Singl wall is how they sho the chimney at Home Depo. I wudnt use singl wal eether. Just dubl wal owval Bvent. I kno it cost more but the Cod say to use it. But wen othr guys cut the header an bang the round vent in the stud spac, its to tite and it rubs. Yuo got to know how the bilders carpenter puts in chimnys. Yuo rich guys gort the monye to put it in rite, the bildrs dont now.