In a cool room in winter, i have an RBI rack that had 3" flex. To keep proper temps that stuff was running HOT. and actually ended up burning on the bottom level and started warping on others. I ripped all of it out... * hassle * I cut the back out of that rack with a tablesaw * since at the time RBIs design was a solid one piece hunk of plastic that could not be removed, not sure what it is now* and added my own strip of 11". Its been running perfectly since i did that, but i dont recommend 3" for cooler setups, or areas where it does happen to get quite cold. Not worth the risk!