Central A/C with a variable speed air handler and the proper control system works very well for dehumidification. If your want, you can add in a whole house dehumidifier with the system.

Since we don't have any ductwork, could someone maybe explain how central a/c is added in this situation? We have an attic but no basement ...

1) you would probably put the air handler in the attic and compressor on the roof or running a line down the side of the house.

2) If it's cool, the A/C won't dehumidfy because it won't need to run, so you need the dehumidifier.

What about adding a fan that draws air up from the lower level?

If we add A/C upstairs only, would I be correct in assuming it would dehumidify the upstairs air, which would then float downstairs through the stairway and maybe help solve the downstairs humidity issue?

As for having a fan to draw up the downstairs air, we do have a whole-house-fan that we run to pull air in from outside. We don't use it much with the downstairs windows open (only for upstairs cooling) because downstairs, it seems the night air is so moist, it makes the humidity issue even worse. I do have to believe it draws up some of the downstairs air when it's on, but that does not improve the humidity levels at all.

Personally, considering our slab appears to be dry as a bone, I am thinking the downstairs humidity problem is due to the warmer, moist air from upstairs mixing into the downstairs and since the downstairs is cooler, the RH is even higher. The only problem with my theory is that warm air doesn't sink, it rises, so I don't know how it would be getting downstairs, except for us dragging it with us when we walk down the stairs.

What else could be causing this moisture? We were sure it was the slab, and now that it's not, I don't know what to think.

Sorry for the long post. Additional thoughts and advice would be most appreciated!

I'm no expert, but from my experience with my system, an AC alone will not get you desired results. You seem to be needing a whole house dehumidifier as suggested.

There are many stretches of days where there's no call for AC but dehumidification is still important. It only takes a few days of this to grow mold. Look into a ventilating dehumidifier for the whole house. You can install it alone without a central AC system.

Your house seems to be "leaky" and allows too much air infiltration. Maybe look into sealing it up. Then you can tackle the problem better. good luck

Why would you add AC upstairs only? If you run the ducts to the basement, that will ensure that cool, dry air gets down there as well. If you have a return in the basement, that will help pull that air back up to be cooled/dryed.

BTW, how do you know the slab is not the source of moisture? The moisture is evaporating off of it as it comes up through the slab, but you're not going to see puddles on the slab--it's evaporating and causing the moisture!

Another thought -- what if you put the dehumidifier in another part of the lower level besides the master bedroom? Then get a ceiling fan in the MBR to give you a light breeze. The DH will continue to dry all the air, but the heat won't be so obvious in your room (nor the noise). To try this out, just go buy a small oscillating fan for $15 and set it up in the bedroom, while running the DH in the other part.

Thanks, cissado & heimert. We did a moisture test on the slab twice and did not show an obviously "wet" slab (as would have been evidenced by a show of condensation on the test article -- there was no condensation). I know that a slab will "breathe" water vapor but it's still pretty extreme for a slab that's technically dry.

We now have A/C AND a large stand fan in the BR, and have put the dehumidifier outside of the BR. The bedroom doesn't seem to get below 73 degrees with this arrangement, relative humidity running in the 60's. In fact we are now running two dehumidifiers on this level of the house -- one in the utility/playroom (which gets closed off at night) and one in the family room just outside the MBR. The one in the family room fills and has to be emptied 2x/day at a setting where it gets the RH to hover around 60-65%.

The reason for not installing AC downstairs as well as up is the difficulty of running the ductwork down from the attic. We don't have existing heating ducts due to HWBB heat, and because of the open floorplan, there aren't a ton of options to run AC ducts. We have neighbors who AC'd the upstairs only and found that the downstairs benefitted from the "falling" of the cool air down the very open stairway.

Honestly, we just want to come to the least intrusive/invasive/costly arrangement we can find. Right now our house sounds like it's going to take off, what with the A/C, the stand fan, two dehumidifiers, a whole-house fan and two more stand fans in the kids' bedrooms. Seriously. Ridiculous!

You need something like this... http://www.thermastor.com/Santa-Fe-Rx/

Running several off the shelf dehumidifers and constantly emptying may mean you need something stronger. This is a very good product from what I hear. They have a few options including ducted designs. good luck

PS Even with controlling your (high) humidity, you still can benefit from sealing your home from leaks that allow the humid air to get in. You may be fighting a losing battle with air infiltration.

Thanks, cissado. I checked out the Santa Fe RX last night and it looks like a pretty good option for us, but of course, they don't list the price and I'm guessing that's because it's pricey. I plan to contact them today to get a ballpark figure.

My husband brought home a 1,500 BTU window AC unit for upstairs to see if we can partner that with a couple other upstairs window units to achieve reasonable cooling up there. We're hoping that cooling the upstairs will help with the overall humidity problem, but it's all a grand experiment. It's supposed to get hot and humid again on Friday so we'll put it to the test.

Perhaps I'll also call our local electric/gas company to have them come do an energy audit, check for leaks and all that. The house was built in 1969, so it probably could be improved.

I appreciate your input -- glad to have a forum like this!

Do you have all your windows closed in the whole house when you are running the dehumidifier? If you are running the whole house fan while running the dehumidifier you are wasting your time and money. To lower the humidity you need to close the house up and run the window AC and/or the dehumidifier.

See, that's what I've been wondering. In a house where the basement closes off to the rest of the house, I'll bet you can keep the windows open and still get good results with just the dehumidifier. But our downstairs doesn't close off, so we're spending all kinds of energy basically getting nowhere.

Since the upstairs hasn't been air conditioned and it's routinely too warm (80s in the summer, but no more humid than the downstairs), we have kept the windows open up there at night with the whole-house fan on because otherwise it's just stifling. Downstairs tends to stay in the 70's, so if it's dry enough, it's comfortable. It's the "dry enough" part that's been difficult.

Last night we tried turning off the whole-house fan and just having a few windows open upstairs (kids' rooms, kitchen) to see if that did anything. Still had to empty the dehumidifier this morning, so I don't think it had much impact. I don't see as we have any choice but to try air conditioning the upstairs, at the very least, and we're going for that particular gusto starting tomorrow.

In the meantime, it's really so distressing not to be able to take advantage of the cool night air with the whole house fan. I'm bummed.