Actually, night time the humidity is higher. It might seem less since the temperature drops.
That's not exactly true. The
relative humidity of that cooler outdoor air is lower, but the
absolute humidity of the outdoor air usually (but not always) falls a bit at night as the radiational cooling into the night sky brings objects down to the outdoor dew point, causing moisture in the air to condense out onto leaves/grass/roofs, etc. When there is solid cloud coverage there is very little radiational cooling, and very little change in the absolute outdoor humidity levels as measured by wet bulb temp or dew point. When the sky is clear radiational cooling can bring surface temps to well below the ambient air temps, but it can't drop below the dew point temp of the proximate air.
To control a ventilation fan as a dehumidifier the controls need to track & compare the DEW POINT temperatures of both the outdoors and indoors to know whether night ventilation would provide any dehumidification.
An ERV does not dehumidify any more than a dumb ventilation fan does. But it will somewhat equalize the absolute humidity between the incoming and outgoing air streams.
To go to the extreme, mini split AC units are very low cost and some are made for the DIY'er. If you get one with a heat pump, you can use it in the winter for a little heat if needed.
A right-sized mini-split will definitely dehumidify & cool the space. But
oversized mini-splits have a lousy sensible heat ratio, since the compressor only needs to runs at it's minimum modulated levels with a warmer coil to keep up with the sensible heat load, leaving the space clammy-cool rather than dry-cool. A lot of DIYers will drop a 2 ton mini-split in to a garage or shop space thinking "bigger is better", and that allows them to cool the place off quickly, but it doesn't dehumidify adequately, and it doesn't operate as efficiency cycling on/off rather than modulating with load.
That said, most mini-splits come with a "DRY" or "DEHUMIDIFY" mode, which keeps the coil appropriately cool for maximum dehumidification. Most mini-splits do NOT control to either a humidity or temperature when operating in that mode, but some do. Fujitsu mini-splits will control to the temperature setpoint even in DRY mode, and Daikin's Quaternity series has independently settable temperature and relative humidity setpoints. Most of the DIY mini-splits are re-labeled Midea units, all of which have a DRY mode that just keeps chugging away no matter how cold it gets indoors.
If the garage has double-hung or single hung window, Midea has a series of pretty-good modulating window-shaker AC units that have a DRY mode, ( and a pretty big turn down ratio too.) It's essentially a "cooling only mini-split in a can". (The hand held remote is identical to the remotes shipped with most Carrier and Midea mini-splits, and even has a "HEAT" mode indicated on the remote, but the unit itself is cooling-only.) I recently installed
the 1-ton version in my open-loft home office that is under-served by the (ludicrously oversized) central AC. The office looks out over the kitchen and has an open stairwell to the living room. With the Midea set to 74F and the central air set to 76F the window unit carries the bulk of cooling load, and except during the most torrid days keeps the indoors to under 55% RH @ 74-76F. When the outdoor dew points are in the 70s switching it to DRY mode for several hours still holds the line on humidity, even if the central air never cycles. But if left in DRY mode overnight it'll be pretty cold in the kitchen in the AM. Depending on how much window area, how air leaky, insulated, & large the garage is the 1-tonner is probably enough (and not too much) for controlling both heat & humidity. A 500-600' garage. (It keeps up with my 2400' house until the low to mid 90sF outside.) It's pretty quiet too- not as quiet as a better-class mini split, but by far the quietest window unit out there, and cheaper than a 3/4 ton DIY minisplit. (Rated at CEER 15 it's substantially more efficient than my old 10 SEER central air too.) There were availability issues and some price gouging going on early in the summer, but those seem to be easing. I only got the 1-tonner because at the time the street price of 8000 BTU units were higher than the 1-ton (1), and hard to find (at any price.)
