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  1. #1

    Confused Insulating Ducts with Bubble Foil AND Fiberglass Insulation?

    Hi- I just finished cleaning and sealing all return and supply ducts with mastic and insulating the attic floor to R-30. Now I'm about to start the duct insulation and the sealing electrical/drywall gaps etc.

    I have a couple concerns about the duct insulating and future condensation.

    I live in Fort Worth, TX so it gets hot and can be humid. I have this crazy idea of using bubble wrap insulation (I'm not here to debate it's worthiness at all) and then using fiberglass with aluminum backing on top of the bubble wrap.

    My thinking behind this has nothing to do with the insulating qualities of of the bubble wrap since I don't plan to do all the labor required to get the air gap between the duct and the foil. I'm thinking about the bubble wrap for it's vapor barrier and air sealing potential, the r-1* that it would provide between the duct and fiberglass is just a bonus in my reasoning.

    If I wrap the ducts with the foil bubble wrap insulation (marketed as radiant barrier) directly on the ducts while sealing it with aluminum tape and then wrap over that the aluminum backed fiber glass insulation, aside from this possibly just being unnecessary and obsessive, is there any detrimental affects that could occur from condensation (rust/mold) within the layers if applied as mentioned?

    I can clarify and post additional information, if needed. In my head this sounds like a decent idea, but I just don't know if it's a complete waste or even worst, if I could create a mold or rust problem in the future or ruin the r-value of the fiberglass from this configuration.

    I've got a couple more days of brainstorming before the actual work gets done so all possibilities are welcome.

    Thanks in advance.

    Eli.

  2. #2
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    ou may have this backwards

    With the vapor barrier in mind, I think the FG wrap first then the bubble wrap as a second vapor barrier,in contact with the Wrap Vapor barrier is better.

    Please understand that it is very difficult to maintain the integrity of the Wrap Vapor Barrier so how is it that the duct doesn't sweat unless there is a large tear in barrier? It is because a small/ expected amount of leakage will put relatively dry air in the insulated space between duct and vapor barrier.
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  3. #3

    maybe... but

    Well... I was thinking I could have it backwards too, but here is my reasoning for putting the bubble foil FIRST:

    1) People typically use the bubble foil by itself and I'm guessing that in many installations they fail to get the air space between the bubble foil and the duct OR overtime that airspace collapses for various reasons and there is direct contact of one of the foil's vapor barriers with the duct. The manufacturers of the bubble foil even recommend using the bubble foil as the material for the spacers to get the air gap in many cases... so, if condensation that would cause rust would be common from this, I probably would've heard about it or read about it in another thread somewhere?

    2) If I put the fiberglass first, I'm running the risk of future owners ripping everything out and doing tons of unnecessary work because they see the bubble foil on top and automatically think "snake oil" and "garbage" ... or I'm running the risk of an inspector failing the job because they don't understand what's actually happening ... or even worst, I'm running the risk of one of the future HVAC contractors coming in and selling the future owner on a reinsulation job that is actually much more expensive and inferior to what I can do right now if I can just get my science straight before beginning the work.

    The attic gets really hot right now (I have plans for a radiant barrier, but need to get the ducts finished first). I've been taking daily temperature readings at 140F around 3pm and the air pushing through the ducts will be coooooold once I get the unit all attached and powered up. The fiberglass insulation that was on the ducts already had plenty of small pin holes and the previous owner/installer didn't wrap any insulation under the plenum due to awkward positioning which creates extremely demanding physical labor. So... it is somewhat comforting that you say there is an expectation of the fiber glass's vapor barrier being imperfect... with this in mind, if the bubble foil outer layer were to receive condensation at the fiberglass contact, it probably would dry out quick enough or be as if it were in direct contact with the duct metal making it no more of a condensation hazard than it would be without the bubble foil in place?

    Just thoughts.

    I was also thinking that if the bubble foil goes on as the outer wrap the moisture problems could be worst because the bubble foil acts as a near perfect vapor barrier... so any moisture that gets in there won't find a way out too easily when it can just sog up the fiberglass or if it can sit between the fiberglass foil and the bubble foil.

