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Musings of an Energy Nerd

Sealing Ducts: What’s Better, Tape or Mastic?

Make sure you choose products that provide durable sealing

Duct mastic seals leaky seams. Mastic can be applied to leaky duct seams with a paint brush or gloved fingers.

Most residential duct systems have numerous leaks that waste energy and lead to room-to-room pressure imbalances. Unfortunately, though, few building inspectors outside of California bother to enforce existing code requirements that residential duct seams be sealed with mastic or high-quality duct tape.

Most model codes, including the International Residential Code (IRC), include duct tightness provisions:

  • The 2006 IRC section N1103.2.2 requires that “Ducts, air handlers, filter boxes and building cavities used as ducts shall be sealed,” while IRC section M1601.3.1 requires that “Joints of duct systems shall be made substantially airtight by means of tapes, mastics, gasketing or other approved closure systems.” Hardware-store duct tape is not an approved tape.
  • Section 403.2.2 of the 2004 International Energy Conservation Code (IECC) requires that “All ducts, air handlers, filter boxes, and building cavities used as ducts shall be sealed.”

To learn how to test residential duct systems for leaks, see Duct Leakage Testing.

All about mastic

Most energy-conscious builders seal duct joints with mastic. Mastic is a gooey, non-hardening material with a consistency between mayonnaise and smooth peanut butter. Duct joints should always be secured with #8 sheet-metal screws before seams are sealed with mastic.

Sealing duct seams is messy work, so wear old clothes. The mastic is spread over duct seams with a disposable paintbrush, putty knife, or your fingers. (If you spread mastic with your fingers, wear rubber gloves.)

Gaps in ductwork or plenums that are over 1/16 or 1/8 inch wide can be sealed with mastic as long as the gap is first reinforced with fiberglass mesh tape. If you’re using mastic to seal seams in fiberglass board ductwork, use fiberglass mesh tape for all joints.

Sources of mastic

Manufacturers of mastic include: Hardcast (Versi-Grip 181 mastic), McGill AirSeal (Uni-Mastic 181), Polymer Adhesives (AirSeal #22), RCD Corporation (#6 Mastic), and ITW/TACC (Glenkote mastic).

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  1. Tom Gotschall | | #1

    Great Seminar on Sealing!
    I'm going to share this with all my HVAC-related contractors and consultants.

    Conveying the need to use the right product at the right location has been a frustration of mine for a long time. Your article is spot on.

    Brad Turner of Southface Energy Institute likes to encourage that mastic be placed "thick as a nickel". This conveys that "painting it on isn't enough, guys."

    Use of tape mesh with the mastic has always helped in my observation also.

    The problem with tapes, especially in field conditions... is that it won't stick whenever condensation is present... an ongoing problem whenever trying to tape with systems operating.

    I had a "devil of a time" opening my evaporator for inspection and servicing after my HVAC guy placed mastic on the access panel. Manufacturers of HVAC equipment need to better respect the need for ready service access and provide better instruction for taping and mastic appppropriately.

    Thanks much.

  2. Roger Terry | | #2

    UL-181 used on Ductboard
    Seams on fiberglass board have to use fiberglass scrim and mastic in order to be a UL-181 Closure

  3. GBA Editor
    Martin Holladay | | #3

    Response to Roger Terry
    Thanks, Roger.

    After I spoke with Roger Terry on the phone, I clarified his point: If you choose to use mastic to seal the seams of fiberglass board ductwork, you cannot meet UL 181 requirements unless you use fiberglass mesh tape along with the mastic.

  4. Sean McClintock | | #4

    Rigid Foam as Air Barrier
    I love reading your blog! It's been very informative as I work with my architect and builder to design/plan our backyard cottage in Seattle.

    We have chosen to apply taped and sealed rigid polyiso as the air barrier. The idea we are trying to follow is that the foam is the one and only air barrier. If it's sealed tight, then there is no need to seal the sheathing to the framing, the drywall to the framing, etc. One completely tight air barrier is all you need, right? We're going to do a blower door test after the foam is applied and sealed and then again after the dense-pack cellulose and drywall is installed just to see how much difference the dense-pack cellulose makes in air movement.

