#1.Totally possible for diy.

#2 Spray foam gains popularity every day and is my first choice. Since you included spray in options I assume you have or can rent the required equipment.

#3. Interior insulation only works if done during mfgring/fabercating but duct is rarely insulated both inside and outside.

Ridgid foam would be a pain and spray foam expensive for such a small area so i think cavity fill fiberglass batts would be the best choice and its DIY.

Do you have access to the outside of the ducts? That's really the only practical way to insulate them.

Yes I do. So only insulate the exterior end? What product do you recommend? @sktn77a

Thanks @sktn77a Here is how the exterior end of the duct looks. I assume that's where I need to do the most insulation. Any suggestions after seeing this? It's an odd space and we wanna keep the cold and bugs/pests out as well. .

I thnk we're talking at cross purposes here. You need to insulate the outer surface of the duct (in this case, behind that wall), not the exterior end.

@sktn77a oh so when you said "Do you have access to the outside of the ducts? That's really the only practical way to insulate them" you meant to literally wrap the exterior of the metal duct in insulation?

If so, thats not possible to remove the ducts. I'm more imagining stuffing the duct with cavity fill fiberglass insulation to keep the air from coming in... Does that make sense and will it help do you think?

"stuffing the duct with cavity fill fiberglass insulation to keep the air from coming in "

You mean on the inside of the ducts?

@sktn77a yes exactly. Won't that help? Insulating the outside of the duct won't work. There's no space to fill unless I removed the duct itself and the cold air getting in isn't that bad.

@sktn77a to clarify, we previously had these ducts used for exhaust from our furnace. We replaced the furnace with heat pumps so we don't need the ducts any more. They are simply being used as a channel for the piping from out tankless hot water and mini splits to run ouf of the house. See photo. So insulating the inside of the duct isn't a problem since we no longer need fresh air or exhaust for our furnace.

Ah, OK, sorry, I misunderstood what you were actually wanting to do. Just stuff them full of some form of blocking/insulating material (fiberglass would work).

@sktn77a Ok! Makes more sense. sorry for the confusion and thanks for all the help.

@sktn77a hi there. Last question. Mind giving me advice which insulation batt to get?

https://www.homedepot.com/b/Building-Materials-Insulation/Batt/N-5yc1vZasbsZ1z0utp9

I see various R# ratings (R15, R30, etc). Any idea which is best for this job? My local Home Depot staff isn't very helpful.