It would effect your Reflex score, not your AC, unless you were wearing light or cloth armor. Reflex is your ability to dodge an attack. AC is the ability of your armor to withstand damage (an enemy miss isn't necessarily a miss, just a hit that inflicted no damage). So bad dex, I would think, shouldn't be taken into account against heavy armor. Only poor reflex and poor initiative. In light or no armor (with someone with dex AND int of 8 couldn't possibly justify wearing, unless imprisoned) there would indeed be the -1 to AC.