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Bringing ducts inside the building envelope with an “inverted soffit”?

lightnb | Posted in Mechanicals on

Many articles on this site and others talk about bringing the ducts inside by creating a conditioned attic space (eg https://www.greenbuildingadvisor.com/blogs/dept/musings/creating-conditioned-attic, https://www.greenbuildingadvisor.com/article/how-to-get-your-ducts-inside-the-building-enclosure), and it seems that all of the energy experts agree that ducts in an unconditioned attic are inefficient.

The two solutions I see the most often are either (1) place the ducts in a soffit inside the house or (2) seal and insulate the attic. Personally, I think traditional soffits are ugly, unless well-disguised into the architecture. But sealing an entire attic with insulation on top is cost prohibitive both during new construction and as a retrofit as it requires re-roofing the house to get foam on top of the decking, or installing thick layers of expensive spray foam to get to the required R values needed by code.

So my question is, why don’t people use an “inverted soffit” design (see attached photo), where they build the soffit going up into the attic, rather than down, so there is no protrusion into the conditioned room, the ducts are in a semi-conditioned soffit, but the insulation still sits on the floor of the attic, allowing much cheaper and easier fiberglass insulation to be used?

I’m attaching a picture illustrating what I’m calling an “inverted soffit”. It seems like a cheap way to improve the efficiency of the HVAC system, by extending the thermal envelope just high enough to include the ducts, too. It could also be done by most handy DIY types as a retrofit for a modest material cost.

Is this an established practice, or is there a reason why this shouldn’t be done or isn’t allowed? I image that some care would need to be taken with regard to moisture that can happen when “double insulating” ducts, but there should be a way to design around this problem, such as leaving a gap between the ducts and the surface around them?

We are in the southeast, about an hour out of Atlanta, GA. Zone 4, on the edge of zone 3.

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Lightnb,
    There are a lot of variations which are quite similar to what you describe. The best have a roof design which incorporates a larger boxed-in area that can hold not only the ducts, but whatever mechanical equipment they are connected too as well. A good example of this is Stephen Sheehy's house which was featured in a guest blog.

    The problem with boxing in each duct, which is illustrated in your drawing, is getting the enclosure continuous and air-tight. You need some material on all four sides, and a way to seal the ducts where they penetrate the ceiling. That's tough in new construction, doing it as a retrofit would be a nightmare.

  2. Expert Member
    Dana Dorsett | | #2

    Clearly not a solution for retrofitting, but plenum trusses are pretty straightforward to build and air-seal new construction:

    https://www.greenbuildingadvisor.com/article/use-plenum-trusses-to-keep-ducts-out-of-your-attic

  3. lightnb | | #3

    Is there any reason why plenum trusses would be required? Could the same not be accomplished by adding framing to regular fink/W style trusses?

    Do you still use insulated ducts inside this plenum box?

    This is a new construction situation. We already have space for the mechanical equipment in a closet inside the envelope.

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