Sometimes I beat around the bush too much in these tech tips, so let's get down to the nitty-gritty! (As Nacho Libre would say.)

Humidity inside a home should be maintained between 30% and 60% relative humidity.

I like to shoot for 50% in humid climates when possible (and by possible, I mean financially feasible for the customer because anything is possible).

If you have no idea what I'm talking about, I suggest you watch THIS video.

Causes of High Indoor Relative Humidity 

• Low Heat Load / Short Equipment Run Time / System Oversizing
• High External Humidity Drivers / Humidity Entering the Home
• High Internal Humidity Drivers / Humidity Being Generated Inside the Home
• Poor or No Spot Ventilation in Kitchens and Bathrooms (or it Isn't Being Run)
• High Evaporator Coil Temperature / High System SHR / High Evaporator Dew Point Temperature
• Insufficient Total Dehumidification Capacity 
• Low Space Temperature
• Relying on the A/C alone to Dehumidify

That is the list of everything that causes high relative humidity in a home or building. Total humidity drops when you pull out more water than you put in, and it increases when more moisture enters the space than you pull out.

Before we cover what to look for and how to fix it, let's first address some common fallacies that often crop up.

Bad Advice = Just Get it Colder in The Space 

Truth = Lower Temperature Alone Means Higher Relative Humidity

The HVAC system can help dehumidify a space. That happens when the evaporator coil runs below dew point, and water leaves the pan and goes out the drain. We call that process “latent heat removal,” and it's our friend when we're looking to drop the RH% in a space.

Sensible cooling decreases the space temperature; while that's a necessary part of comfort in most seasons, it's the enemy when it comes to dropping indoor RH%.

When air is cooled without being dehumidified, the relative humidity in the space actually INCREASES because the lower the temperature, the less water vapor the air can contain before turning into liquid water.

When we dehumidify with cooling equipment, it is the water leaving the drain that matters (latent heat removal), not dropping the temperature of the space (sensible).

For dehumidification:

Getting water out (latent heat removal) = good

Dropping room temperature (sensible heat removal) = bad

Bad Advice = Add More Insulation to Drop The Humidity 

Truth = Adding Insulation Will Decrease The Heat Load and Generally Increase the Relative Humidity 

For an air conditioner to pull out humidity and drip it down the drain, it needs to run. It needs to be warm enough in the space for it to run.

When you add typical insulation to the ceiling, floors, and walls, you decrease the heat load without changing the humidity load. That will result in the RH% going up.

Some insulation materials, such as closed-cell foam, will also act as an air & vapor barrier; it helps to block moisture from making it in. That can help reduce humidity, but it is the air/vapor barrier portions that do it, not the insulation.

Bad Advice = If Humidity is High, It's The Air Conditioner

Truth = Many Humidity Issues are Caused by Abnormally High Moisture, Not the A/C

The air conditioner needs to be properly sized and selected with sensible and latent capacity that matches the building design. There are many cases where homes aren't built or lived in exactly to design and cases where the weather doesn't act as the models predict.

In Florida, we have a lot of hurricanes and tropical systems. In these cases, we get tons of moisture, high winds that create a big pressure differential across our homes and force it in, low sensible temperatures (so the A/C doesn't run much), and power outages that keep it from running for days in some cases.

For months afterward, owners will complain of condensation, biological growth, high relative humidity, etc., and everyone tries to “solve” the issue by messing with the air conditioning. These tropical weather events increase the amount of moisture in the home while impacting the ability of the equipment to remove the moisture at the same time.

My own house is another example of an extreme internal moisture condition. I have great insulation, good vapor and moisture barriers, and excellent HVAC equipment (if I do say so myself).

However, I have ten kids. My wife and I also homeschool, so they are home most of the day. We live in the country, so we do tons of laundry (lots of dirt and mud), and we cook three meals a day at home.

Needless to say, our home has internal moisture loads that no model will be able to account for. That is why we added a whole-home dehumidifier to keep that humidity in check.

Final case study:

Many years ago, I had a customer who always had high humidity in the main living area, and the vents in the ceiling would sweat. I kept going back and messing with the equipment over and over, and nothing I did seemed to help. I finally asked another tech, and he laughed and said, “They have a pool, don't they?”

Sure enough, they did have a pool. “How did you know that?” I asked.

He smiled and said, “They are leaving the slider open when the kids play in the pool to keep an eye on things, or they are in and out all time. That's why the issue is always in that room.” I'll be darned. He was right. You may be able to use a data logging humidity sensor to find these sorts of client-caused intermittent issues.

