Knowing that Sig Forum is the premier site for getting technical questions answered, I come hat-in-hand with the following. (And please accept my thanks in advance.)
1.) What should the relative humidity INSIDE one's home normally be (for a centrally heated & cooled home)??
2.) IF one believes that the relative humidity inside the house is excessive, what would be the most common causes of that? - And exactly what should one do to correct the situation?

Please know beforehand that I have already consulted "the internet", but that I value the opinions of Sig Forum members more than I value that. THANKS in advance. FredT

Interested in the responses myself

Drainage of the coil and air box is imperative. Any moisture trapped in there will just recirculate into the house I believe

Anywhere from 45-60% I would think depending on time of day, how often the ac runs, location, etc...

You think the humidity is too high but is the house cool enough? What is your thermostat set at?

If your house is warm, the ac isn't running long enough to pull out the water from the air.

If your house is cool enough, but still humid, your system is oversized I would think.

If you have to freeze out your house to get the humidity down, then your system is definitely oversized.

40-60% is the general range.
75 degrees at 50% is the sweet spot.

Assuming your humidity is high?

Reasons:

Dirty filter
Plugged drain line
Oversized air conditioner
High infiltration (lack of vapor barrier)
Improper charge (losing capacity)

What temperature do you keep your house?

Oversized equipment is a common problem...in the old days they used the formula of 600sf per ton.
Today's houses are much tighter and more energy efficient and the size of the HVAC units have been getting smaller per given unit.

The ideal relative humidity is between 45 and 50% according to most "experts".

For us keepers of guitars, we prefer it a little higher.

I have a monitor that registers the crawlspace temp and humidity as well as the indoor. Currently 75 degrees in the crawlspace and 86% humidity (South Carolina, gotta love it!probably about 95% outside the crawlspace) Inside temp is 73 and humidity is 56%.

Sorry, these answers will not be very helpful. I am a commercial HVAC contractor and have been in the business my entire working career. I do not do any single family residential work and have not for the past 15 or 20 years, but I do live in a house and I think I still have a reasonable grasp of the concepts as they apply.

1. It depends. The vast majority of homes have no direct control over the humidity level. Some have a limited ability to add humidity in the winter and many have air conditioning that will remove humidity in the summer, but the air conditioner is typically (but not always) controlled by space temperature and not humidity. The gold standard for comfort is 50% humidity at 74 degrees + or - 2 degrees. In the cooling season less humidity is normally well tolerated, in the winter a little more is usually fine.

2. There are a lot of possible causes for high space humidity and it would be impossible to guess what may be causing yours. Most likely you will need someone to come investigate the issue. In my area there is very little call for specific dehumidification, other than natatoriums, pot grow facilities, or large machine shops, but besides these specific cases the main cause of excess indoor humidity has always gotten back to water infiltration. Ground water seeping through a slab or basement walls is most typically the problem, but there are many ways it can find it's way in. Even an improperly vented clothes dryer could cause issues, there are just too many possibilities to guess.

I stayed out of this thread because I'm no expert ... but after morning coffee, being a little more daring ...

1] 50% or a little higher. Fact, a violin I restored over last winter sounded like poo, but after taking it to the client - we were meeting up at a YMCA lobby just off their natatorium - the tone dramatically improved over just a few minutes as the wood soaked up humidity. I was even a little scared the swelling wood would pop newly-glued seams. False fears, thank God.

2] Excessively high humidity? High water table? Increase ventilation of unconditioned spaces, perhaps. Broken bathroom fan? Open more windows on occasion. Hard to say without more information.

My basement is a bit on the humid side. Not mushroom-growing humid, but I can smell it. No ventilation, underground. No water ever, at the bottom of a long hill, but the soil is virgin packed sand, and standing water drains well. Humid air is heavier, I recon, and it makes its way downstairs and gets trapped?

I have a lot of "stuff" including firearms, precision measuring instruments, reloading tolls, optics, powder, etc., and etc. which can be damaged by high humilities. I don't have any musical instruments but plenty of wood.

My aim is to keep my shop between 45 and 50% RH. The basement I aim to keep around 40%.
This requires the operation of dehumidifiers in both places.

