Results 1 to 11 of 11
  1. #1
    Join Date
    Jun 2007
    Location
    Central Virginia
    Posts
    31

    Question Mastic vs tape for sealing air ducts?

    I am getting two quotes for sealing the metal air ducts in our basement. The goal is to reduce air leakage and save $ on a/c.

    #1 says he uses foil tape (which has the sealant on it) because it is faster to install and doesn't crack like mastic "paint" would- his example was the return air handler metal does flex a bit. He also said he will use the tape to seal around supply and returns on main living area.

    #2 says he uses mastic because it seals better; he will use spray foam to spray from the basement around the supply and return areas. He is also giving quote to air seal the plate/sills around basement perimeter against air filtration.

    Any input re using the two materials? What about method of sealing air supply and returns- at main level with tape or from below with foam? The only place I can see where the movement of metal might cause a crack would be just at the air handler unit. Maybe it really doesn't matter-just personal contractor preference? I have 1 quote and am waiting for 2nd one.

    One more question. I have a hot water heater in basement and the #2 contractor noticed the vent going into the pipe was undersized for the water heater size. It is 3" but should be 4" he said. The vent isn't drafting all that well if the bathroom fan on the main floor is on, all doors shut, and he said this should help.

    Thanks for the input. I'll probably be deciding in the next week or two which one to use, and get on with it.

  2. #2
    Join Date
    May 2011
    Location
    Orlando FL
    Posts
    7
    I have used both foil tape and mesh and mastic.The foil first then mesh and mastic,and the spray foam at the bottom should work well also

  3. #3
    Join Date
    Jan 2010
    Location
    Athens GA
    Posts
    1,234
    #2 is far and away a better way to go.
    Putting the right size flue on your water heater is a safty must,but if you have negative pressure in your house that will not solve the problem.
    It sounds like you need fresh air introduced into the house especially the basement.
    Now that could be as elaborate as an ERV or as simple as an outside air pipe run to the combustion area of your water heater and your furnace.You could also install a direct vent water heater which would be the best thing as long as you have a two pipe 90% plus furnace.

  4. #4
    Join Date
    Mar 2005
    Posts
    121
    Mesh and mastic applied a nickel thick shouldn't crack. Sounds like you need combustion air.

  5. #5
    Join Date
    Jun 2007
    Location
    Central Virginia
    Posts
    31

    Hmm

    The 90% furnace is vented to outside. As far as negative pressure, one of the problems was that one of the two returns in the basement area was mounted right on the return air handler. During the test, the meter they were using showed something like -3.5 to -4. but by covering the vent with plastic, the numbers were reduced to something like -2.5. I don't remember the exact numbers nor the type of hand held device they were using, but they did say they exhaust or air inside the water heater flue showed little or no CO2, just other gasses. They (they = testers, separate from contractors #1 and #2) said that by sealing the return duct mounted on the return air handler made it "marginal".

    So now y'all have me a bit worried - house isn't all that well sealed, but I am going to have some done in attic and around sill plates. Are you thinking that with a larger flue for water heater + sealing the 1 return, that won't be enough? I will also have the chimney itself sealed - not using it, but don't know if that makes it better or worse. The testing people will come back to check, but I want to make sure everything is safe. Just installed first of two CO2 detectors (finally!). FYI, basement has 1 supply on one side, 2 supplies (ceiling height) and one return-near floor.

  6. #6
    Join Date
    Jan 2010
    Location
    Athens GA
    Posts
    1,234
    Look you have a house.Lets pretend your house is a cardboard box.You have so many cubit feet of air in that box.You have 1 furnace,you have 1 water heater and you have two bathroom fans? and maybe a cooking hood all taking air out of your box.If the infiltration into the box isn't the same as what is being exhausted then you have a negative situation in your box.If its a negative situation the box will pull air down the chimney or any other opening you have to the outside.a negative situation could ,I say could overcome the water heater venting or maybe even the furnace venter motor.This type of situation could bring dangerous gases into your house and if you breathe enough of it you could get very sick.
    Now if you had a direct vent water heater it would bring in all the air it needs for combustion.That is also why you should have a second pipe from the outside to your furnace to bring in fresh air for combustion.
    These last two things would at least remove the dangerous gas but you still might have a problem with your house fans and need a fresh air intake.
    I am 100% in favor of insulation but that is not your problem.Instead of letting air out through the roof or walls you need to have somethibng to bring in air to your house because you have so many things pushing air out of your house.
    Im hope you can understand what I'm trying to tell you.

  7. #7
    Join Date
    Jul 2010
    Location
    georgia
    Posts
    562
    just a homeowner, but I'd go with quality USA made mastic tape (mastic tape, not just metal tape).

    properly applying mastic is very time consuming, and I dont think many folks do it correctly due to that. Mastic should dry completely before applying insulation. Applying mastic to existing ducts can be difficult if you cant get 360 degree access. Joints can separate while drying if disturbed.

    Get the mfg's directions for mastic tape and make sure they follow them. some kind of surface prep may be needed before applying.

  8. #8
    Join Date
    Dec 2005
    Location
    Cincinnati, Oh
    Posts
    4,923
    Quote Originally Posted by b26440510 View Post
    just a homeowner, but I'd go with quality USA made mastic tape (mastic tape, not just metal tape).

    properly applying mastic is very time consuming, and I dont think many folks do it correctly due to that. Mastic should dry completely before applying insulation. Applying mastic to existing ducts can be difficult if you cant get 360 degree access. Joints can separate while drying if disturbed.

    Get the mfg's directions for mastic tape and make sure they follow them. some kind of surface prep may be needed before applying.
    Mastic tape is crazy expensive. If you have to "prep" the duct before application, then the labor is crazy expensive also.
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

  9. #9
    Join Date
    Feb 2011
    Location
    Orange County, in a Galaxy far far away...
    Posts
    277
    Hardcast mastic tape and generous amounts of "pooky" is my favourite way to seal ducts and plenums. Expensive but effective. Anything else is prone to leaks in future.

    Just my humble opinion...
    You cannot cheat an honest man. But that doesn't stop people trying!

  10. #10
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,702
    Was a duct blaster test performed? If you were going on a diet you'd step on a scale before you began, right? Otherwise you could neither set a target nor know how well you did. If you have no initial leakage number, there is no way to tell just how effective either method is. Both approaches are highly dependent upon work quality, and since air leakage is invisible this pressure test is crucial.

    If these guys don't do this testing, how do they know how good a job they did? By how pretty it looks?

    The water heater draft safety issue can be easily addressed if it becomes an issue when they are done with the air and duct sealing work. Please let us know your results?

    If a blower door test was done, could you share the CFM50 # and the square footage of your home? Would also love your annual gas and electric consumption.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  11. #11
    Join Date
    Mar 2011
    Location
    North Eastern NC
    Posts
    80
    If sealing is what you are paying for, then go mastic and you can expect it to be sealed. The quality stuff does not crack. The big box store stuff will...

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event