Humidity (and heat)

Trying to mitigate the mildew in a large library I inherited. I've got an air purifier running full time. This seems to have helped a little. I thought the next logical step was to lower the humidity. At 72 degrees, the humidity is 55%. Library of Congress says temp should be 70 or below, and humidity of 35%. So I bought a good dehumidifier, put it in there, and got the humidity down to about 45%, but the temperature shot up to 82 degrees! No good at all! What did I do wrong? Is there a way to dehumidify a small library (160 square feet) without roasting the books? Thought about an AC unit, but when it gets cold, I won't be able to run it without dropping the temp too low. Maybe just vent the dehumidifier? But then air in the room will have to come from somewhere, laden with moisture, which might create a vicious cycle.

Your note says "Library of Congress says temp should be 70 OR BELOW" - therefore you should not be worried about getting temp 'too low.' There are many benefits to low temp, low humidity with the main one being inhibition of mold, fungi and other troublesome microbes. I say go for the AC unit.

I think the ideal temperature for book storage is approximately 55 degrees.

burrrr. Wouldn't want to read in 55 degrees. I tried different ways of using the dehumidifier. Tried to sneak up on a low humidity to see if I just tried to lower it too fast. Nope. Just lowering the humidity 5% raised the temp about 8 degrees. There must be some law of thermodynamics I am fighting here. Gonna return in this weekend. I may try a smaller unit before putting out for an AC unit.

There are some dehumidifies that kick off a lot of heat, while others don't. I encountered that problem myself, and bought a different dehumidifier, which was fine. Unfortunately, the package labelling says nothing about it, but I did find it mentioned deep within the instruction manual.

The usual small dehumidifier (dh) is just a heat pump, as is the usual small AC unit. The dh passes air over a cold radiator core, then over another core to remove heat from the pump outlet. The air is then blown into the room and it has warmed up a bit. On an AC unit the air from the first core is returned to the room but it is now cooler. Air over the second core is returned to the outside.

Air will only hold so much moisture measured from 0 to 100%
But the actual moisture at 100% varies with temperature and decreases as the air temperature drops. Thus as the air flows over the first cold core it is cooled so it can no longer hold the moisture and the water flows down to a pail or drain. Dry air flows over the second core and picks up the heat and then blows into the room. Now the room air temperature goes up and the new 100% is higher plus the moisture is in the pail so the RH reading goes down.

IN the AC unit the first core works the same way but the air is returned to the room and not reheated. But now the room air is cooled so !00% is less moisture. The RH reading will change very little for some time even though the water flows down the core and is passed outside.

In my motor home AC, from a temperature of 100+, the RH would hardly change for a half hour or more.

You could also pass air through a chemical to lower the RH but I would guess the cost is higher.

To lower both the temperature and the RH an AC unit would be my first choice.