Insulating duct run in vented crawlspace

Magus

I have an addition to my home that is about 3.5 feet above ground level. The area underneath is vented. My question is insulating heating duct that runs between the floor joists. One side is attached to the floor above with one side against the floor joist and the other side has about an inch of space. Since I can’t get to 2 sides of the square duct, my thought was to just buy a sheet of Styrofoam and cut it so it would fit across the floor joist, attach it and caulk if necessary. My concern is moisture, any comment are appreciated.
My first post, please be patient if edict is incorrect.

#CARRIERMAN

Hi Magus

Most of the time rectangular duct is inulated on the inside. If yours is not, it sounds like you may not be able to. But heres the problem with uninsulated ductwork, in the winter time its not so cruical other than heat loss. In the summer time is when it becomes a problem. You and I live in the same climate area. In the summer time the temperature differential and the humidity cause the ducts to sweat. If you incase unisulated duct it will actually cause moisture to build up inside the cavity causing mold to grow. This is when you run into trouble. Let me know if you need any further assistance on this matter.

Good luck
Rusty

Magus

Hey Rusty,

Thanks (I think) for confirming what I suspected. Since nothing was done right on this addition, its probably 40 years old or so, I’m guessing the rectangular duct is un-insulated. I just finished replacing all the insulation underneath since it was reversed and while I didn’t find any mold on the wood, the insulation was discolored in many spots with black mold. The rectangular duct runs to the wall and then T’s into round ducts that run under the joists to the floor vents. I guessing the real solution is to replace the square duct with round, run it under the joists and then insulate the new and old ducts.

Thanks again for your comments,

Joe