If your house thermostat is keep at a steady temperature, ours is at 77, your RH will tend to rise during night time hours when your AC does not operate much. Fluctuating temps and RHs are a bad combination for rust.

The State of Florida hired me in the late 90's to develop an Indoor Air Quality program. I am extremely familiar with the answers to the questions you are asking.

First: WRT to the average relative indoor humidity, it is between 42% and 44% in an average Florida home(based on 3200 samples). This will obviously be different if you live in a different climate. This "average" home has A/C running 9 months a year in a very humid environment.
If you live in a more temperate area, your indoor humidity actually may be a bit higher as you will use less A/C to cool down your indoor environment (thereby reducing the humidity) in the summer months. During the cooler months, your humidity may actually be far less ( older folks will remember getting shocked with static electricity after walking across the carpet in the winter time).
Without the A/C running, the indoor environment tends to have higher humidity than the outdoor readings as water vapor is added from showers,cooking and laundry. This can be as much as 2% to 10%.
Every home is different and so are lifestyles as well as outdoor environment situations, so to know your actual indoor relative humidity it would be necessary to measure your actual conditions.
Personal comfort is dependent on both temperature AND humidity. At higher humidity levels (50+), the human body "feels" good at a narrow range of temperatures...typically 74-75 F, but at a humidity level of 30% the "feel good" range increases to 70 - 79 F. Therefore, for comfort you strive to keep the humidity generally lower.

Second: If you think that your humidity is excessive you first should determine exactly what the humidity level IS. This is best done with a digital hygrometer. Do not use or even think about looking at the common analog meters as they are totally unreliable.

If your humidity is high (+45%) and the A/C is being used...then the A/C is not removing enough water from the air. This could be caused by improper sizing of the A/C unit (not large enough OR too large) or the unit is not functioning properly. You are only removing water from the air when the A/C is running. So, if the does not run enough hours of the day...then it will not remove enough water.

Also consider A/C duct leaks. If there are leaks in the duct work, you could actually be pumping or sucking outside air into your home which would dramatically increase humidity.

Another common mistake is to set the A/C fan switch to the "ON" position which has the effect of keeping the blower motor continuously running. This typically raises the relative humidity about 10%.

There are many issues and circumstances here to discuss. If you like, please feel free to contact me for further discussion.

Taking just a moment to thank everyone for the helpful responses. I had resisted getting into the details of this situation because I didn't want to bias anyones' answers or get anyone started down the wrong track. My details briefly are these: I had noticed several small things that tripped me off that the humidity inside was excessive. Crazy as it sounds, one of them is that I noticed I've been sweating indoors much more than usual. Now that may be a function of the meds I have to take but it is extremely irritating nonetheless. Another "clue" was "sticky" floors... you know what I mean - all things are clean & yet when you walk across the floor your slippers sound like you're ripping an envelope open. But all the while the temperature has been reliably & consistently low. So I finally put 2 and 2 together & guessed that the RH was higher than it should be - or has been. Not wanting to go off half-cocked I bought a digital combination thermometer & hygrometer.... and sure enough found that the RH was sitting at about 73% (with temp at about 74). Called a church member I know who is skilled in HVAC (made his living doing that for more than 40 yrs). He soon found that the drain tube for the condensate (at the unit itself) was improperly plumbed when the house was built some 16 yrs ago. (no "trap" was installed in that condensate drain pipe)... I FINALLY pulled out an old dehumidifier - one that was still in good shape - and put it to work in the house. FINALLY, finally got the RH down to about 55%, but the NOISE of the dehumidifier is too much to bear. But STILL I think that we should be able to get the RH down to a lower level than that. Granted, we are in Middle Tennessee, and we HAVE had some wet weather at times over the summer... but I do NOT remember having excess moisture in this house during any of the previous 15 years. I have explored the internet for answers & info, and what they say confirms to me that my suspicions are probably correct. I just feel that there is an additional component to this problem. I will keep looking; and I especially want to thank all who took the time to answer my questions. If you think of anything else I would like to hear of that also. Thanks again. Fred

Watch the high humidity... its perfect for growing mold and mildew.