NEWBIE Generic Benroy 1 1/2" spars.....1 1/2" insu

Hello all GREAT SITE! I'm building the generic benroy and installed 1 1/2" spars. My question is....Do you fill the 1 1/2" void or use 3/4" insul board? 

or t n ttt for short (tnttt.com)
https://www.tnttt.com/