<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">What is a wet bulb temp? also, does this imply that every time one doesn't foil the meat is dried out? </div></BLOCKQUOTE>
wet bulb temp is like the "real feel" temp inside your smoker. When news stations report the temp they often include "real feel" which bumps up the temp if it's humid, and down if it's dry. It's not a perfect analogy because pork doesn't feel, but its surface is moist and is effected by evaporative cooling (like a sweating person).
225* air can hold a lot of humidity, so any moisture will be evaporating off the butt and cooling the surface temp. The current hypothesis is that the temp at which meat stalls at, is approximately your wet bulb temp in the smoker. The center of the butt cannot get hotter than say 175*, because even with the smoker running at 225* the butt's surface is cooled to 175*. The only way to break the stall is for something to change, either the surface of the butt or the internal conditions of the smoker. Foiling the butt creates a 100% saturated condition inside the foil and a dry surface that will no longer be subject to cooling.
I'm curious what would happen if you kept mopping the butt, keeping it moist, and kept it in the stall for many hours until tender. When I run cuts of pork shoulder or belly in my sous vide rig, I can get them to tender in 12 hrs at 175*. So, I suspect it's possible to use the wet bulb stall to your advantage, if you had the patience.
Not foiling meat doesn't automatically mean the pork will be dry, any more than foiling would automatically mean it will be moist. There's far more variables here than just foil.