A temperature split of 12-14 degrees is probably a low refrigerant problem. The humidity rising means the coil is not getting cold enough to create condensation and remove the humidity. This also suggest low refrigerant. It could be something else, but I highly doubt it is an uninsulated return duct or dusty blower motor problem.

Any tech who does a diagnosis without connecting a set a gauges is wasting your time. When was the last time the AC got a preventative maintenance check? You need to find a competent and honest HVAC company who will diagnosis the problem properly.

@mike_home We just bought the house a couple weeks ago, so we are quickly seeing the difference between contractors to trust and ones to not, and this one definitely was leaning the wrong way there. They did preventative maintenance last week on it, but obviously did not do much else. We're already looking into another company to inspect this.

Hi, Josh,

Your temperature and relative humidity data confirm sensible cooling only. You probably have a combination of issues including inadequate cooling along with infiltration of humid outdoor air. You have identified a source of moisture input from the basement. A couple of questions:

What is the relative humidity in the basement?

Does your fireplace require continuous venting such as one with gas logs? If not, can you install rigid insulation to block any back drafting of air down the chimney?

Was the duct work replaced in 2016 when the furnace and a/c were? Is there evidence of mastic or foil tape sealing joints in the ducts?

You need to take the supply and return air temperatures at the air handler to see if the system is cooling properly. If you get a higher differential this way then the problem is with the ductwork (leaks/insulation). If the differential is the same then either the system just isn't very efficient or it is low on refrigerant. If that's ice on the indoor coil then that suggests low refrigerant.

For any given water content in the air, relative humidity increases when temperature decreases. So if the air is being too cooled quickly, that may explain why you feel the humidity is higher.

@Charles Ross Homes Just noticed I said basement in my post, I meant to say crawlspace.

All good questions. I cannot answer the duct work replacement, that's unknown to us. The ducts in the attic that I've seen so far do not have any sealing joints evident, so probably a good idea to do that.

I believe that the dehumidifier down there does display the relative humidity so I'll check on that next time I'm down there, but I think that it is significantly less humid in the crawlspace.

Fireplace is not gas log, it's wood burning. Where exactly would the rigid insulation get installed in the fireplace? Do you mean by essentially covering up the main opening to the room with the insulation to seal that up when not in use?

Also, I don't think that the vents to the crawlspace around the outside of the house are sealed yet since installing the dehumidifier, should those be blocked off by Styrofoam? I've seen conflicting opinions on this.

Check for leakage of the supply vents in the garage. Any leak can draw humid air from the garage and into the house. It also means any carbon monoxide from the running car can also be pulled in. Make sure you have working fire and carbon monoxide detectors in the house.

What is your location. An outdoor temperature of 80 degrees is not that hot. The real test is when the outdoor temperature is 95 degrees. The AC may run continuously and never reach 72 degrees.

@mike_home I'm in the Midwest. Oh yeah I have absolutely no faith that this AC will reach 72 on a 90+ degree day if it can't cool 74 to 72 on an 80 degree, low/mid humidity day, hence my concerns here.

Will check on supply vent leaks, and planning to install detectors this weekend.

Hi, Josh,

You shouldn't have any HVAC supplies in the garage; that would violate the mechanical code.

Joints in duct work should be sealed with mastic or UL approved tape designed for that purpose.

Your crawlspace can be a source of moisture that gets pulled into the your home by a combination of stack effect and an HVAC system operating at negative pressure due to inadequate return air flow (most do.) If you're running a dehumidifier and the crawlspace isn't a sealed, conditioned crawl space, then you're spending money to dehumidify the outside world. I recommend you convert the crawl space to a conditioned crawl. One of the principal sources of moisture is improper grading within the first 10 ft of your home; there should be 6 in. of fall in that distance. Storm water should be directed well away from the foundation.

You should be able to fit a piece of rigid insulation to seal the fireplace throat (entrance to the fireplace flue)--not the opening of the fireplace into the room. I suggest you include a ribbon that hangs down in full view as a reminder of the need to remove it before lighting a fire in the fireplace. Make sure any fresh air intakes for combustion air are closed, too.