"I was just speaking with a fireplace construction expert, and a food smoker himself, discussing my similar situation. I have trouble getting my offset smoker to get up to cooking temps. After looking at my design, he explained that I had too small of a flue which kept the heat and smoke from moving into and through the smoke box rapidly enough. The ratio of the firebox opening to the flue size should be at least 10:1. He explained that the smoke and heat can only get into the smoke box if it able to move from the fire box through the smoke box and out the flue. He also noted that, unlike a fireplace with a chimney, smokers of this kind do not rely on any "real" draft effect and if the air/smoke flow from the fire through the smoke box is constrained all the heat will remain in the firebox."
In my original build, I utilized a 4" by 10" inlet air steel slide damper and a 2" dia. steel pipe exhaust port that I purchased from a BBQ Smoker Accessories Web site for the flue. The opening in the baffle wall is 4" by 12". Until speaking with that expert, I did not understand the need for a bigger flue. Although I had smoke, it took forever to get cooking heat into the smoke box with out creating a very large fire.
Today, I went to Lowes and purchased the following: A 6" galv steel round duct starting collar for $4.48, a 6" galv steel round flat plate damper with key for $4.45 and a 6" galv steel round Rain cap for $10.28. After removing the 2" pipe exhaust port, I cut a 6" dia hole in the roof and installed the foregoing listed items. I used foil faced "Seal Tight" flashing to seal around the collar to keep rain water from leaking into the smoke box. I happen to have this flashing but any suitable sealant could have been used just as well. It is not the best looking flue, but I want to report....IT WORKS! I'll probably paint it black.
I start the fire using paper and lump charcoal. Once lit, I added three pieces of firewood, each about 4 to 6" in length. As soon as the fire began to burn well I closed the firebox steel plate lift-up access door and the temperature in the smoke box rose to over 300 degrees in just about 15 minutes. I was preparing to smoke a pork loin and was able to throttle the temperature down to 225 degrees by closing the flue damper about half way and closing the inlet air damper about a quarter of the way. It maintained the 225 degree cooking temperature for the two hour cook with little flame and no need to add any firewood.
My smoke box is constructed of wood and sided with T1-11 pre-primed OSB siding. The box and the box door is well sealed and does not leak smoke. I lined the interior with 1" foil faced foam insulation board (foil face in). My smoking grates (2) were fabricated locally using 16 gauge expanded metal attached to an 1" by 1" by 1/8" angle frame. I lined the firebox with a 1/4" thick coat of refractory cement and painted the exterior of the entire smoke house and masonry foundation/firebox with a waterproofing elastomeric paint. This is important to prevent the masonry block from absorbing any moisture. When heated by the fire the moisture in the masonry will turn to steam, expand and destroy the block. I use a drop in expanded steel fire basket in the firebox which I also purchased on line from the same Smoker Accessory site.
I hope this information will be useful to anyone contemplating building a back yard smoke house with an offset fire box. All of the designs I had reviewed prior to producing my own design, did not give any information regarding air inlet opening, baffle opening or flue sizes. I think it would be a great idea if some mechanical engineering wizard out there would put together some helpful data reference charts for these kind of builds. Would be a great addition to this site.