We recently completed an addition (~1100 sq ft) as well as converted our garage to a climate-controlled workout room (~450 sq ft). The HVAC contractor installed a new heat pump system that supplies only these areas. The rest of the house is supplied by an older traditional AC/furnace system.

The humidity in all areas supplied by the new heat pump is ~10-15% higher (ranging from 50%-59%) than all areas supplied by the older unit (41%-44%). Temperature control seems ok, other than the workout room being several degrees higher (don't yet have blinds up on the west-facing windows of that room so I attribute it to that). However, the master BR registers are the only ones where the air seems to come out with what I view as typical force.

Some other details - The addition & workout room are on a slab, while the existing house is over an encapsulated crawl space. We have seen small amounts of insulation on the workout floor and registers even though the insulation was blown in after the HVAC was completed.

Thoughts on the cause(s) of the higher humidity?

This has nothing to do with your problem - but I am considering doing an addition similar in footprint to yours and was curious as to what this cost you?

The 1100 sq ft addition consists of a master br/ba/closets, powder bath and new laundry room. Bricked in the garage, added HVAC, and did polyaspartic coating on the floor. Added a large finished carport/porte-cochre that extends out from the old garage and is tied into the roofline so it looks like part of the house. Trim/finish out is fairly basic to match the existing late-60's ranch finish of the rest of the house.
All in $250-$300k.

Experienced HVAC guys - What is a reasonable expectation for humidity in new construction with a completely new HVAC system where the thermostat is set to 75, with an average high of 90 with 70% relative humidity?

TheRatt87 said:

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Experienced HVAC guys - What is a reasonable expectation for humidity in new construction with a completely new HVAC system where the thermostat is set to 75, with an average high of 90 with 70% relative humidity?

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Need more information to give you the best opinion. Do you have a single speed, two-stage, or inverter/variable speed system? How many systems? Do you have fresh air intake? Single story or two story? Is your insulation spray foam, batt or other? What time of day and where in the house are you taking the readings?

Generally, ideal target is 45-50%, realistic is <55%, exceeding 60% continuously is not good.

New addition and separate converted garage currently physically separated via doors from the rest of the house.
New Rheem 2-stage system supplies the addition & converted garage, while a second 20+ yr old system supplies the original house.
There are return air vents in both the new addition and the converted garage. Don't know about fresh air intake.
Entire house is single story, with blown in fiberglass insulation.
New addition & garage are on a slab. Existing house is over an encapsulated crawl space.
Humidity everywhere and at all times in the new addition & converted garage is high 50's to low 60's, and 10+% higher than the existing house which is mid 40's-50% max everywhere and at all times.

HVAC contractor is claiming that 55% is realistic expectation, despite every manufacturer (including Rheem) indicating that room humidity "comfort level" should be 40-50%, and if not, consider a new system (which I obviously have). HVAC contractor cannot explain the lower level in the existing house, as he claims he never sees that level.

I dont think he is being totally honest because I have owned multiple homes, and have never felt the level of humidity currently in the addition & converted garage.

Thanks for any feedback & guidance.

Having a slab, blown in, all new, assuming ~475sf/ton then history says

BS
HVAC contractor is claiming that 55% is realistic expectation. HVAC contractor cannot explain the lower level in the existing house, as he claims he never sees that level.

&

True that
Ikanizer
Milwaukees Best Light

Until you report back after thorough reconnaissance, we should all save time/effort with solutions

Is your two stage running the whole time in first stage, or is it bumping up to second stage? If the latter, you may try adjusting thermostat settings to extend first stage runtime before second stage is called for.

Speculating, but I would not be surprised if your encapsulated crawl space dehumidifier is assisting your 20yo HVAC in driving down vapor in the original living area.