Mini-split v Package Unit Humidity Control; Sizing and Configuration
I have a 1920’s house in Tallahassee, Florida – 2×4 frame construction with shiplap siding, no vapor barrier or insulation except in the attic and one wall that was installed later (originally a screened in porch), and a fair number of windows. I have sealed the floor and walls of the dirt crawlspace using heavy plastic with vapor-barrier tape at the seams, but there is still appreciable air flow between the foundation wall and the girders. The main floor is ~1,600 sq. ft.; approximately ~600 sq. ft. of the attic has been finished into a bedroom and bath. We tend to just keep windows open for four months out of the year in the spring and fall, heat occasionally over the 2-3 months of cool-ish weather we get, and generally keep the AC on between June and September unless it gets cooler than average The HVAC system (presently a 4-ton gas/AC package unit for the main floor, which I believe was grossly oversized) and a 1.5 ton heat pump for the upstairs) needs to be replaced. I am considering a Mistsubishi heat pump (MXZ series ) with a 2-ton ducted MVZ air handler for the main floor and a 1 or 1.5 ton wall unit upstairs.
Humidity has been an issue – the interior humidity generally almost matches the exterior when the AC is off, and given that we set the temperature to ~76-78, often stays high even when the AC is on. This sometimes leads to mildew growth on leather and cloth. My question is whether my newly proposed configuration would tend to provide better humidity control than the current package unit, and whether the Mitsubishi systems allow humidity control independent of temperature control. If anyone has input on the sizing and configuration, that would be appreciated, as well.
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Unless you can air seal the walls better all air conditioning would be just bailing against the rising tide. The Mitsubishi's can be set up for "Dry" mode with the remote/thermostat, which will improve it's latent cooling (moisture) performance compared to typical 1-2 stage package units, but on days when there is little to no sensible (temperature) load it won't be perfect.
The only mini-splits that have fully independent temperature and moisture control is the Daikin Quaternity series, which is a wall-coil type, not a ducted mini-split.
If you can dense-pack fiber insulation (either fiberglass or cellulose) in the wall cavities the air infiltration levels will drop- usually by a LOT, and the more you can air seal the upper floor ceilings the less stack effect drive the house will experience. If you can also fit the code-min R30 of fiber insulation in the attic (cellulose is usually more effective than fiberglass for open -blown fill) the whole house cooling load is likely to drop below 2-tons (if it isn't already), including latent load. With a kneewalled attic room situation it's usually more effective to seal off all venting to the mini-attics, air-sealing and insulating on the under side of the roof deck with 8" of half pound density open cell foam. That's a lot more expensive than 8" of cellulose on the attic floor, but it will air seal a lot better,and the air sealing factor is key to keeping the humidity well controlled.
I agree with Dana. Your indoor humidity problem has nothing to do with your equipment specifications. It has to do with air leakage.
The only way you can improve the situation is by sealing the leaks in your floor, walls, and ceilings.
Thanks for the replies. I would be concerned about insulating the walls unless I was really opening them up and could install a vapor barrier, as well, but I will take another look at the attic with your suggestions in mind.
With regard to the floor, is it likely better to address the crawlspace walls and the gaps between the foundation and girders, or the floor itself? One of my first jobs on this house was to take out all the fiberglass batting that was between the floor joists because it was damp and in bad shape overall. But I would consider going in with mineral wool, either using batts between the floor joists or rigid boards against the walls.
Air sealing work does not require insulating. It requires sealing cracks and gaps with caulk, tape, canned spray foam, or (if the hole is very large) a material like drywall, plywood, or OSB. The work is sometimes performed when the house is being depressurized with a blower door.
In US climate zone 2 don't really need or particularly want a vapor barrier for the walls- you need an air barrier, to limit the air-transported moisture. Mere vapor diffusion through walls is more than an order of magnitude slower in terms of the rate at which moisture is getting in.
Insulating doesn't air seal to the standards normally needed in New England, but if you can't air seal more properly there is still a LOT of air infiltration reduction benefit to dense-packed fiber insulation, and that can be done without opening up the walls. For a US climate zone 2 location dense packing the walls and air-sealing with caulks / tapes /foams where it doesn't involve major house surgery should be enough for managing the latent load problem.