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Fixing a leaky basement duct chase/soffit run properly

jeffwatson | Posted in General Questions on

I have a small, less than 1000sqft single story house with finished basement heated with a natural gas furnace.

The supply & return registers are on the floor of the main floor, so the ducts run under the floor joists.

Since the furnace room is in the back of the (rectangular) house, there’s a soffit that travels to the front of the house, which runs perpendicular to the floor joists. This soffit contains both the main trunk supply and trunk return air ductwork that the registers branch off from. The soffit is enclosed with drywall & the ductwork is not visible.

In the furnace room, ductwork is exposed (sheet metal rectangular) & I’ve spent some time sealing those up with mastic & foil tape. I’m under the impression that if the exposed work was this leaky, that the ductwork within the soffit is probably in the same leaky condition. I doubt there is any barrier above the ducts and that the soffit is just connected to the floor joist/perimeter walls’ air space.

1) Am I correct that this current setup is a bad setup? Can anyone explain how a system with these parameters would perform?

2) Is the fact that the supply & return ducts are parallel to each other in this single soffit a problem, even if they weren’t leaky?

3) Is my only option to tear down the drywall, seal the ducts, and put up new drywall? Any alternatives? And anything more I should do if I want to fix this the right (aka green/energy-efficient/properly-working-system) way?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, duct seam leaks usually don't result in a significant energy penalty, as long as the ducts are located inside the home's thermal envelope. If your basement walls are insulated, your basement is inside the home's thermal envelope.

    Your case is different, however: the soffit complicates things. If the soffit communicates with the joist bays and your rim joists, then duct leaks inside the soffit will either pressurize the soffit (if we're talking about supply system leaks) or depressurize the soffit (if we are talking about return system leaks).

    If the soffit is pressurized, conditioned indoor air will be forced out of leaks in your rim joist.

    If the soffit is depressurized, outdoor air will be sucked into the soffit from leaks in your rim joist.

    Either of these phenomena is undesirable.

    Is this problem worth fixing? It depends. If you know that the builder did a very conscientious job of sealing air leaks at the rim joist area, you may not need to worry. Similarly, if your energy bills are affordable, you may not care about this problem.

    If the problem bugs you, or if you know that your house has signs of moisture problems or rot, you may want to seal your duct leaks. Unless you decide to use the Aeroseal approach (which is expensive), you'll have to remove the drywall on your soffit to seal your duct leaks.

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