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  • insulating Duct work

    Ok here is the question.

    Several years ago we had central air installed. We installed our own duct work. Fiber duct and all the metal boots were wrapped in foil backed insulation. we also used flexible duct to all the vents.

    When the guys were installing the central air they told me the insulation on the boots was unecessary on the metal boots.

    We have since added 2 wall stacks to the system these are metal so they fit in a 2x4 wall. I noticed before we closed up the wall the 2 metal ducts would get covered in condensation. I'm going to assume that when closed in the wall they will continue to condensate.

    Correct me if I'm wrong or not but these wall stacks and boots need to be wrapped in insulation also correct?

    If so does this insulation need to be the foil backed? or can it be the pink loose fill you can buy at lowes and home depot?

    Also still more. Murphys laws says that we had to cut through a floor joist to get these 2 wall stacks up. we have sistered(sandwiched) both sides of this cut joist. there is very little room to get any insulation in these areas to cover the metal wall stack and metal boots. Would great stuff (you know that wonderful foam in the can that every home owner loves to spray everywhere) fit the ticket here? Seems like it should get everywhere needed to cover the metal boots and wall stacks.


    Final question.. Can anything be done about condensation on metal vents?

  • #2
    Originally posted by superdairyboy View Post
    Ok here is the question.

    Several years ago we had central air installed. We installed our own duct work. Fiber duct and all the metal boots were wrapped in foil backed insulation. we also used flexible duct to all the vents.

    When the guys were installing the central air they told me the insulation on the boots was unecessary on the metal boots.

    We have since added 2 wall stacks to the system these are metal so they fit in a 2x4 wall. I noticed before we closed up the wall the 2 metal ducts would get covered in condensation. I'm going to assume that when closed in the wall they will continue to condensate.

    Correct me if I'm wrong or not but these wall stacks and boots need to be wrapped in insulation also correct?

    If so does this insulation need to be the foil backed? or can it be the pink loose fill you can buy at lowes and home depot?

    Also still more. Murphys laws says that we had to cut through a floor joist to get these 2 wall stacks up. we have sistered(sandwiched) both sides of this cut joist. there is very little room to get any insulation in these areas to cover the metal wall stack and metal boots. Would great stuff (you know that wonderful foam in the can that every home owner loves to spray everywhere) fit the ticket here? Seems like it should get everywhere needed to cover the metal boots and wall stacks.


    Final question.. Can anything be done about condensation on metal vents?
    Any exposed metal duct should be insulated with a vapor barrier insulation,
    like ten foil or vinyl covered. Usually in a wall ,if the duct is sealed in the wall
    so you don't have air movement around the duct in the studded space, the duct will not condensate. later paul

    Comment


    • #3
      so esentaily you are saying any metal duct that is enclosed in a ceiling or wall or even a drop ceiling where there is no air flow it should not condensate.

      However in an exposed setting say like a warehouse or cellar where it may be exposed it will condensate and thus should be covered?

      Comment


      • #4
        Originally posted by superdairyboy View Post
        so esentaily you are saying any metal duct that is enclosed in a ceiling or wall or even a drop ceiling where there is no air flow it should not condensate.

        However in an exposed setting say like a warehouse or cellar where it may be exposed it will condensate and thus should be covered?
        If a duct is in a tight studded space and sealed in that space then it would not condensate. If you have a space between a drywall ceiling and a drop ceiling with ducts in there then you should insulate the ducts. a drop ceiling is not sealed so you would get air flow and there is a lot of air up there. All metal ducts should be insulated if the are going to be in a cold area or have air con. air going through them. In a tight studded space where the duct can be sealed
        into that space, like a wall, you can get away with no insulation. So insulate all metal ducts you can get too or you could get condensation. later paul

        Comment


        • #5
          got it very good thanks for the info.

          Comment


          • #6
            Ducts in Unconditioned Spaces

            Only ducts exposed in unconditioned spaces are required to be insulated and vapor sealed.

            An 'unconditioned space' is that part of the house that is not heated or cooled (like attics, crawlspaces or unfinished basements). Ducts run outside the heated and cooled building envelope outside of the structure are also considered to be in unconditioned space.

            Interior walls are part of the conditioned space and ducts within them do not require insulation or vapor retarders at any time..

            It is not true that ducts in a tightly sealed space will not condensate. Water vapor will pass through most building materials (like drywall) regardless of how well sealed the stud space may be.

            It is also not true that ducts installed above a drop ceiling should be insulated. If this area is part of the conditioned space no insulation or vapor retarder is required or necessary.

            Comment


            • #7
              I did notice last night that there was condensation on the duct that will be in the drop ceiling. the ceiling is not up yet. So I will insulate those parts.

              Not sure if I will open the wall up or not to insulate the 2 wall stacks.


              Brian

              Comment


              • #8
                Originally posted by manhattan42 View Post
                Only ducts exposed in unconditioned spaces are required to be insulated and vapor sealed.

                An 'unconditioned space' is that part of the house that is not heated or cooled (like attics, crawlspaces or unfinished basements). Ducts run outside the heated and cooled building envelope outside of the structure are also considered to be in unconditioned space.

