I'm looking at insulating a house that has 2x6 studs. I am going to put R-board on the outiside with an r value of 5 and fiberglass on the inside. As far as fiberglass goes I have noticed that there is an R-19 that most people use around here and also an R-21. The R-19 is made to fit a 6 1/4" cavity. The R-21 is made to fit perfectly in a 5 1/2" cavity. My question is why does everyone use the R-19?