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Insulating air conditioning ducts

Constantin Member Posts: 3,782
... I wonder though at which point it will be quicker and easier to insulate the roof rafter bays + the other attic walls, close the vents, and make the attic part of the conditioned space. You gain useable space in the place as a bonus.

Sealing the ducts is a must though. It is staggering how leaky they usually are.

So far, our unvented roof is working like a champ, and the very real issue of duct losses via unsealed seams is therefore not as pressing (because the losses occur inside the conditioned space).


  • chuck_6
    chuck_6 Member Posts: 107
    Insulating air conditioning ducts

    We have standard air conditioning on our second floor - the air handler is in the attic along with the sheet metal duct work. We have ventillation in the attic. Is it a good idea to wrap insulation around the duct work? If so, what is best - R13, R19? I don't if "cooling loss" would apply like the heat loss in uninsulated pipes during the heating season.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Insulation? Absolutely!

    Firstly, seal the ductwork thoroughly and scrupulously, all cross-joints, longitudinal seams and double on corners, with a brush-on mastic (not duct tape). I like the water-based Hardcast Products (Iron Grip) but there are others. This alone is an excellent investment. Once sealed, run the fan and crawl around to find any leakage you can feel or hear and seal those too.

    Secondly, because the attic is ventilated means nothing in terms of cooling the space from a practical perspective. Sure, it will be 110F not 160F, good strides. But especially because it is vented you have the humidity issue. Your duct will drip like you read about.

    So once sealed, insulate the ductwork with bona-fide duct wrap and with an FSK (Foil-Skrim-Kraft Paper) vapor barrier on the outside.

    Thickness per most model energy codes are R-5 when in conditioned space (1.5 inch fiberglass essentially) and R-8 (2-inch plus) for ducts open to the weather as your seem to be.

    Personally, if I wanted nice, thick insulation without sagging, I would use rigid fiberglass board in two or more layers with staggered seams, to about a 2-inch overall thickness. Then go over that with 1.5" foil (FSK) duct wrap. Roll and seal all joints, stapled, then cover with aluminum foil tape.

    Sure, you can go with R-11 or R-19 batts, all well and good. But the practical attachment and the ability to seal the vapor barrier jacket becomes more complicated because of the bulk.

    The vapor barrier is everything.... mess that up you might as well not have tried anything. Condensation will form within the trapped areas between duct and insulation. This is why I favor a rigid board insulation when so exposed..

    My $0.02

  • Brad White_9
    Brad White_9 Member Posts: 2,440
    All that making the attic part of the living space will do

    is to keep the temperature gradient down. You would still have to insulate the ductwork because it will still be (or should be!) below the dewpoint of the space it is serving. If it is not that cold it will not dehumidify.

    Unvented roof? Have to get over there one of these days, Constantin... I know I have said this before, just have to find the time..


  • Constantin
    Constantin Member Posts: 3,782
    You're undoubtedly right...

    I suppose the roof area would still be warmer, even with the benefit of insulation in the rafter bays. That said, the requirements for insulation in the conditioned space on the ducts will go down because the gradients would (presumably) be reduced also.

    As for the humidity question, I presume that would have to a lot with infiltration and proper ventilation of high-humidity sources inside the home, i.e. showers, ranges, etc. If you cut down infilatration and the resultant stacking effect, then perhaps the humidity in the attic space would be close to those of the spaces below, i.e. low, perhaps low enough not to cause any dew to appear on the metal ductwork.

    I have a bit of exposed ductwork in the basement, have yet to see any dew establish itself. I presume it has to do with the cool temperatures down there and the leakage, which conditions the space even as only other zones are calling for cooling. But that is another story...

    If the attic becomes part of the daily use space, the homeowner may in fact like some leakage... keeps the place cold and spares the expense of sealing the ductwork... ;-P

    As for the unvented roof, I don't think it's that big a deal. I simply followed the logic of Joe and the other folk at building science corp and decided that my home would benefit from the same construction detail. I know there are some that expect my roof to fail in no time, but my plastic "slate" has yet to cup, crack, deform, or otherwise announce that it's unhappy/too hot. Presumably, the local climate has something to do with it.

    In places like Nevada, I'd choose a higher-temp-rated roofing product but would keep the unvented roof. The conditioned space up there is great for AC air handlers, ducting, and stays freeze-free. This is of particular interest to folk like myself who have a fire sprinkler system.

    Now, as for your visit, you've been soooo delinquent, I might as well send you a "180 days past due" notice. :-)
  • Big Ed
    Big Ed Member Posts: 1,117
    #181 Tape

    I thought the code changed in which you have to use the #181 silver tape to seal the duct work? Maybe just a local code change ?

