Low refrigerant, a dirty coil, and a fan set to “ON” instead of “AUTO” produce the same result.
Is 65% humidity in a house too high?
Yes. The EPA recommends staying below 60%, so 65% is above the threshold where mold, mildew, and dust mites become a real risk. It also makes your home feel several degrees warmer than the thermostat reads, which is why people at 65% often lower the temperature and still feel uncomfortable.
Why is the humidity higher in my house than outside?
Relative humidity is measured against air temperature, and cool air holds less moisture than warm air. When your AC cools outdoor air without removing enough water from it, the reading rises. Add the moisture your household generates through cooking, showering, and laundry, and indoor can easily exceed outdoor.
Will a dehumidifier fix high indoor humidity?
A portable dehumidifier helps in one problem room, and a whole-home dehumidifier tied into your ductwork can solve it across the house. But if the cause is a short-cycling AC, a duct leak, or a drainage problem, a dehumidifier is treating the symptom. Find out why your system isn’t keeping up first.