    One detail that I'm not 100% confident about though is sealing the foam board to the concrete stem wall. The sheathing will be flush with the stem wall allowing the foam to extend below the point where the wall meets the mud sill. What do you recommend using to provide an air seal between the polyiso and the concrete? I think the builder is planning on using some Sikaflex product.


  5. GBA Editor
    Martin Holladay | | #5

    Response to Sean
    Although you are posting your question on a blog about duct mastic and duct tape, it seems that your question refers to another blog. But maybe my guess is wrong; in case you missed the earlier blog, you should read it:
    One Air Barrier or Two?

    Your polyiso should be sealed to the concrete with a high-quality caulk. In addition, I would air seal all of the following cracks: between the concrete and the mudsill (with sill seal), between the mudsill and the rim joist (with a gasket or caulk), between the rim joist and the subfloor (with construction adhesive or caulk), and between the subfloor and the bottom plate (with a gasket or caulk). Alternatively, many of these air leaks can be addressed from the interior with spray polyurethane foam.

  6. homedesign | | #6

    "Mastic" for Plywood Joints and Edges
    I realize this blog is about mastic & ducts
    Perhaps there is room for a hybrid blog.

    Combining mastic (gooey stuff) with an Airtight Plywood Approach.
    Not a continuous liquid applied membrane as in Joe Lstiburek's dream wall.
    But, Rather the limited use of Gooey Stuff at panel joints and openings/transitions.

    In the place of tape or peel&stick
    Mastic-like stuff (Liquid applied membrane)seems to be more flexible/reliable/tolerant compared to tape and construction adhesives, Eh?

  7. Sean McClintock | | #7

    Wrong Post!
    Sorry about that! You're right; I meant to post that question on the One Barrier or Two? entry. I accidentally put it on the wrong post. Thanks for the advice!

  8. jim green | | #8

    heat ducts
    we have slab floor which our heat ducts are in the slab , they are always filling up with water, which we have to pump out, cant use heat while water is in there because it makes to much humitidy, please is there any way to fix this problem? thank you jim green

  9. GBA Editor
    Martin Holladay | | #9

    Response to Jim
    The problem you describe is unfortunately fairly common in homes with under-slab ductwork. Someone must have thought (a few decades ago) that installing ducts under slabs was a great idea; experience has taught us that it wasn't, and homes are no longer built that way.

    You can try all the usual remedies for drying the soil around a house -- improve the grade of the soil around your house so it has a slope away from your foundation; install gutters and conductor pipe that carries roof runoff away from your house -- but these measures are unlikely to solve your problem.

    Your ducts should be abandoned. That means that either new ducts need to be installed in interior chases or your attic, or you need to install a boiler and baseboard radiation.

  10. GBA Editor
    Martin Holladay | | #10

    Response to John Brooks
    The products exist to do what you suggest. Sto makes two products -- a mesh tape + "gooey stuff" -- designed to seal air leaks in sheathing seams. The products are called Sto Mesh (or Sto Fabric Tape, a similar product) and Gold Fill. (I described the use of these products in "Housewrap In a Can".)

    Once you install Sto Mesh and Gold Fill over your sheathing seams, you have an air barrier. However, you don't yet have a WRB. If you want a WRB, it makes sense to continue with the next step suggested by Sto, and cover the entire exterior wall with Sto Gold Coat or Sto EmeraldCoat.

    However, if you prefer to use housewrap as your WRB, I can't imagine there is any reason you couldn't just use the Sto Mesh and Gold Fill to cover cracks and create an air barrier at the sheathing plane.

  11. chris | | #11

    homeowner rebuilds duct work using rivets screws and mastic
    My hvac system was horrible to say the least. I was shocked and horrified by what i found in the attic and in the furnace closet. First of all the furnace never worked. No matter how much i turned up the heat, the house would never get warm. In addition the filter was horrible. I was loosely placed in the bottom of this furnace with a giant gap at the end. The furnace was an old60% unit that was probably running at 40% with a rusty old heat exchanger. I bought a 95 percent efficient furnace to replace the old rust bucket. I tried to use a contractor but their prices were completely unreasonable. I thought i'm a hands on guy i will do it myself !
    I looked up the codes and followed them to t. In addition i bought a 10 inch electrostatic filter. Along with a steam humidifier.

