The county inspector dinged me on a couple of things for my bonus room renovation. One of these things is wrapping the register boots (endpoints) in insulation. I've seen registers with this insulation already installed, but he mentions that batted insulation is fine. The reason for this is to supposedly reduce the condensation leaking outside the boot and onto the wooden frame, joist, and sheetrock. Any idea what you folks do to accomplish this? I have some old R-6 flex duct insulation and some foil tape and was planning to wrap the batt around the boot and tape it up with foil tape.

3 Answers 3


You're thinking about this too much. The goal is to reduce air contact with the metal. Simply get yourself some R-11 or R-13 fiberglass batts and pack the joist or ceiling space loosely around each vent component. Condensation is greatly reduced by doing so, and your obligation is fulfilled.

  • Thanks. I tried doing this and it looks damn ugly and I'm not sure if I'm packing it too tight and losing the insulating quality of the batts. Do inspectors usually care if you use insulating ductboard or something similar to this? Would be much easier and I can easily assure no air between the boot and foamboard.
    – saigafreak
    Feb 22, 2016 at 15:39
  • I doubt you could reduce the R value of an R-11 batt to below the R-6 that insulated ducting would have even if you jammed it tight. That's not a concern. What does "ugly" have to do with it? I appreciate craftsmanship as much as anyone, but... it's insulation. :)
    – isherwood
    Feb 22, 2016 at 15:43

you have to understand the dynamics involved. the goal is not just insulation, but intactness of the vapour barrier. you can have all the insulation in the world and it will do you no good if you allow nice warm air to enter into the insulation pocket. the warm air cools as it permeates the batt and eventually starts to cool. once it drops to the dewpoint of the air, the trapped moisture in the air precipitates out and now you have wet insulation. when the cold air in the uninsulated space reaches the wet insulation you are going to get ice. then the cycle repeats, ever closer to your inner heated space. if it warms up, the ice melts and now you have wet insulation. gravity and capillary action draw the moisture through the batt and eventually you have moisture up against the wood and drywall. if you use batting and don't ensure the air barrier is intact, you will make a problem for yourself. you can do it with batting and very careful use of sealing tape and caulking. why bother? polyurethane foam is used for a reason. its a great insulator that doesn't lose r-value over time and its airtight right out of the gate. just foam it. the foam blocking or forming i described is just to minimize the amount of liquid foam you have to use. you can use straight spray foam if you want as well, its just more costly. your time and money.


its a little hard to tell what you have going on. i am assuming these are ceiling hot air vents in a room below an insulated attic space. use 2" styrofoam board to build a "form" aroun the register termination (tighter the better to use as little foam as possible). you want to do it so the adjacent joists and the foam act like a box around the ductwork. then inject sprayfoam into all the places you can get it. this way you have an r7 or higher (depends on the foam) box that is not only insulated but airtight. this is the best way to do it and here in ontario (and it meets the new revised code specs) we have lots of these types of reno special heating systems that run insulated ducts through an attic.

  • Inspector actually specifically mentioned using batted. Not the pink stuff but that silvery stuff you find inside R-6 or R-8 flex duct. He also told me that he didn't want to see construction screws connecting joists to rafters but whatever. Have you tried using batted to cover the boot before?
    – saigafreak
    Feb 19, 2016 at 16:50
  • The material in flex duct insulation is also fiberglass, in most cases. It's just typically a coarser variety, which I assume is cheaper to produce. Batts are a finer fiber to reduce itching and discomfort during install.
    – isherwood
    Feb 19, 2016 at 17:44

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