                Interior walls are part of the conditioned space and ducts within them do not require insulation or vapor retarders at any time..

                It is not true that ducts in a tightly sealed space will not condensate. Water vapor will pass through most building materials (like drywall) regardless of how well sealed the stud space may be.

                It is also not true that ducts installed above a drop ceiling should be insulated. If this area is part of the conditioned space no insulation or vapor retarder is required or necessary.
                We are talking about two different things here. one is code on where it is required to insulate duct. The other is where to insulate ducts for adding air con. These are two different subjects. You are right that the code says insulate all ducts in a unconditioned space. In some areas like where i live in northern Mi., Me and most inspectors will say that we have to keep our crawl
                space from freezing so we don't insulate our ducts in the crawl space.
                Now when insulating the ducts that will have air con. going through them.
                Any ducts that have warm air around them and cold air going through them
                can condensate. This means, point blank, all metal ducts need to be insulated to stop condensation, weather or not the ducts are in the treated area or not. The ducts need to be sealed and a vapor insulation around them.
                Later Paul

                Comment


                • #9
                  Absolutely Wrong

                  "This means, point blank, all metal ducts need to be insulated to stop condensation, weather or not the ducts are in the treated area or not. The ducts need to be sealed and a vapor insulation around them."

                  Absolutely wrong.

                  Air conditioning ducts do not require insulation and a vapor retarder when they are in conditioned spaces. Ever. Period.

                  Why?

                  Because condensation only occurs when there is a difference in temperature that allows for the duct to fall below the dew point of the air in which the duct passes.

                  If the duct passes through a conditioned space, it is by definition, at the same temperature and humidity level as that space!

                  And therefore, the duct surface will never fall below the dew point.

                  It is scientifically impossible for that to happen.

                  That is why the International Mechanical Code and International Residential Codes both state:

                  M1604.3.4 DUCT INSULATION. "Duct insulation shall be installed in accordance with the following requirements:

                  1. A vapor retarder having a maximum permeance of .05 perm in accordance with ASTM E 96, or aluminum foil with a minimum thickness of 2 mils, shall be installed on the exterior of insulation on the cooling supply ducts that pass through NONCONDITIONED SPACES conducive to condensation."
                  That is why the International Residential Code and the International Energy Conservation Codes both state:

                  N1103.2 and 403.2 DUCTS. "Supply and return ducts shall be insulated to a minimum of R-8. Ducst in floors ahll be insulated to a minimum of R-6.

                  EXCEPTION: DUCTS OR PORTIONS THEREOF LOCATED COMPLETELY INSIDE THE BUILDING THERMAL ENVELOPE."
                  Ducts inside the conditoned space, inside the building thermal envelope, by definition, CANNOT SWEAT.

                  And why mechanical and energy codes do not require insulation or a vapor retarder on them in these locations.

                  -------------------

                  "You are right that the code says insulate all ducts in a unconditioned space. In some areas like where i live in northern Mi., Me and most inspectors will say that we have to keep our crawl
                  space from freezing so we don't insulate our ducts in the crawl space."


                  The above comment and practice is so bizarrely wrong, I am at a loss to know how to even comment on it.

                  If you keep the insulation off of your ducts in a crawl space to 'keep it from freezing', you are actively conditioning that space and are therefore required by all know building and mechanical standards to:

                  1. Block off the ventilation
                  2. Insulate the walls (and possibly) floors because it becomes a conditoned space
                  3. Fully condition the space by fully heating it, cooling it, and dehumidifying it just as you would any other part of the habitable space in the stucture.

                  If you are really doing this, and your inspectors are allowing this, none of you know what your doing up there in Northern Michigan....and I find that hard to believe....Because your state adopts and uses the Codes from which I just quoted.

                  The better explanation is, and with all due respect, that you simply don't understand what you're suggesting, because it flies in the face of all known Codes and standards of building and mechanical practice...

                  And I will simply stop there.
                  Last edited by manhattan42; 06-06-2010, 07:01 PM.

                  Comment


                  • #10
                    Originally posted by superdairyboy View Post
                    I did notice last night that there was condensation on the duct that will be in the drop ceiling. the ceiling is not up yet. So I will insulate those parts.

                    Not sure if I will open the wall up or not to insulate the 2 wall stacks.


                    Brian
                    Is this room air conditioned?

                    If so, then there is no need to insulate the ducts and what you may have seen is condensation that only temporarily occured while the system was bringing the temperature and humidity in that room into balance.

                    Other wise it is not required to insulate the ducts above the drop ceiling.

                    The air temperature will reach equilibrium with the duct temperature after a few minutes and no more condensation will occur.

                    Comment


                    • #11
                      Originally posted by manhattan42 View Post
                      Because condensation only occurs when there is a difference in temperature that allows for the duct to fall below the dew point of the air in which the duct passes.

                      If the duct passes through a conditioned space, it is by definition, at the same temperature and humidity level as that space!

                      And therefore, the duct surface will never fall below the dew point.