    After i removed the old furnace, the fun really began. The main transition plenum was barely attached to the rest of the duck work. No mastic or duck tape was used. A 16 inch duct had a 5 inch gap on 1 side , where a piece of duct tape was flapping in the breeze. Home runs in the attic would fall apart in my hands as i tugged on them. Yep no screws just duct tape!?!?
    I rebuilt the entire ductwork system Using dlp mastic Along with screws and pop rivets after disassembling the entire system in order to clean out the fiberglass dust and that was drawn in due toi the gaps and links. In my humble opinion hvac industry needs a good kick in the butt. I m pretty sure that most houses in my block are in the same condition. It was a lot of work but i know i did the job better than any hvac contractor would do.

  12. GBA Editor
    Martin Holladay | | #12

    Response to Chris
    I urge you to be very cautious about running a humidifier in your home. Humidifiers cause far more problems than then solve.

    If your indoor air is too dry, it is usually an indication that your home is very leaky and would benefit from air sealing work. In many cases, installing a humidifier in a leaky house just makes things worse (and in some cases leads to major mold or rot problems).

  13. Bohdan Boyko | | #13

    duct mastic
    Good article Martin. Just two comments.

    As I recall, UL 181 foil faced tape was rated as a sealant only when applied on duct board with an iron and squeegeed.

    All duct mastics for residentialsystems are water based. Unfortunately, I have come across some red mastic which is designed for commercial use; the redstuff is petroleum based and should be avoided.

  14. Mark Siddall | | #14

    Over here, in the EU we (well some of us), are using EPDM seals on our metal ducts.

    Lindab Safe (
    Flaktwoods Veloduct (

    Anything like this your side of the pond? (These are a little pricy so I've also been wondering about whether there is a manufacturer that does EPDM sleeves/ collars that could tightly grip around tubular ducts - anything like this over your side?)


  15. GBA Editor
    Martin Holladay | | #15

    Response to Mark Siddall
    We have the same products over on this side of the pond. The brand I'm familiar with is SpirAmir (Dublin, California):

    These are sections of galvanized steel ductwork with built-in EPDM O-rings for sealed seams.

  16. Tom | | #16

    Great post- don't forget Manual J
    Thanks for this informative piece. It makes perfect sense and demonstrates that it's the little things that can make a difference. I recently had a community member ask a question about this very subject and I have linked this article to my recent post regarding how to size your HVAC system using Manual J at

    Tom at Millennial Livng

  17. TGray | | #17

    duct seal on old ducts
    My ducts are 1960 era galvanized. I was going to get out the mastic and brush and mesh but it just seemed so much easier to use silicone caulk along the relatively tight seams. My HVAC guy did not have a problem w/ this. Anybody find fault w/ that? Fast simple done.

  18. GBA Editor
    Martin Holladay | | #18

    Using silicone caulk to seal duct seams
    Others have used that technique, especially for the lengthwise lock-joints in round galvanized ductwork. But I'm skeptical about the longevity of the seal when you use caulk at the seams between sections of duct.

    Every time someone bumps against the ducts, it stresses the joint. Over the long run, mastic will be much more likely to stay airtight than caulk.

  19. Jim | | #19

    Sealing an A/C coil above a hot air furnace
    I've recently had a 95% hot air gas furnace installed along with an A/C system. The system is an upflow model, which pushes air past the A/C coil and into the duct work. The A/C coil sheet metal housing itself leaks warm air (when heating) badly. What should I use to seal the "door" of the coil? It currently has sheet metal screws securing it closed, but hot air leaks at the seams of this door. Can I caulk it? Or should I use mastic here?

    Thank you.

  20. GBA Editor
    Martin Holladay | | #20

    Response to Jim
    The article above addresses your question: "High quality duct tape — not mastic — should be used to seal holes in a furnace or air handler. As energy expert Bruce Harley notes, 'Mastic would render the cabinet unserviceable.' ”

  21. Mike C | | #21

    Questions and Responses
    Jim , I live on a slab that the ducts and main plenum filled up with water. I dug 270 feet of drain tiles and the first big rain the plenum had water in it. Not as much but still most of the ducts had been blocked because of decay - collapsed. I filled all the ducts up with cement but if you do don’t fill them up to the top of the flooring like where the tile is. My son helped me and good thing I caught it because I would be slowly chiseling it out today.