  4. #4
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    In cases like these it would be hard to find someone who tried this before and who knows based on experience, so you'd have to rely on physics. Warm air holds more moisture than cool air, and condensation occurs on duct surfaces when a large volume of air is cooled enough by the duct surface. Faced insulation helps because there's only a little bit of air hugging close to the surface of the duct that can cool enough for condensation, plus the facing prevents more humid air from coming into the insulated space. Let's say you put foil first. The outside of the foil would be warmer than the inside, so the air trapped inside the fiberglass layer would not get as cool as if it was directly against the duct without the inner foil layer. Therefore condensation is even less likely. Based on physics I doubt you would have condensation problems, in fact I think your idea would help because you have a double vapor layer preventing new humid air from getting close to the cold duct.

  5. #5
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    You expressed a concern about future owners thinking your ducts are bad because they look different. If you could get a duct leakage test from a professional, and also a static pressure test, that would document how good your system is. And likely would reassure future owners. I have reasonably high confidence you will apply lots of mastic and the result will be good sealing. This is the kind of thing I expect a homeowner will do well because he really cares about the result more than the time spent. No disrespect meant to HVAC professionals, but this is a job with a high labor/payment ratio is it not?

    I sure wish we could invent cheap wireless moisture sensors and apply them to see when the dreaded condensation actually happens. We could learn more by comparing actual results to theoretical, where it most matters.

    Best of luck -- Pstu

  6. #6
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    I've seen ductwork with double vapor barriers
    the insulation condensates between the vapor barriers
    and the insulation value is zero.

    I wouldn't ever do it. even on hard pipe the ductwrap should
    be single layer..lots of folks insulate over existing ductwrap
    as an 'upgrade' this actually causes more problems as the
    hard pipe has lots of leakage sites and this moisture is drawn
    into the duct system.

    not a good idea...and in my hot humid climate...a really really
    bad idea.

    imo it is a lot of labor for no benefit.
    your mastic sealing will provide the air seal.

    best of luck
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  7. #7
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    Quote Originally Posted by energy_rater_La View Post
    I've seen ductwork with double vapor barriers
    the insulation condensates between the vapor barriers
    and the insulation value is zero.

    I wouldn't ever do it. even on hard pipe the ductwrap should
    be single layer..lots of folks insulate over existing ductwrap
    as an 'upgrade' this actually causes more problems as the
    hard pipe has lots of leakage sites and this moisture is drawn
    into the duct sys
    I believe what you observed is true, but I have a hard time believing this based on physics unless there were some leaks in the ducting (in which case it's a problem with leaky ducting, not because of double layers of insulation). Can someone explain this based on physics to place the blame on the double insulation? The laws of physics don't change, so I tend to think this result occurs because people who "upgrade" by adding another layer are failing to check the metal duct work for leaks.

  8. #8
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    I like Energy rater's explanation

    She is not talking about double insulation, but rather double vapor barrier with duct wrap and like the general Building Science rule about double vapor barriers capturing moisture, double wrap does just that.
    I do think since ther is no Fuzz in the bubble then the either way the duct/bubble or wrap FSK/bubble would be a double barrier and no sandwich like the double wrap problem

    Hows that explanation sound to Energy and Scorpion?
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  9. #9
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    Thoroughly sealing the ducts is the first priority. Using a good non-hardening mastic is best and using a backing where there are large holes is better. Once the duct is thoroughly sealed, you can use a duct blaster to check it for additional, undiscovered leaks. Get it as tight as possible, then insulate.

    The concept of moisture barrier and thermal efficiency are two entirely different issues. The moisture you should be worried about isn't what's in the ducts, it's what's in the attic! If your ducts are tight, there is only thermal transfer about which to be concerned.

    Think of it this way. The duct is tight, so nothing is leaking out or in. It's just a conduit through which the room air will travel.

