Humidity Level New Construction

What kind of insulation do you have in the home? The hvac might not be running long enough to remove enough moisture from the home.

Also how new is new, it is possible that the home is still removing moisture due to the humidity from construction

Check behind the thermostat and make sure there isn't a large hole for the wires to come through. They are pretty sensitive and will read high if there is an oversized opening behind them.

I just moved into it, built this year.

The attic has minimal batt insulation to pass code, I’m adding more insulation next month, was recommended to increase to r49

Also my attic is hot as heck, I only have soffits and minimal ridge vents.

Is whirlybirds with ridge vents a good idea? Most sites say combining them isn’t effective. Again I don’t know much about this, just been researching the past few days

I believe this is the exact reason DR Horton is getting sued. Their homes don’t meet the humidity level proofing we need in south LA. Hence, mood growth

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I only have soffits and minimal ridge vents

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If you don’t have enough soffit/ridge/edge ventilation your home can be too “tight” and suck moisture in through other spaces.

Is your soffit vented or non vented?

The first thing I would do is get a standalone hygrometer and confirm the thermostat is giving you accurate readings. If the humidity level is actually ~65% throughout the house, you’re likely going to feel pretty uncomfortable in the space unless it’s <69° or so.

Take readings in multiple areas to see if there’s a huge dynamic in the area near the thermostat, which could point to an air infiltration issue from the attic or crawl space, if applicable— especially while the A/C is running. Your findings should give you a good starting point.

as mentioned above, verify the accuracy of your t-stat first. My t-stat humidity readings are off by 18%. Hygrometer functions seem to be the most difficult to get consistent with residential devices.

Is your house a 1 story or 2 story home? based on 3.5 tons, I would hope you are a single story home. If not, a well built 2-story home should be in the 2.5 to 3 ton range if not even lower.

If your t-stat is accurate, the first thing you can tinker with on your own is the CFM/ton setting of your AHU or furnace. Setting this to 350cfm/ton should be able to get you <50% with a properly sized system. If you get to 350cfm/ton and you are still having high humidity issues, you will probably want to have your builder come out for some warranty work. You could be dealing with a basic oversized system or you could have high infiltration in the house/ductwork.

With regards to system sizing, when it is peak design temps for your area, you want your system running 90-100% of the time from like 3pm-6pm. I'd guess your design temps are somewhere around 98-100F. If your system is running less than 75% in that situation, system size is likely contributing to the problem.

Also, be sure you do not use constant fan ON when in cooling mode.

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The first thing I would do is get a standalone hygrometer and confirm the thermostat is giving you accurate readings. If the humidity level is actually ~65% throughout the house, you’re likely going to feel pretty uncomfortable in the space unless it’s <69° or so

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Definitely do this first. This inexpensive hygrometer is recommended by HVAC techs on HVAC Talk forum, I have a couple and they are accurate, and can be calibrated.

Mine will creep to 60 during the summer when it is raining. The rain cools off the house so the air runs less plus it ups the surrounding humidity so I know why it happens. I usually stay around 50+/- 5% during the summer months though.