Insulating liner with a 9x13 flue tile

Hello, getting a liner and insert installed soon. The installer measured the flue tiles to be 9x13, outside dimension. When asking about insulating the steel liner, he said 90% the time he does not do it. He usually insulates the top only. He said he would put insulation on the bottom if I wanted. Back to the liner, There is not enough room for an insulated wrapped liner, and he has never used vermiculite or blown in insulation. Can I use a smaller diameter liner to connect to the 6" stove? It is a 27' chimney, on outside wall of house.

Thanks.

I am not sure if this guy is WETT certified. He says he has been installing for over 10 years. He said he may have to ovalize the liner to fit it in, thats without the insulating. I will ask him about using a 5.5 inch insulated vs. a 6" non. I have to believe that would fit with no problem. Like I have heard so many times on this site, do it right the first time! It is a pretty clean shot down the chimney, with a slight bend as it exits the flue tile into the smoke chamber. To clarify, the entire chimney run is 27', the flue tile is approx. 22 ft'. He has never done a damper seal at the bottom (so he says). But he offered to use non face insulation if I wanted. I guess thats better than nothing. Should I use the firmer stuff like roxul?

Does anyone know the inside dimensions of a 9 x 13 flue tile? Installer said even without the insulation, it would be tight especially if there is anything protruding, like mortar, from the seams. Should I insist on an insulated liner even if it means going with a 5.5"?

Thanks!