Rigid liner extension above masonry

[Riteway]

The instructions for my insert (Drolet 1800 Escape) "strongly recommend" adding a minimum of 12" rigid liner between the between the top of the masonry chimney and rain cap, in order to ensure an "optimal draft." Besides making the chimney slightly taller, anybody know how this would improve draft? I need to add an extension anyway so I can meet the 12' minimum chimney height requirement, but I was just curious about the scientific rationale for this. Seems like the extension would slightly cool the flue gases before they are emitted; not sure if this is a good thing or not.

 

[begreen]

Sounds like a good question for Drolet support.

 

[fire_man]

I added 30" Single wall rigid above the masonry chimney and it improved the draft quite a bit. The stove used to backpuff badly before the extension.

Yes I know all about the cooling due to its not the proper class A pipe.
I sweep it twice a year and there is only a very light coating (<1/8") of fluffy soot but I burn super dry wood.