info on raised flue

[treefinder]

my buddy has a lightening 3x8 raised flue and we were wondering in the back do you fill in the back under the flue pan up to the pan or do you leave it all open behind the ramp ?

[maplwrks]

Fill it right up level with the rails---scoop out about 6-10 inches immediately before the stack.What are you going to use to fill the arch?.....sand? vermiculite? This scooped area is important, as the amount you scoop out for your smokestack will effect the way your rig boils. This will force all heat and fire through your flues.

[tuckermtn]

If you are speaking of bricking/insulating, then yes-

On mine (early 1990's vintage) I did brick and arch board up the "ramp" ...near the top of the first baffle (his may or may not have one based on vintage) I switched to ceramic blanket- ran this right up under the raised flu, using vermiculite to fill the void under the ceramic blanket. Just before the stack, I let the ceramic blanket drop down to allow the gases to go up the stack...