                      It is scientifically impossible for that to happen.
                      Well, being an engineer, I know science (theory) and reality don't always agree.

                      I have such a case of a metal boot that I had thought would be OK that is condensing even though it is still open to the rest of the house that has had the AC running all summer.

                      I found it because another one that had already been closed up with a false ceiling had damaged said ceiling. 2 shorter and smaller diameter runs in the same dropped ceiling don't seem to have the problem (I pray). Or they are also soaking the insulation like the one that is still open has been doing. Thankfully (???), the water has been trapped within the flex sleeve so it hasn't dripped on that ceiling (it's still open for now to an adjacent room).

                      I'm thinking my only option to insulate these boots is with Great Stuff as the OP had pondered for his stacks over a year ago since I can't readily access the one on all sides at least to do any taping of seams.

                      My real question is how do you seal the "seam" where the flex ends at the elbow? Spray the GS over the exterior a couple inches down? I figure I can drop the boot out of the floor and work through the hole to spray all around it in a few passes and then go around the rim of the boot on the last pass and fasten it back in so the foam forms to the underside of the floor.

                      Comment


                      • #12
                        Originally posted by topshot View Post
                        Well, being an engineer, I know science (theory) and reality don't always agree.

                        I have such a case of a metal boot that I had thought would be OK that is condensing even though it is still open to the rest of the house that has had the AC running all summer.

                        I found it because another one that had already been closed up with a false ceiling had damaged said ceiling. 2 shorter and smaller diameter runs in the same dropped ceiling don't seem to have the problem (I pray). Or they are also soaking the insulation like the one that is still open has been doing. Thankfully (???), the water has been trapped within the flex sleeve so it hasn't dripped on that ceiling (it's still open for now to an adjacent room).

                        I'm thinking my only option to insulate these boots is with Great Stuff as the OP had pondered for his stacks over a year ago since I can't readily access the one on all sides at least to do any taping of seams.

                        My real question is how do you seal the "seam" where the flex ends at the elbow? Spray the GS over the exterior a couple inches down? I figure I can drop the boot out of the floor and work through the hole to spray all around it in a few passes and then go around the rim of the boot on the last pass and fasten it back in so the foam forms to the underside of the floor.
                        The last thing i am going to do is argue with any body. When you run cold air through a duct it can condensate. when you put cold water in a glass it condensates, when you run cold water in a water line it condensates. Answer seal and insulate with a vapor barrier insulation so no air can get to the metal.
                        I have never had trouble sealing, i use nylon bands, with a banding tool that put the band real tight so it seals good. Paul

                        Comment


                        • #13
                          Originally posted by paul52446m View Post
                          I have never had trouble sealing, i use nylon bands, with a banding tool that put the band real tight so it seals good. Paul
                          Banding tools are nice, but my question is for ducts that already have the flex installed but no insulation on the elbow or the boot itself. The ducts are also in close quarters.

                          Comment


                          • #14
                            [QUOTE=topshot;44572]Banding tools are nice, but my question is for ducts that already have the flex installed but no insulation on the elbow or the boot itself. The ducts are also in close quarters.[/QUOTE
                            If you are asking for a easy fix, i don't have one. Your job was done wrong.
                            If these boots go into the ceiling and you can't get to them and you wan't to fix them, then you would have to take them out, pull them back, insulate them properly and re install them. You can take a pole and stuff insulation around them, but if air can get to the out side of the metal and cold air goes through the inside they could condensate, but they should not condensate too bad.
                            Paul

                            Comment


                            • #15
                              Originally posted by superdairyboy View Post
                              Ok here is the question.

                              Several years ago we had central air installed. We installed our own duct work. Fiber duct and all the metal boots were wrapped in foil backed insulation. we also used flexible duct to all the vents.

                              When the guys were installing the central air they told me the insulation on the boots was unecessary on the metal boots.

                              We have since added 2 wall stacks to the system these are metal so they fit in a 2x4 wall. I noticed before we closed up the wall the 2 metal ducts would get covered in condensation https://everesthvac.net/‚Äč. I'm going to assume that when closed in the wall they will continue to condensate.

                              Correct me if I'm wrong or not but these wall stacks and boots need to be wrapped in insulation also correct?

                              If so does this insulation need to be the foil backed? or can it be the pink loose fill you can buy at lowes and home depot?

                              Also still more. Murphys laws says that we had to cut through a floor joist to get these 2 wall stacks up. we have sistered(sandwiched) both sides of this cut joist. there is very little room to get any insulation in these areas to cover the metal wall stack and metal boots. Would great stuff (you know that wonderful foam in the can that every home owner loves to spray everywhere) fit the ticket here? Seems like it should get everywhere needed to cover the metal boots and wall stacks.


                              Final question.. Can anything be done about condensation on metal vents?
                              I was looking through the Graphisoft website earlier today, and i downloaded the tutorials for the Graphisoft constructor, and i never realised how similar to ArchiCAD it is. Anyway, while going through the tutorial I saw that Constructor has a very nice, and easy to use tool specifically made for duct work. Is there anything similar to this available for archicad?

                              Comment

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