    My questions are : I had my brothers friend who does hvac work for a living tell me how to run flex duct in the attic unconditioned space. He helped build the plenum runs and from there we trunked out flex duct. He did not use mastic and I taped all the seems myself with polyken 339 which is 181 ul. It has been three years and the tape holds on the main trunks but where the boots meet the ceiling is the issue. They were taped by my sons and son in law because I am to tall to get in the crawl spaces. My daughter moved out recently which gave me time to inspect the up stairs which is a finished attic if you would call it that. It has its own gas furnace up there. I removed the knee wall on both sides of the attic and to make a long story short I could feel the heat coming from the flex boots when the downstairs furnace ran.

    The issues I have is with the drywall that was cut - I painted it at the time worried about drywall leaching into the air. The HVAC guy had me cut the boot to make a lip on four sides that laid on top of the ceiling drywall so that the register could then screw into it. I premade the flex to boot connections and then they installed them. Now that I have access to them:

    1.] Can I use a water based mastic on the inside of the boot and cut drywall where they meet?
    To seal that up...
    A.] Will the mastic cause any vapors that could be harmful because I am applying it from the inside?

    2.] Should I tape and use mastic on the outside of the boot and drywall on the upper part of the ceiling? In the attic part.

    3.] Should I make some type of fastening system better them 2 screws holding the boot to the register? Any Ideas besides sure boot that has the nailing system already on the boot.

    4.] I will use insulation on the exposed part of the boot - What is the best way to insulate the FLEX DUCT? It has an r6 value but is in unconditioned spaced. I had no other way to run the ducts. Could I bury them in batting insulation as there is some loose insulation that comes up to the top of the ceiling rafters - joists - 2x6 rafters 16" on center. There is a barrier so the insulation does not get into the soffits but any more loose fill could over flow where batting would not fall into the soffits.

    5.] I am more concerned on the best way to seal the boots inside and out.

    6.] I don’t think I can get the flex ducts into conditioned space unless I made another small knee wall and insulated that then. Any Ideas.

    7.] I had to remove some of the attic ceiling drywall because of an old roof leak and the attic furnace plenum runs along the top of the rafters not insulated no sign of tape or insulation and must of been that way for the last 50 years. Square galvanized.

    Its like opening up a can of worms one thing after another. I will have to take down the attic ceiling drywall to address that plenum - Mastic will work good there but insulation again in an unconditioned space.

    I have gable vents on each side of the capecod style house and soffits along both sides plus a ridge vent.

    Can I use water based mastic in the inside of the boot to seal small gap and the drywall.

    Any suggestion would help and sorry for the explanations I thought they might help.

  22. GBA Editor
    Martin Holladay | | #22

    Response to Mike C
    Mike C,
    It's hard for me to visualize the air leaks you describe where the register boot meets the drywall. Most register boots have lips that should be caulked to the ceiling drywall. If yours are different, you may need to replace the boots or seal the gaps with canned spray foam, mastic, or tape.

    I wouldn't worry about using mastic inside a register boot, but some people might. Usually mastic is applied from the outside of fittings.

    Here's the procedure for sealing flex-duct connections:
    * The duct boot or coupling should be inserted at least 2 inches into the end of the duct. The fitting should be attached to the inner sleeve of the flex duct with a drawband (clamp) or #8 screws.
    * Seal the joint between the inner section of duct and the fitting with high-quality duct tape or mastic.
    * Seal the exterior vapor-barrier sleeve with a drawband and tape.

    If your insulated flex duct has an intact vinyl sleeve, it's save to bury the ductwork in insulation. Pile it on.

  23. Mike C | | #23

    Followup With Martin
    Hi, Thanks for the quick reply ...

    I made the lip on the boot that is on top of the drywall. The register screws into it. We did not caulk that lip when we installed the 2 register screws. I was not told to do this but it was an instance where I had to rely on someone else for information at the time.

    Should I remove the 2 register screws and caulk the lip - What caulk would you use - Water base I assume. This is a late 50s house and the ceiling is plastered I have not tested for asbestos. I guess I fear the worse because of the drywall/plaster possibly leaching into the air. There is a leak between the top of the ceiling drywall and the boot. The connections for the flex to boot are done properly.

