My smithy is only 8' x 12'. Have 3 burner gas and a 18" rivet forge the rivet forge well have a 12" chimney. The smithy is made from 2 x 4s and T-111 outside,inside is cement board ceiling with corrugated steel on walls. Floor is gravel,the smithy is off the ground about 3" all the way around except at the doors and 10 cement pillars. The smithy is also completely insulated with R-13.
How would I go about ventilating and how much air will I need.