What to Do About High Humidity

That right there is a mack daddy ventilating dehumidification system. It covers all the bases: ventilation, filtration, and dehumidification. However, not many of us are in the position to pitch a system like that to customers. Installation may not be feasible, and we need to focus on practical solutions.

There are many ways you can combat high humidity, depending on the types of tools you have at your disposal, the severity, the budget, and your client's patience. I'm not going to give every possibility and test, but here is what I would suggest for the average HVAC tech, even if it makes my more hardcore building science friends cringe a bit:

1. Make sure you have a few good-quality psychrometers/hygrometers. I use the Testo 605i as my go-to. Never trust a cheap tool with humidity measurement.
2. Ask the customer how often they cook and note if they have a range hood that vents outdoors.
3. Ask the customer if they use bath vent fans when bathing and showering.
4. Look for roof leaks, proper grading around the home, ponding water, etc.
5. Test the space humidity, temperature, and dew point at various locations around the home. You can often find the source of an issue this way. Keep in mind that the dew point tends to increase as you get closer to the ceiling because water vapor is less dense than air.
6. Check the HVAC equipment in detail. When humidity is a challenge, setting up the equipment for 350 CFM per ton is generally a good practice. Make sure it is all wired properly if it is multi-stage or has dehumidification features. Confirm the system airflow; for newer equipment, using the total system static and fan chart method is usually the easiest for a tech. I use the Testo 510 and 440 dP for this.
7. Inspect the ductwork and seal any leaks. Leaking ducts cause pressure imbalance in the home and can either drive air in or out of the home.
8. Ensure there are no dryer vents, bath fans, or kitchen ventilation leaking or discharging into attics or crawlspaces. Make sure the dryer vent is well-connected to the dryer.
9. Check and measure any incoming fresh through fresh air intakes, ERVs, or HRVs. If it is too much, it may be reduced, but proper calculations—and likely blower door testing—will need to be done before reducing outdoor air.
10. Look for can lights, gaps around boots into the space, holes in walls between the attic and crawl space and the living space, etc. Sealing these can greatly reduce the moisture drivers.
11. Check seals, sweeps, and weather stripping around doors and windows.
12. Assess to see if the equipment may be significantly oversized. If so, then do a Manual J calculation to determine.
13. Discuss supplemental whole-home dehumidification with the customer, especially when the issue is a big priority for them.

The goals in inspecting the home and equipment are to make some of the following recommendations that can reduce indoor humidity when they are appropriate:

• Run or install point ventilation in the kitchen and baths to remove excess moisture at the source when in use.
• Alter habits (like leaving doors open) that lead to moisture issues.
• Install new weatherstripping and door sweeps.
• Seal or install sealed can lights, seal around boots, and seal other gaps between the attic/crawlspace and the home or walls.
• Change HVAC system settings to run longer with a colder evaporator coil (reheat is an extreme example of this).
• Advise properly sizing equipment or installing whole-home dehumidification where appropriate.

Quick caution:

It is possible to seal a building so tight that it can become unhealthy. Whenever sealing is in order, it is best to do a before and after blower-door test on the space. That way, you can decide if you need to bring in outdoor air mechanically.

When this is the case, I generally suggest a ventilating dehumidifier (and an ERV, in some cases) in humid climates; otherwise, you can just make the situation worse.

Also, keep in mind that when you run a colder coil, the equipment, ducts, and vents will be more likely to condensate, as they will also be colder. While a colder coil will decrease the space humidity, it may not be an option if it results in excessive equipment, duct, and vent sweating. Of course, that's situation-dependent and often dictated by where the equipment and ducts are installed. Attics are the WORST for that.

When condensation occurs, you can either drop the dew point (humidity) of the air around it or increase the temperature of the sweating surface. Sometimes, decreasing the dew point of the air is very hard (like ducts in an attic). So, our only choice is to increase the temperature of the duct with either more insulation or warmer air going through it.

Another thing worth mentioning is that variable-speed blowers and multi-stage compression paired with humidity controls can help a lot with the coil temperature and run-time side of the equation. Even then, they aren't a silver bullet to fix all issues. If you over-promise, you may end up with a dissatisfied customer.

Once more, for lower humidity in a home, you want:

• Longer run times
• Colder evaporator coil
• Less moisture coming in from outside
• Less moisture being generated from outside
• Higher indoor temperatures
• Extra moisture removal with dehumidification when required
• Spot ventilation when cooking or bathing

—Bryan