    Brand , do you know the factor for r8 duct work in ventilated attic ? I want to update my work sheet.R6 I use 1.1 .. thanks
  • Brad White_9
    Brad White_9 Member Posts: 2,440

    Not sure on which code you refer too. Here in MA "duct tape" is still outlawed. Another post said it was because of fire retardency, first I heard that. I had understood it was because we all know it falls off.

    Most duct wrap is R 3.3 to 3.5 per inch which is where I get 1.5 inches for R 5. Technically and practically speaking you would need 3 inches for R 8 but would get R 10 by default. Not a bad thing.

    The rigid board has a slightly higher R value, not sure it makes it to 4.0 to allow an R 8 at 2 inches thickness.
  • Big Ed
    Big Ed Member Posts: 1,117
    Type of Tape

    The tape my local inspectors require is not a cloth duct tape.It more like mylar...I am pretty sure its tag number is #181. I have to check later..
  • Ron Schroeder
    Ron Schroeder Member Posts: 998

    I dutifully bow to Brad in his infinite wisdom when it comes to ducts, sealing and insulating you might want to check to see if your ducts have the insulation on the inside, I have a hard time believing that a contractor would put AC ducts in an attic with no insulation. However anything is possible even at that rate sealing and taping is a must.
  • jim lockard
    jim lockard Member Posts: 1,059
    foil tape

    UL181 is a foil tape and is approved by DOE (dept of energy) for taping seams on metal duct work. Vest Wishes J.Lockard
  • jim lockard
    jim lockard Member Posts: 1,059

    Most of that inside insulation is a sound barrier. We do not use it because we perfer are customers do not breath it . J.Lockard
  • Ron Schroeder
    Ron Schroeder Member Posts: 998

    I do not care for it either but when you go out in the field you find what was left not what you want, I also think it is a place to gather too much dust etc. Not to mention trying to clean in a few years
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Very Tru Jim

    The duct liner is primarily for dampening sound and vibration transmission. Quite often, the lining is not continuous and there are often sections where there is no lining, especially at the point where duct sections are joined together.

    In my opinion, ducts whould be wrapped with as much insulation as physically possible, making certain to seal ALL seams. It seems wasteful to spend money to cool air only to have the conditioned air pick up heat from the attic. If the attic is at a temperatuare of 130 degrees in the summer and the space temperature is 75 degrees, you are looking at a supply air temperature of about 55 degrees. This results in a 75 degree delta-t between the air in the supply duct and the air in the attic. No matter how much you insulate, you will always have a certain amount of heat transfer between the attic air and the supply air.
  • Brad White_90
    Brad White_90 Member Posts: 1

    Good points Bruce and Jim on lining. A "rapping" with the knuckles will tell if it is lined or not.

    In the past four years I have designed two major performing arts centers and assisted in another. The acousticians insist on lining. Lining has the same R value as equivalent fiberglass wrap. It has been long standing practice to use 1" liner and it has been practice to wrap only what you do not line (in supply ducts and return ducts in unconditioned spaces). But the 1 inch standard liner as a default, thermally does not cut it by the energy code, so we go to 1.5 or even 2 inch liner. Those ducts get BIG.. And very quiet too.... Add perforated or even solid liner for mass and you have one expensive sheetmetal job!

    As to where they butt, we do specify a 1/4" overhang so they compress but as Professor Siberstein says, there may be gaps. With Ductmate or TDF mastic we have not seen sweating of late but something to watch for.

    Still unless there is an acoustical imperitive, I do not line ducts. But fact is, liner has come a long way from the fiber days and is treated to prevent fungal growth. Wonder what the heck that is!
  • Christian Egli_2
    Christian Egli_2 Member Posts: 812
    New product, new magic ceramic paint

    A salesman has shown me a product I had long set aside in the category of black magic, but this time, he got my attention and showed me pictures and samples. So now I want to know more.

    I'm talking about the ceramic paint (a mixture of latex paint and ceramic fiber - probably a calcium silicate - but I don't know) that can be spread on the outside of buildings, and as a sealant for objectionable fibers on pipe insulation , and all around air handling systems and ducting.

    The color is almond, it is softish, velvety, and applied to a thickness of about 1/16 inch. There are proprietary application procedures.

    Of course, I forgot to pay attention to such details as the name and type.

    Is this stuff worth it? Does it really have the fantastic R-value? Any experience? The cost is higher than fiberglass, but the looks are worth it. That's if you send guests up to admire your attic.

This discussion has been closed.