    The insulation is another issue entirely. So let's have a look at the dynamics. Let's assume the duct temperature is 55F and the attic is 140F. Let's also assume that the dew point of the air in the attic is 90F. We can now see, that if 140F attic air can come in contact with anything that is 90F or lower in temperature, condensate moisture will appear. So the trick is to keep the air of 90F or less, totally away from the attic air. Now we just need to understand that the conduction of warm temperature migrates inward toward the duct (heat always flows from warmer to cooler, right?) at some point, the 140F temperature is going to cool down to 90. Where, at what depth of the insulation that temperature is reached is a function of the R-factor of the insulation. But, if the exterior of the insulation is moisture tight, then there is no moisture to condense out of the temperature in the insulation when it reaches 90F or less.

    So as you should be able to see, it's keeping the insulation moisture barrier sealed so the hot, damp air can't penetrate. Then we don't care about dew point within the insulation as the outer surface will always be above 90F and therefore no surface condensate will appear. If you allow the moisture barrier of the insulation to be penetrated, then you'll indeed end up with wet insulation. And if you use insulation that's too thin (low R-factor) then the surface of the insulation may be 90 or less and condensation will form on the surface.

    So seal the ducts, use a very high insulation R-factor with a good, sealed moisture barrier on its exterior and you won't have any condensation issues. Try and defeat the laws of thermodynamics and, well, just don't do that and you'll be okay.

  10. #10
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    Quote Originally Posted by skippedover View Post
    Thoroughly sealing the ducts is the first priority. Using a good non-hardening mastic is best and using a backing where there are large holes is better. Once the duct is thoroughly sealed, you can use a duct blaster to check it for additional, undiscovered leaks. Get it as tight as possible, then insulate.

    The concept of moisture barrier and thermal efficiency are two entirely different issues. The moisture you should be worried about isn't what's in the ducts, it's what's in the attic! If your ducts are tight, there is only thermal transfer about which to be concerned.

    Think of it this way. The duct is tight, so nothing is leaking out or in. It's just a conduit through which the room air will travel.

    The insulation is another issue entirely. So let's have a look at the dynamics. Let's assume the duct temperature is 55F and the attic is 140F. Let's also assume that the dew point of the air in the attic is 90F. We can now see, that if 140F attic air can come in contact with anything that is 90F or lower in temperature, condensate moisture will appear. So the trick is to keep the air of 90F or less, totally away from the attic air. Now we just need to understand that the conduction of warm temperature migrates inward toward the duct (heat always flows from warmer to cooler, right?) at some point, the 140F temperature is going to cool down to 90. Where, at what depth of the insulation that temperature is reached is a function of the R-factor of the insulation. But, if the exterior of the insulation is moisture tight, then there is no moisture to condense out of the temperature in the insulation when it reaches 90F or less.

    So as you should be able to see, it's keeping the insulation moisture barrier sealed so the hot, damp air can't penetrate. Then we don't care about dew point within the insulation as the outer surface will always be above 90F and therefore no surface condensate will appear. If you allow the moisture barrier of the insulation to be penetrated, then you'll indeed end up with wet insulation. And if you use insulation that's too thin (low R-factor) then the surface of the insulation may be 90 or less and condensation will form on the surface.

    So seal the ducts, use a very high insulation R-factor with a good, sealed moisture barrier on its exterior and you won't have any condensation issues. Try and defeat the laws of thermodynamics and, well, just don't do that and you'll be okay.
    Would your explanation work with 75-80 dew point? I think it would. Because 90 is pretty extreme, like in Arab Gulf island regions. I don't think anywhere in the USA is likely to exceed 80.

    I have a ventilated attic and we see 75 dew point during the summer. Measurements in the attic are well under that -- not explaining why just what is observed.


    Thanks -- Pstu

  11. #11
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    ok so I thought a lot about this last night when I couldn't sleep.
    I try to stay away from all the mathimatical calculations, as most of
    my clients want answers, not calculations or theories. they don't hire
    me to confuse but to solve. sometimes y'all make my head ache with all the
    calculations..I'm just sayin..LOL![

    a comment was made about someone actually seeing the condensation between
    vapor barriers on ductwork. I've seen this several times. then physics to explain...
    I'll let Joe answer this: this is just a portion of the article
    bsi-049-confusion-about-diffusion?

