I've been doing some reading, lots of reading, LOL, and came across an article saying that home designs like ours, two stories with a vaulted entryway and staircase are a nightmare for HVAC. It recommended two systems: one up, one down for real comfort and said it was really more economical than a two-zone single system. We've thought about putting a second unit in the attic, but it's something that probably has to wait until the house is paid off. In the meantime, I've been thinking and finally realized that there is a place we could put another supply duct. It would be on the far side of the door into the garage. The current duct trunk is on the other side of that door. It's right above the closet in the guest room, so we should be able to go through there and straight up to the attic--hope the attic joists aren't in the way. The run would be about 10"-11" x 24" in size and approx 58' long with 2 right angle turns. We're thinking it could handle about 900 CFM and hoping we could take our master bedroom and vaulted ceiling bath off the current duct and supply them with that. Our bedroom gets a little, but our bath is at the end of the current run and gets basically no air. With our master suite off the main trunk, I'm pretty sure the other upstairs rooms would have plenty. Does that sound like a duct that would work? Hubby is also thinking that if we did it with ductboard, he'd be able to do most of it himself, except for the connection to the plenum. He's pretty good with carpentry, plumbing and electrical, but has only worked on a/c in cars. And if I hadn't mentioned it, after measuring our house, we're really around 2,600 sq ft, not the 2,400 sq ft we had thought we had. Sorry, one more thing, if we have 1410 CFM now and add 900 CFM with another duct, going to about 2500 CFM, does that mean we'd need any certain blower size? Or do we just go with what the manual J says, assuming I can find the manual or a tech to do the calcs for us?

You posted in the plumbing rather than HVAC... but here's my $.02: First you recognize this is really odd. You have hit on the major issues: 1. Sounds like you have an oversized AC unit for the bedroom. That would tend to reduce the temperature without reducing the humidity level in the cooling season. 2. You have tested for other sources of moisture including from the floor (are you on a slab or on crawlspace / basement?) 3. You have tested for water in the walls and have a new roof. Now what about the following: 1. have you ruled out plumbing leaks? 2. have you ruled out a clogged condensation line on the A/C unit that allows water to back up into the room somehow? 3. was the master bedroom an addition or significant remodel? 4. There isn't a humidifier on the master bedroom furnace by chance is there? You need to know that you have an accurate way of measuring humidity such as a digital hygrometer. If you are using accurate test equipment and are getting a significant change in humidity from one room to the next while the AC is off for several days, you need to find out why before you do anything else.

All the venting for bath fans and kitchen can go out through new sidewall or gable vents. There is no need to put holes in the roof. You just need to have someone install the proper vents and (preferably) run hard vent pipe the entire way to where it exits. Any long run of flexible vent really cuts the airflow. Screens on gable vents should be replaced or repaired by stapling up new metal vent material. Then hire a pest control company for a one-time treatment to get rid of any residual problems. Then you need to verify there are also soffit vents, and that they are not blocked by insulation. Gable vents are the exit, but you also need to have soffit vents to let air in. Replacing the recessed lighting cans with insulation-contact rated versions is not that hard if you are willing to crawl around in the attic. However, some of those old electricians really got carried away with nailing the brackets in place. Another temporary option is to get good quality HVAC tape (the shiny tape with a peel-off back), and tape the housing very securely to the ceiling drywall. In your mild climate, just stopping air infiltration will be a good start. If these are metal HVAC ducts with insulation on the outside, they can probably be air sealed without really disturbing them, and then they could apply sealer that encapsulates the old protective coating. In most areas (maybe not CA), encapsulation is an approved way to deal with materials that contain asbestos. Bruce

(FWIW, ceiling heights in basement apartments might not be biggest concern. I know of two basement units right on my street that have been renovated within the last few years. Both have had stable tenants paying $1600/mnth and $1750/mnth. This despite the fact that although the units look nice, it is clear that they are not legal, with bulkheads at around 6ft and original-sized windows that are too small. yeah, I know. The reason isn't just about legalities, it's about money. Rental Real Estate is a long term game. There's a number of things that can do you in. Legal is one of them, but the cost to entry is much lower. The other con is that real estate is hot right now, been hot for quite a while. So what happens when things aren't so favorable? This isn't to say that if you do xyz, legal etc. That you won't have trouble. Crap happens. Life gets in the way. As you said you don't want to be told what to do. This situation you got is complicated, you do all the work upfront pay for it, then the process of getting paying tenants etc. Not everyone is cut out to be a tenant, not everyone is cut out to be a landlord. If you catch my drift. There is no amount of planning, doing that can prevent life happen type scenarios from happening. I've seen too much... I've condensed this down. Rental real estate can be 'easy'. But often times things get exagerated in terms of how much you make. Especially if you carry debt. This reno is likely going to cost well into the 30K range, maybe more no matter which or how you decide. Unless you do the work yourself, which takes more time. That's a lot of money to stick out there with no realization of ever turning a profit. Remember, this looks much easier from the outside looking in. Tenants don't care about half of what you think they might. Until they do care. As long as a place is clean, nice looking, nice area, nice schools many times that will do it for most. But things go bad after a tenant moves in. I feel the floor removal method is the better way mainly for the access to redo the plumbing runs and get the place height legal. (is it perfect, no not really. It's a choice that opens more options when it's far easier to fix things not thought of when the house was built.) The HVAC stuff is secondary for a basement. There will be little need for conditioning a basement. If the HVAC is problematic for the upper level then obviously before you finish the basement would be a good time to shore up those issues. I think dehumidification / some AC / little heat is really all you need for a basement. If this is your first endeavor into rental real estate, you've got your work cut out for you. It's always adviseable to do 'easy' first. This is complicated for a wide variety of reasons. Not saying you can't do it, but after trying it... well I first rented a home over 30 years ago. It was a complete disaster. Tenants stop paying 3 months after moving in. It took me the better of 20 years to get over those mistakes, and the visual nightmares as a result. Trust me when I say this is not easy, not for the faint of heart. If it was easy, everyone would do it. It's glamorized primarily because RE often is sold because of how glamourous it is. That's misleading when you're knee deep in crap after you've bought it. If this is a "house hack" kind of project, you're slightly better off in terms of getting your money back. But it's easily a 3 - 4 year project of working for nothing, dealing with tenants etc. Meaning this investment is going to take at least 3-4 years of your life before you show a nickel of profit. That's provided there isn't some unplanned "life happens" event that effects you or your tenant. There's far more to consider here, it's not meant to discourage you but as you noted the likely illegal rentals in the area... they're likely illegal due to? money. The whole reason you are contemplating doing this.