    Should I caulk the lip on the underside and stick it to the top of the drywall and screw the register back in. Then as over kill mastic the top edges of the drywall and boot?

    what do you think about the drywall leaching into the air?

    Thanks Martin..

  24. GBA Editor
    Martin Holladay | | #24

    Second response to Mike C
    Mike C,
    I think you are overthinking this.

    You can caulk the gap between the drywall and the register boot with any caulk you like. I'm partial to polyurethane caulk or silicone caulk.

    If you remove the ceiling register, you may be able to caulk this crack from below by reaching into the register boot. If the register screws to the lip on the boot, the sheet-metal screws should pull everything together.

    Otherwise, there's no reason you can't seal the crack from the attic using canned spray foam.

    Since you have no evidence of any asbestos in your plaster, I wouldn't worry about drywall "leaching" into the air. If you are the kind of person who worries a lot, you can always send a sample of your plaster to a lab for testing. However, as long as your plaster isn't "friable" and flaking into particles or dust, I wouldn't worry. If there are any raw edges that bother you, just tape them.

  25. Mike C | | #25

    Response to Martin
    Thanks Martin,

    I found good information from this little post and it was very informative. I thank you again.

    I did paint the drywall/Plaster edge and did tape a few originally but they got ahead of me and I missed a few that I should of taped.

    One last question: I also have Ductwork for the Attic Space that was converted to livable space. The ductwork is 50 years old at least. It is run through the very top peak of the house which is about 1.5 ft from the vent ridge. It has been there for all this time not insulated and that part of the roof looks good. The vent ridge was installed in the early 90s. The ducts are dirty and needs cleaned.

    Should I just take it down clean and mastic it then add an r6 insulation. The furnace that is in there now is an upflow which is about 15 years old. There is about 650sq ft up there even with the peaks in the way. It would be impossible to reroute the ducts and build soffits in the conditioned space?

    I also ran into this Black insulation that felt like plastic - Balsam - Wool - if I could contribute anything is this link which Balsam-Wool is not asbestos and has a picture if you ever run into it.

    I can do Electrical, Computer Networking, Plumbing, Drywall Finish, Tile, Other Flooring and Framing.

    There are lots of unknowns in HVAC and Longevity because of Air flow but I think some people have a good handle on it. As new products are available and things get greener we will all have better air quality and a healthier enviorment.

    Thanks to all who contributed to this!

    Thanks again Martin!


  26. GBA Editor
    Martin Holladay | | #26

    Third response to Mike C
    Your plan to clean the ducts and then install mastic and duct insulation sounds fine.

  27. Jim | | #27

    Fiberglass scrim - is this the same as joint compound tape?
    Hi Martin,

    Could you advise whether the fiberglass mesh tape or "scrim" is the same as fiberglass joint compound tape?

    I'm planning to seal all of the ducts in my house, which the HVAC guys said is 100% fiberglass board butted together and connected with foil backed tape. I'm planning to remove the tape and apply the mesh tape and mastic in each section. The ducts are also completely wrapped in that silver bubble wrap insulation as well, so plan to cut off a section, use the mastic, wait for it to dry then wrap with the original bubble wrap stuff.

    Thanks for the advice on sealing the AC coil housing - I will use the foil tape.


    Thanks for the advice

  28. GBA Editor
    Martin Holladay | | #28

    Response to Jim
    Fiberglass mesh tape is sold for a variety of purposes -- fiberglass boat repair, auto-body work, fiberglass tub/shower repair, and drywall work.

    As long as the tape you use is fiberglass mesh tape, it will work fine. Just be sure you aren't using paper drywall tape.

  29. Cj | | #29

    Mastic Keeps Dissolving in Rain - What to do if exposed?
    Hi, We are getting a bit tired of the annual application of mastic to our exposed metal round ducts. The heat of the summer blasts the mastic then the the rain comes and washes it away from the areas that are facing the sun. We then get leaks every winter with the rain. We are getting a little tired of the annual scrape and re-application of mastic. Is there something we can use that's tougher and more resistant to UV and not water based? These ducts are on a flat roof on top of a 3 bedroom house in the Los Angeles area. We saw 118 degrees this summer and 14 inches of rain last week. All the ducts leaked water into the house as the mastic I applied 9 months ago was badly weathered on the top side of the ducts.

