Adding an armored kilt to heavy armor has no effect.
The armored kilt is made of a thick cloth skirt with bars of steel hanging down from the waist and a ring of horizontal steel plates just above the hem. An armored kilt can be worn separately as light armor, or it can be added to other suits of light or medium armor. Adding an armored kilt increases a suit of armor’s armor bonus by +1, but it adds 15 pounds to the armor, lowers the maximum Dex bonus by 1, and increases the armor’s weight category (from light to medium and from medium to heavy). Adding an armored kilt to heavy armor does not provide an armor bonus increase.
Sean K. Reynolds says, “Adding an armored kilt to an existing set of armor has the effect of creating a new type of armor, just as studded leather is leather with studs, chainmail is a chain shirt plus leg and arm coverings, and half-plate is chainmail plus plates. Once you add it to armor, don’t think of it as a separate piece that you can add unique properties to, just consider the new armor as a whole. Just as you wouldn’t say ‘I want to add magic to the studs of my studded leather armor’ or ‘I want the plate parts of my half-plate to be adamantine’ or even ‘I want to add enhancement bonuses just to the padding of my full plate,’ don’t try to single out the armored kilt as a separate piece.” See the source thread for additional comments.
Pathfinder Companion: Adventurer’s Armory. Copyright 2010, Paizo Publishing, LLC; Authors: Jonathan Keith, Hal Maclean, Jeff Quick, Christopher Self, JD Wiker, and Keri Wiker.