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Community and Q&A

Retrofit duct sealing (sealing on all sides)

TimTuckerCom | Posted in Green Building Techniques on

I’ve read through the Tape vs. Mastic article and have looked at various how-to guides for duct sealing, but I feel like I’m missing something.

In our home, most of the ducts run along the basement ceiling, including some within floor joists.

It makes sense how I’d apply tape to the sides and bottom, since those are easily accessible, but how would the tops get sealed?

I can see taking duct work down (high labor) or aeroseal (high cost) as possible options to tackle the problem, but the return on investment for ducts that are already within the conditioned space seems hard to justify.

Is there a secret trick I’m missing here, or am I best to just seal off 3 sides of most ducts and hope for the best?

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  1. walta100 | | #1

    In my opinion if the ducts are in the basement they are inside the conditioned space and the leaks are irrelevant.

    The only way it would be worth spending time on them is if the rooms at the far ends of the ducts were colder than the rest of the house and adjusting the dampers cannot even things out. Note if you can even out the temps and your thermostat setting remain unchanged your energy usage will increase.

    If your ducts are in unconditioned attic or craw space only then do they pose a problem.


  2. TimTuckerCom | | #2

    In our case, basement is conditioned, but only rim joists are insulated.

    My aim at the moment is to try to improve overall air sealing & insulation in the house and cut down on any CFM losses in the duct work so that we can size down for a new furnace when the current one eventually needs replacing.

    All that aside, I would assume the base technique of retrofit sealing would be the same regardless of whether the ducts are inside the envelope or not, wouldn't it?

  3. user-2310254 | | #3


    I will respond to bump your question. I suspect that some sealing is better than no sealing if you want to optimize your ducts. But you might want to consider if a mini split would work as your next system. If so, you would end up abandoning your existing ducts.

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