Metal duct-work in unconditioned space is not a good way to go.

The high mass of the metal allows it to absorb a lot of heat (or lose a lot of heat) that the system must then overcome.

Duct-board has a much lower mass and less of an impact.

Thanks for the reply, the next question than would be what is the most efficient insulated ducting currently on the market, I would ratrher listen to the opinion of an tech than to manufactuer claims

Do you have AC? Do you want it? Consider talking with a local building expert or independent energy rater. In my location, the advice would be to seal the attic and spray-foam insulate the roof deck to keep your duct leaks inside the living space. I live, however, in the humid Gulf South. I don�t know if that is advisable where you live.

Heat gains into the attic and into the ducts are mostly radiant, anywhere. Here, the spray foam greatly reduces air infiltration along with the moisture that comes with it reducing AC load. A radiant barrier will not approach the savings.

I include that detail to illustrate the kind of thinking that needs to be done. Michigan will be different. That is why you need a GOOD local energy rater. You are contemplating drastic changes to your home. They can help you identity the most cost-effective ways to improve your home in addition to the HVAC system. There are sometimes programs that subsidize the cost of evaluation.

Will a heat pump work or is it too cold? Can a high-velocity, forced-air heating system be installed as sleeves in your current ducts? (The ground has a pretty moderate temperature compared to your current attic.) Could you snake hydronic heating pipes (PEX) though your current decrepit ducts and use them to feed radiators in the rooms? That would be sweet if you only need heat. If you want AC, consider mini-splits. Heat pump, mini-splits could provide heat when the temps are moderate and you can use the hydronic for the coldest weather.

Use 3 inch thick duct board.

Apparently 3 inch is not easily available in some places, so go with the thickest stuff you can get.

how long can your flexible ducts be that snake through attic