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  1. #1

    Metal Duct vs Duct Board

    I live in a single story house built in the 1920's that is approximately 1500 square feet. It's elevated, on blocks, about 3 feet from the ground and the AC/Heating ducts run beneath the house, with vents opening in the floor. Currently, I have metal ducts running beneath the house. I'm not sure how old they are, but I do know they are leaking air because the crawl space beneath my house is very cool and conditioned, while the inside of my house doesn't drop below 80 degrees (it's over 90 outside here in lower Alabama right now) in the summer (it also doesn't warm in the winter).

    I've noticed the current metal ducts running beneath the house have several holes throughout, some that are big enough that a cat climbed inside recently (and then pushed through a vent and into the house). Also, the ducts are not wrapped in any insulation at all.

    I have a few questions: First, should the ducts be removed and replaced? Or just repaired and wrapped in insulation? Second, if they should be replaced... should I go with duct board or metal ducts?

    I've received a couple different quotes. The metal duct was about 4x the cost of duct board. However, I've read a lot here at this site about the pros of metal duct vs the cons of duct board, so I'm a bit reluctant to go with duct board. But, I'm also wondering if metal ducts are truly worth spending 4x the cost of duct board?

    Thanks.

  2. #2
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478
    If its under a house, without doubt, go with metal. Vermin can chew right through ductboard.
    If done properly metal ducts will outlast you.
    If the ducts are sized properly just taping, patching, and insulating would be sufficient. Seeing how they werent sealed and insulated to begin with I would have my doubts as to whether or not they are sized correctly.

  3. #3
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,841
    Quote Originally Posted by ar_hvac_man View Post
    If its under a house, without doubt, go with metal. Vermin can chew right through ductboard.
    If done properly metal ducts will outlast you.
    If the ducts are sized properly just taping, patching, and insulating would be sufficient. Seeing how they werent sealed and insulated to begin with I would have my doubts as to whether or not they are sized correctly.
    I totally agree. In addition, if moisture is a problem under your home, you might consider having the metal ducts spray foamed to insulate and seal them. I'd still recommend all joints have mastic before spraying however. We often use spray foam in homes along the coast where periiodic flooding into crawl spaces is an issue. properly applied, theh spray foam keeps the ocean water at bay and the ducts are not exposed to the salt. Even better, the heating systems don't even have to shut down. The furnaces are in the attics connected to the basement system through a chaseway.
    If YOU want change, YOU have to first change.

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  4. #4
    Join Date
    Jun 2007
    Location
    Raleigh, NC
    Posts
    31
    I have used both metal and duct board, I believe galvanized metal, that is insulated, by wrapping the outside is best, not insulated from the inside and yes it is expensive. The question is how long do you plan on keeping the house, sounds like till the end, go with the metal, IMO

  5. #5
    Join Date
    Sep 2003
    Posts
    1,311
    If ductboard is installed so that it will not come apart it should cost more than metal. Sounds like someone gave a price for ductboard that will give you trouble. Go with the metal. There's no problem with just patching and insulating the existing.

  6. #6
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    what town do you live near?
    what state?

    so, get rid of the moisture in crawl!
    seal & insulate the crawl!
    see BUILDINGSCIENCE.com

    are ducts rusted?
    look, using a battery operated lantern.
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  7. #7
    Join Date
    Jun 2007
    Location
    Raleigh, NC
    Posts
    31
    I know guys that can install duct board the right way, it's a skill that I have not seen many people with, but it is by far cheaper, galvanized metal that is wraped is very expensive, not the insulated from the inside, it holds mold just like duct board.

  8. #8
    Join Date
    Sep 2003
    Posts
    1,311
    Quote Originally Posted by ArtThib View Post
    I know guys that can install duct board the right way, it's a skill that I have not seen many people with, but it is by far cheaper, galvanized metal that is wraped is very expensive...
    Depends on. For the black ductboard (yellow is a no-go for me) the ductboard costs 183% more than metal and insulation. Yellow ductboard cost 150% more. The only ductboard that I trust to stay together is siliconed at the longitudal seams and sealed with the iron-on tape. Others may just use self-adhesive tape and yellow ductboard and there's a chance of fall-apart. Perhaps the quality of ductboard duct is why mine costs more.

  9. #9
    Join Date
    Jun 2007
    Location
    Raleigh, NC
    Posts
    31
    That may be true I have not seen black duct board be for, but the duct board I've used is stapled, taped and mastic, it is not coming apart but like I say most people who use the stuff do not know how to put it together, thats why it gets a bad name!

  10. #10
    Join Date
    Apr 2002
    Posts
    11,808
    If you run metal through unconditioned space, it is so important to seal them, else you turn your heating/cooling system into an expensive exhaust fan, that heats/cools the air before exhausting it.

    Besides wasting money exhausting air that you are spending your energy dollars to condition, it makes the home go negative and causes outdoor air to infiltrate in.

    Leaky supply ducts are a double edged sword.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

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