Fogging HVAC

So I want to fog my HVAC system with concrobium biocide for mold. I know it is not an ideal fix but it’s the most affordable option… The mold outbreak is in the basement and fogging the HVAC is just one step in my remediation plan.

I have a gas furnace and a/c unit. I want to fog the system without getting fog on the main and top level of the house. It is a 3 story house. There are 2 vents in the basement, 9 on the main floor, and 3 on the top floor. I will leave the basement vents open, cover half of the other vents with plastic sheeting, and vent the other half outside with ducting hose.

My concern is I will create a build up of pressure in the ducts and force a leak. Is this a valid concern? Any advice would be greatly appreciated.