    Please advise,



  30. GBA Editor
    Martin Holladay | | #30

    Response to CJ
    The answer is in the article on this page: butyl duct tape.

    Read what I wrote: "Foil-Grip 1402 consists of 12 mils of butyl adhesive (similar to the adhesive used in some flexible window flashings) with a 2-mil aluminum-foil top layer. Hardcast recommends Foil-Grip butyl tape for use with galvanized steel duct, duct board, or flex duct. The manufacturer claims that Foil-Grip 1402 is rugged enough to use outdoors."

  31. XBpPpFuZSp | | #31

    Thanks for the GLENKOTE shout-out!
    Any chance you can update the link to our website? We are at (not at taccint as your post lists).

    Dan Shugrue
    Market Development

  32. GBA Editor
    Martin Holladay | | #32

    Response to Daniel Shugrue
    I have fixed the Glenkote link.

  33. Jayne_l | | #33

    drawband for flexible duct
    Are the drawbands for sealing flexible duct joints anything different than plastic cable ties that I can buy anywhere?
    Thanks for the great blog. It opened my eyes to the importance of fixing the horrible installation at my son's house. There are no drawbands at the joints. Just old cloth duct tape that is falling off.

  34. GBA Editor
    Martin Holladay | | #34

    Response to Jayne Masternak
    Both plastic and steel clamps (drawbands) are available for flex duct. Here are some Web sites with photos:

  35. FastSteve | | #35

    Conditioned Space
    I see no mention of the distinction between unconditioned space and conditioned space.
    I'm kind of old school. I think ductwork in an unconditioned space needs to be sealed tight.
    In a conditioned area, assuming reasonable ductwork standards, sealing this is a waste of time, energy and resources.
    If I'm building a "Green Home", there will be no ductwork in an unconditioned space to begin with and therefore I don't need any sealant at all.

  36. GBA Editor
    Martin Holladay | | #36

    Response to Steve Oxler
    You can call it old school if you want, but your building code official has another name for it: a code violation.

    According to section M1601.4.1 of the 2009 IRC, "Joints of duct systems shall be made substantially airtight by means of tapes, mastics, liquid sealants, gasketing or other approved closure systems. Closure systems used with rigid fibrous glass ducts shall comply with UL181A and shall be marked181A-P for pressure-sensitive tape, 181A-M for mastic or 181 A-H for heat-sensitive tape. Closure systems used with flexible air ducts and flexible air connectors shall comply with UL 181B and shall be marked181B-FX for pressure-sensitive tape or 181B-M for mastic. Duct connections to flanges of air distribution system equipment or sheet metal fittings shall be mechanically fastened."

    I understand the argument in favor of skipping mastic on ductwork located within the conditioned space. If the house is very well insulated and has an airtight thermal envelope, unsealed ductwork can work. But if the thermal envelope is leaky, duct leakage can pressurize joist bays and send conditioned air outdoors through envelope cracks, or can pull in outdoor air via the same mechanism.

    Moreover, leaky ducts may result in distant rooms getting less airflow than the duct designer intended.

    So my advice is, follow the code and seal your ducts.

  37. [email protected] | | #37

    Duct Insulation?
    Lot's of good info here, Thanks so much. My 1979 home was built with little attention to energy efficiency. I am gradually improving it. There are unsealed, uninsulated ducts in the crawl space. I know this is a big no-no, but other than converting the crawl to a basement there is no way to get the ducts into a conditioned space. So, once I seal them, what is the best method/type of insulation? Or, are there ducts that come with insulation built in? My immediate concern is the round ones which I would be willing to replace if there is a pre-insulated alternative. The large rectangular main ducts are well wrapped with faced fiber glass. Please excuse me if this has alread been covered else where, my searches were unsuccessful.

  38. JIM BAERG | | #38

    Duct Sealing
    Thanks for the very helpful article. I was unaware that flex duct liners need to be Zip-Tied AND sealed with tape or mastic. Here's a couple of add'l suggestions;
    I would recommend using a ZipTie tool which will tighten the strap much tighter than hand tightening.
    Our crews often use 2 or 3" magnetic rolls to make a cover over the filter door. Easy to remove when changing the filter. I like silicone caulk for any duct-to-furnace cabinet attachments.
    Also, sealing return air duct-work, filter doors, etc. in furnace rooms is especially important if the house has an open combustion gas furnace or HWH to lessen the chance of down drafting.

