Have HVAC Questions? Ask an HVAC Expert for Answers
You need to work out the inside area of the existing ducts and then match them up to their metal equivalent. If you would like help with that let me have the original dimensions and I will work it out for you.
Yes by tapering down this ensures that the flow rate stays constant. Supply is 15 x 8 = 120 sq inches / return 18 x 8 = 144 sq inches. So any ducting that when you measure the length x depth equals the two sizes respectively. I would however allow for some insulation to keep the heat/cold inside the ducts.
You are insulation to stop condensation, so rockwool or glass fibre. I think you will struggle to taper the metal duct so I would try it for a couple of days if needed you could always put a restricter in at the end of the duct. I think however it would work fine, its not like you are going up twenty floors.
you can reduce the size of the duct by making the hole smaller right at the end.
Question, why dont you use plastic ducting ?
I'm not sure that you can use it to code in your state but here is a link to an article that you might care to read --