Hi DIY, I recently started work on upgrading the attic insulation in my three level mid 70s townhouse. All of the upstairs HVAC ducts are run through the attic and the main duct that feeds them comes up from the 1st floor to the attic through a slightly oversized void space running vertically straight up through the house. There's about 8 inches of extra space between the HVAC duct and the walls of the void space on all sides. I can feel significant air movement coming up from the void space into the attic. Because this void space is directly against the concrete firewall between my house and the neighbors house I feel that I need to a) seal the HVAC ducting so it doesn't leak b) fill the void space with insulation to reduce heat loss through the bare wall. My questions are this: is there a way to seal the vertical HVAC duct in the void space (I'm assuming that's why I can feel air coming up)? I can not reasonably fit/lower myself into this vertical void space. The second question: if sealing the duct is not reasonable will blown in cellulose be dense enough to block air movement when packed slightly? Thanks all! via /r/DIY http://bit.ly/2TFWEm1