  39. Pgager | | #39

    Loose boot advice
    I have an unattached duct boot i attic above a ceiling register. I assume the register screws were used to hold down the boot. I dont think the boot was ever attached properly. The ceiling is plaster and cracking around the old register screw holes.
    Is there a way to attach the boot from BELOW ( ie from room not from attic). I want to attach and seal because there is wildlife droppings/nesting around the loose boot. I cannot go up there to clean anytime soon. Ideas I have come across include glueing boot flanges or wooden shims, but not sure how that would be done from below. After solid attachment I would like to seal seams to avoid contamination and energy loss. Sealant ideas appreciated.

  40. GBA Editor
    Martin Holladay | | #40

    Response to Ptr Gag
    As far as I know, if you want to repair this problem from below, you will have to make an access hole in the plaster sealing. At that point the duct can be attached to the boot, and the seam properly sealed, before the ceiling is repaired. Plaster repairs aren't that difficult.

  41. Sofiane | | #41

    Reviving a somewhat old thread.

    I need a precision, should every transition one section of duct to another be sealed, as when one straight run is made of two parts of duct that are mechanically fastened? It seems that only the transitions from straight to elbow are sealed in my system. Thank you in advance for your input.

    1. Expert Member
      MALCOLM TAYLOR | | #42


      Not 0nly should each section of duct be sealed to the next, but the seams on each duct should also be sealed.

  42. nasmith29 | | #43

    FIrst off, thank you for this great article. Secondly, I have a mastic question I can't seem to find a universal answer to I hope you can help. My home is from 1925 and most ceilings are plaster. Basically, i used duct mastic to seal the boots to the attic floors and then i sealed the boots to the ceiling of my living space (prior to the grills being reattached). Basically - in one room, I had plaster pretty broken up, but i was able to still add mastic to it. However, it was so broken up that the grill had been caulked shut, and now that i've sealed it with mastic (not very well as you can see in the picture - it needs another coat on top of the fiberglass tape), i have no idea how to reattach the grill without caulking it again. Is this my only option? Should I instead remove the duct mastic from the ceiling somehow and try to fix the plaster prior to sealing it? Please help! (And btw - the cracks in the picture are already sealed - albeit poorly from a previous owner lol).

    1. GBA Editor
      Martin Holladay | | #44

      NA Smith,
      Your question is unclear. Assuming that boot is sealed to the plaster, and the duct system is relatively airtight, you don't need an airtight seal between the grille and the boot. All that is necessary is that the grille be attached to the ceiling framing with screws, and that the grille sits close enough to the ceiling to be aesthetically acceptable.

      1. nasmith29 | | #45

        Wow. I had no idea I was double working myself. I hope when I remove the mastic from the ceiling that it doesn’t make it more difficult to put plaster up! Thank you.

  43. yd | | #46


    Have there been any advancements regarding tapes since the article was published? Are butyl mastic tapes still the best alternative to conventional mastic sealant, and if so, is Hardcast Foil-Grip 1402 still the best option for sealing rigid ducts?

    It seems there have been some good developments with acrylic tapes, particularly for flashing tapes. The temperature ranges appear to be broader, which seems interesting in my case for installation on ducts in an unconditioned attic.

    I’m torn between the butyl mastic tape Hardcast Foil-Grip 1402 UL723 and the acrylic tape 3M 3340 UL181 A-P. I see that the standard doesn’t necessarily affect the tape’s performance.

    I want something high-performing but also proven to last long-term. I hear both butyl and acrylic can dry out over time.

    I have two 6-inch rigid ducts to install in an unconditioned attic—one with a 9-foot run for the kitchen fan and another with a 7-foot run for the bathroom fan. I’m not sure if I like the idea of having butyl in a duct over my kitchen oven.

    I am in climate zone 5a, with attic temperatures ranging from -2 to 5 degrees Celsius (28.4 to 41 degrees Fahrenheit) this time of year, and because of that, I can’t use conventional mastic.

    Any input is appreciated.

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