    Diffusion is supposed to be easy because all you need to know is one equation—Fick’s Law—and you are done. Ha. Trouble is that the equation is only an approximation, and we can’t really measure the material properties to make the equation work anyway because the properties change with both temperature and vapor pressure. And there are many different materials. Too bad for us that there are temperature differences and vapor pressure differences across assmblies. Too bad for us that there are many materials making up assemblies. Too bad for us that everything varies all over the place all of the time.2

    And what we call diffusion isn’t diffusion anyway; it’s a combination of diffusion, capillarity and surface diffusion with simultaneous flows of varying magnitudes sometimes in opposite directions. And the physics is not clear or agreed upon or understood. Consensus? For that you have to go to the IPC.3 You won’t find it here. But in the end, it doesn’t matter to me because I’m an engineer, and I don’t need to know all of this as long as I know what happens in the end.

    Here it is. Ready for the answer? All the action happens at the surface.4 That’s it. That’s the magic insight. That’s what you gotta know. A blind man could see it. OK, maybe not. Work with me on this. Ever wonder why dew point calculations never make sense? Think about it. They say that condensation happens in the middle of wall cavity insulation. Ever see any there? Huh, huh? Never. So dew point calculations are a lie? Not really, but folks tend to get the wrong impression with them. It turns out that context is everything. For starters, ASHRAE Handbook—Fundamentals has the dew point calculation thing right, but no one follows the actual ASHRAE procedure. And certainly no one pays attention to the assumptions and limitations for that calculation. Ah, that assumption and limitation thing again. For yuks, you ought to go read Fundamentals.
    __________________________________________________ ________________________

    in this article they show the calculations that show where condendsation 'should' occur in a wall assmebly..but where does the end product actually appear? at the warm surface of the wall.
    the action occurs at the surface....
    now apply that to ductwork. for the sake of simplicity lets apply that to hard pipe ducting.
    we all know that stuff leaks like a bear unless every seam and joint is mastic sealed.
    the old duct wrap that is removed to do the mastic sealing (in retrofit jobs) shows each leakage site as dirt/dust trails that match up exactly where the seams and joints that are not sealed are. so that is duct leakage...

    the condensation occurs at the duct where the insulation is not in contact with the duct..
    a good example is where the duct leaves the plenum..mid day this time of year in my hot humid climate this is the area that condensates. it may or may not run down the duct into the insulation..but usually just drips down the plenum.
    if it does migrate into the duct insulation..the heat of the attic dries it thru the paper/foil backing of the ductwrap...BUT when you add an additional layer of wrap the condensation forms between the foil/paper of the first layer, and the fiberglass of the second layer. this doesn't migrate out as quickly. why?

    well I"m sure there is a calculation for that too..but my guess would include that the second vapor barrier is in much better shape than the old ductwrap. where the old torn ductwrap would help diffuse the moisture..the new one being in better shape does not allow this diffusion as quiclky. it may take several days for the moisture to diffuse.
    which would be ok..but the dewpoint doesn't occur several days apart, but every day/night.

    we all know a little about codes.
    code dictates that there can not be two vapor barriers in one building assembly.
    one vapor barrier on walls for example. the condensation is one of the reasons for this
    as it was explained to me years ago and last year again concerning attics (another story)

    so take this wall assembly that has little pressure driven differences
    and think about it on ductwork with mechanically induced velocity of air movement. put this mininally insulated duct in the hottest part of the attic, add that second vapor barrier..and you have created quite the situation.


    these are my observations. over the years I've gone through two copies of Joe's hot humid climate builder's giude, been to building science site more than a few times and had lots and lots of conversations with folks much smarter than I.
    lots of times things you see in the field can't be 100% explained by physics
    or theories. in the field things are not perfect, too much human error, building failures
    and a dozen other factors that make a difference.
    it is best to keep an open mind and not lock yourself into any one answer, as conditions
    are seldom the same.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  12. #12
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    Quote Originally Posted by energy_rater_La View Post
    if it does migrate into the duct insulation..the heat of the attic dries it thru the paper/foil backing of the ductwrap...BUT when you add an additional layer of wrap the condensation forms between the foil/paper of the first layer, and the fiberglass of the second layer. this doesn't migrate out as quickly. why?

