Your contractor needs to fix the problem. It is not normal and removing the insulation is not a solution.
FYI,
This is an unfinished attic, without climate control atm.
The roof shingles were recently replaced, but not all of the plywood decking underneath was replaced.
I would take some burning sticks of incense and let it smoke all around where the cold air return joins the ac unit in the attic. You will see pretty fast if attic air is being pulled into the AC.
I don’t understand how the attic environment is making it’s way into the house. The unit should pull from the returns, through the unit and out the outlets. I don’t see how you would be getting attic smell in the house. It is basically a closed system unless no returns were put in. In that case it would pull air from the basement if that is were the unit is. Is the unit in the attic? I never heard of that being done in the NE before.
It sounds like you have been doing all the duct taping on the supply side. What you need to look at is the return ducting and how it is sealed. Also, you may need to have the dusting cleaned. Once you get a lot of dust or insulation material into the duct work it will take a long time to be blown out during normal use.
From the air handler to the vents you have positive pressure. From the return grills to the air handler you have negative pressure. Check the return ducts and make sure they are all taped. Applying mastic to all joints in the system in not a bad idea either.
It sounds more like the air supply/return problem, not the ductwork.Where is the air return filter?Is it in the celing or did they come down an interior wall?
Problem is more likely to be associated with the return ducting or the point where the return duct meets the air handler. Leaks at these points suck in attic air and pull it into the house. Leaks in supply ducting will usually not have such an effect.
Many power companies have a low or no cost program where they test the duct system for leaks. Mostly concentrate on the supply ductwork, though. In humid climates, especially, leaking supply or return ductwork causes huge problems, besides the energy wasted.
When ducts are installed, most of the time the interface between the duct boot and the ceiling is not properly sealed. Which can allow attic air into house, after all it’s not just the AC, there are a whole bunch of new holes in the ceiling. Seal with aluminium tape, caulk or foam.
Best approach I’m aware of for most climates is to turn the attic into a semi-conditioned space by moving the insulation to the underside of the roof with spray foam. Must be done properly and building science implications worked thru, or Bad Things can happen.
Excellent site for anybody with a home. Huge amounts of free information. Technical level varies.
www.buildingscience.com
For info on foam insulation.
http://www.icynene.com/
Probably should have replaced all of the older insulation as well.Since dust will collect on it through the years.Another issue would be to assure the ductwork was completely insulated or condensation would build up and ruin the ceilings.That could be what that musty smell is.
Good luck with the problem.
Get the HVAC contractor back. Problem sounds like it is in the return side not the conditioned side. I would start where it is pulling the air back. Suspect you will find it is being pulled through a unsealed wall or ceiling cavity somewhere.
The other thought I had was that you may be using the wrong tape. Duct tape is not for ducts. You need special tape that is suited for sealing ducts. Duct tape will fall off and not make a proper seal.
Sounds like the attic wasn’t properly vented, who was your roofing contractor?
Hoses are likely leaking in the attic creating positive air pressure in the attic and with the attic not properly vented you are getting the smell and dust coming in around your registers.
I had a new house and had a guy install satellite tv in my attic and crushed my hose to one of the rooms, you have to watch these contractors like a hawk.