I live in the state of ma. My question is should I use flexible duck or metal duck to bring heat to a secon floor. Flexible duck for inside wall.

No flex in walls.

Metal duct has a smoother inside surface then flex duct and is less restrictive to air flow then flex, if the metal duct is insulated on the out side then it is noisier then flex, the metal duct will cost you more but will be a better installation for the long hall. I suggest having them insulate it on the inside for a quieter system and for heat loss or gain. Good luck, Mike

Insulating metal duct on the inside can cause mold entrapment if it ever gets any moisture in it and with central A/C this can be a problem. It can never be cleaned correctly or disapate mold problems unless you take out all the internal insulation in the metal duct to clean it. Also insulating metal duct on the inside also reduces the square inches available for air flow and that reduction will have to be taken into account for your CFM calculations.

External insulation does not present any of the above problems. Forty years in the industry taught me much but each person is entitled to his opinion.