    well I"m sure there is a calculation for that too..but my guess would include that the second vapor barrier is in much better shape than the old ductwrap. where the old torn ductwrap would help diffuse the moisture..the new one being in better shape does not allow this diffusion as quiclky. it may take several days for the moisture to diffuse.
    which would be ok..but the dewpoint doesn't occur several days apart, but every day/night.

    we all know a little about codes.
    code dictates that there can not be two vapor barriers in one building assembly.
    one vapor barrier on walls for example. the condensation is one of the reasons for this
    as it was explained to me years ago and last year again concerning attics (another story)

    so take this wall assembly that has little pressure driven differences
    and think about it on ductwork with mechanically induced velocity of air movement. put this mininally insulated duct in the hottest part of the attic, add that second vapor barrier..and you have created quite the situation.
    Here's the part that doesn't make sense to me:

    Double vapor barriers are bad in most home construction because this arrangement traps moisture; there is no place for the humid air to escape if the wall gets wet and then it condenses inside the insulation - we'd all agree on this. However HVAC duct wrap with FSK facing is already a double vapor barrier - on one side the barrier is the duct itself, and the other barrier is the FSK facing. So if we say that it's bad to have an inner bubble foil layer followed by an outer fiberglass FSK faced layer, I don't understand that. Vapor is trapped anyway, even without the bubble wrap on the inside. I am thinking specifically about the original poster's question and assuming that the duct work is not leaking and the new insulation is not messed up, etc.

    The pros on here have observed mold problems inside double insulated duct work. I highly suspect that the cause of this is 1) leaky duct connections and/or 2) the existing insulation before the second layer is dirty and that dirt provides food for the mold to grow. I still have a hard time believing that the primary cause of the problems with double insulation on ducts is due to the double vapor barrier. Otherwise we wouldn't use FSK faced insulation to trap moisture between the duct and the FSK.

  13. #13
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    Quote Originally Posted by ScorpionLeather View Post
    Here's the part that doesn't make sense to me:

    Double vapor barriers are bad in most home construction because this arrangement traps moisture; there is no place for the humid air to escape if the wall gets wet and then it condenses inside the insulation - we'd all agree on this. However HVAC duct wrap with FSK facing is already a double vapor barrier - on one side the barrier is the duct itself, and the other barrier is the FSK facing.
    If the duct insulation is applied correctly, the outer jacket is the vapor barrier, and all of its seams are taped so vapor cannot easily migrate into the fiberglass insulation.

    I have metal ducts at work moving air through them at around 20 degrees F. They are wrapped in fiberglass with a mylar reflective vapor barrier jacket. They live in a subbasement flowing normal room temperature and humidity air (dew point averages in the 50's). Only where the outer jacket has been damaged is there a problem with moisture condensation. This is because with the outer jacket torn away, moisture migrates through the fiberglass and hits the metal surface, condensing readily.

    I still have a hard time believing that the primary cause of the problems with double insulation on ducts is due to the double vapor barrier. Otherwise we wouldn't use FSK faced insulation to trap moisture between the duct and the FSK.
    It happens for the same reason it occurs in wall cavities. With insulation and vapor barriers on both sides, the drying potential is greatly reduced. With a wall cavity having vapor barrier only to the exterior, it can dry to the interior. Sure, a duct is different, in that if its inner surface is metal there's little opportunity for vapor diffusion through metal. What then would be the dynamics necessary to make a difference between a duct with two vapor barriers vs. one? My guess at this point is that with only an outer vapor barrier, there is enough duct leakage to keep enough air moving through the fiberglass to prevent condensation. Air moving through fiberglass reduces the insulating properties of this fiberglass. This increases the drying potential. If you then wrap the duct sheet metal itself (thereby negating any duct leakage into the fiberglass wrap) with a vapor barrier, then wrap the vapor barrier in fiberglass, then add yet another vapor barrier, any moisture that permeates the outer layer (and it will since no sealing job is perfect) gets trapped in the insulation, since there's little drying potential. It therefore accumulates over time since the moisture drive is always inward when outdoor weather is hot and humid.

    Just as hot goes to cold, greater moisture goes to lesser moisture, higher vapor pressure to lower vapor pressure, etc.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

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