My 18oz. tank is definitely the warmest kilt I own. Not much difference to me between the 4 yd. and 8 yd. in warmth. Also not much difference between the 13 0z. and 11 0z. The interfacing and lining materials make more of a difference. The 11 oz. kilts look bad after sitting on them in hot weather. 13 oz. hold their pleats much better.