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Best insulation for spiral duct?

How would YOU insulate spiral duct AC plenum from cooling loss and condensation in an UNconditioned hot moist attic?

Mike T. of Swampeast, and others here , use Spiral Duct for the plenum of the high velocity systems. I plan to use it.Went to see some today.

It comes in 10 foot pipe sections.
I think I will nestle the pipes on 2x4 blocks on the attic floor. And wrap pipe in fiberglss insulation and sheet plastic sealed with acoustical sealant [doesn't dry out], and finally heap my existing insulation, R38, over all of it.

And perhaps spread some sort of trough of plastic sheet a few inches below the pipe to trap and let evaporate any drips. Hmm, and hold up insulation a tad above the trough so it stays mostly dry. Sheesh, what is better?


  • Brad White_9
    Brad White_9 Member Posts: 2,440
    The best thing you can do

    is to seal the living cr*p out of the ductwork, even the spiral seams. (We specify it in just those terms, sort of :)

    Even spiral ductwork has minute leaks along the runs but they add up. A good brush-on mastic is best, applied to clean metal. A rag with some 409 or TSP solution then a rinse-wipe and let it dry to get the grease off.

    A small leak will cool the insulation in spots enough to get condensation.

    I would get good quality (Knauf or Manville) duct wrap, R-8 if you can get it, with reinforced foil on the outside. If you are running a Unico or other High-V system, these run at colder temperatures, so thickness and integrity are twice as important. Make sure you have at least R-8 in that case. Do NOT use two layers of R-5 unless the inner layer has the vapor barrier removed. Save the vapor barrier layer for the very outside.

    Roll the seams and staple, then seal with aluminum foil tape, lengthwise and around the joints. An occasional band of plastic tie strap is a good thing every five feet or so but do not snug it because it will compress the insulation.

    At support points (your blocks), I would add another layer of bare insulation underneath, right on the duct, before wrapping. This will compensate for minor compression. Then go ahead and lay your attic fiberglass atop the duct. It will not stop moisture of course but your properly insulated an barriered duct should be fine.
  • joe_94
    joe_94 Member Posts: 39
    Interesting that spiral seeps a bit.

    Thanks for good tips.
    Interesting that you have seeen small leaks in the spiral.
    Guess I'm in for some goopy times with sealer. joe
  • Brad White
    Brad White Member Posts: 2,398
    It shows up during

    pressure testing and especially when it is outdoors. I dead-headed about 150 feet of spiral round and flat oval once for pressure testing. At 6 inches pressure, the leakage was extrapolated to about ten percent of airflow and about 6 CFM per 100 SF of duct area. Once sealed thoroughly with hardcast it went to practically zero leakage, maybe 4 cfm total, our ability to measure. Who knew?
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    I have to reiterate

    I have to reiterate what Brad said about wrapping a duct twice with R-5 isulation. You must remove the vapor barrier from the inner layer.

    Doing anything less would be like applying SPF 15 twice thinking that you will be getting SPF 30 protection. (Don't laugh)

This discussion has been closed.