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Musings of an Energy Nerd

Solutions to the Attic Duct Problem

If you’re used to putting ducts in the attic, moving the ducts indoors probably won’t be easy

A vented unconditioned attic is a terrible place to locate a furnace and ductwork. Attics are almost as cold as the outdoors during the winter, and can be much hotter than the outdoors during the summer.
Image Credit: Image #1: Christopher Fuller - GBA

In last week’s blog, I discussed the practice of burying attic ducts in deep insulation. Since burying ducts is simply one of several ways to address the energy waste associated with the decision to locate ducts in a vented unconditioned attic, it’s worth taking a fresh look at all of the possible solutions to the attic duct problem.

Energy experts have been advising builders for at least thirty years that it’s a bad idea to locate ducts in vented unconditioned attics, yet the practice persists. Why? Building scientist Joe Lstiburek blames the problem on architects and builders, and he’s partly right. But it’s also fair to blame regulators, most of whom are too timid to approve code changes that would make the practice illegal.

According to NREL researchers David Roberts and Jon Winkler, moving the ducts from a vented attic to a new location inside the conditioned space will reduce electricity used for cooling by 15% to 20%, and will reduce the size of the needed air conditioning equipment by 0.5 to 1 ton.

In this article, I’ll look at a variety of solutions to the attic duct problem — a topic that I first tackled seven years ago in an article titled Keeping Ducts Indoors. The presumed reader is a designer or builder who wants to know the best way to avoid the energy penalties associated with attic ductwork.

In my 2011 article, I listed five possible solutions to this problem:

In this article, I’ll also discuss three additional approaches:

I wish I could announce that bringing ducts inside a home’s thermal envelope is easy. In fact, it isn’t. If you are used to building slab-on-grade homes with attic ducts, you’ll find that there is no easy, inexpensive way to move the ducts indoors. All good solutions are…

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  1. Dennis Miller | | #1

    Why does the soffit or plenum approach require airtight ceilings? Is it so that the plenum/soffit doesn't become a channel that supplies air to a fire? or some other reason?

  2. User avater GBA Editor
    Martin Holladay | | #2

    User 7073758,
    First of all, can you tell us your name? (I'm Martin.)

    There are several reasons why a ceiling should be airtight. The main reason has nothing to do with whether there is a soffit with ducts in it. Ceilings should be airtight because you don't want conditioned air from inside your house to leak into your attic or the great outdoors. (All of your building envelope should be airtight -- that means your ceiling, your walls, and your floor.)

    If you have a soffit with ducts in it, and there is a small duct leak due to imperfect mastic at a duct seam, the entire soffit can become pressurized. If there are any tiny leaks in the home's thermal envelope -- for example, rim joist leaks or ceiling leaks -- the pressurized soffit will assure that lots of air escapes through a small hole in the thermal envelope. Everything needs to be airtight -- the parts of the soffit that represent the home's air barrier, as well as the duct seams.

  3. Mark Hays | | #3

    To Martin Holladay and GBA readers: We recently completed a down-to-the-studs remodel of a 1950s era home in Massachusetts. This project highlighted another key factor: If you upgrade insulation for the entire house, the overall size of the HVAC / heating system can be reduced -- often significantly. Fewer duct runs may be needed, with smaller diameters and fewer registers. We wrapped this house with 2" thick XPS (carefully air sealed), sealed and filled all of the stud bays, and flash 'n batted the roof joist bays plus a final layer of polyiso foam board. Result? I ran ducts through all of the floor/ceiling joist bays, with just one supply/return per room. The furnace and HVAC condenser were 60% of the size the contractor recommended. (Yep, I did a Manual J and he did not.) In summary, upgrading insulation and air sealing in general is always a good idea -- that can help with other challenges, e.g. HVAC and heating systems. Plus you win-win-win with recurring heating / cooling costs every year, which keep going up-up-up.

    Two more notes: (1) This house was built shortly after WW2, probably by the returning GI who bought the land. He built it with 'radiating' joists, generally centered around the chimney. This made it much easier to run ducts from a central 'engine room' in the basement. Architects and builders should consider this. Provide a path for ducts; it won't cost much more. (2) I know that Mr. Holladay is opposed to foam board for good reasons. That said, and with remodels in particular, foam board is often the best / only solution. We are not working with deep walls, and customers have thinner wallets. Fortunately, Owens Corning responded to pressure and picked a new 'blowing agent' in 2012 that sharply reduced the GWP (Global Warming Potential) for Foamular XPS. OC also created a complete environmental report for this product -- which covers materials, manufacturing, transportation, etc. costs. So kudos to experts like Mr. Halladay; your critiques changed corporate behavior. (And polyiso is better when cold temps are not a issue.)

    I hope this is helpful,


  4. User avater GBA Editor
    Martin Holladay | | #4

    Congratulations on your house. I'm glad to hear of your success story.

    While Owens Corning XPS may have a blowing agent that is less harmful to the ozone layer than the blowing agents used 30 years ago, before the Montreal Protocol kicked in, I don't think that Owens Corning has yet address the problem of the global warming potential of the blowing agent used in their XPS. If it's advertised as having an R-value of R-5 per inch, it's the bad stuff.

    In general, I'm not against the use of rigid foam. I just steer green builders toward EPS or polyiso, rather than XPS. For more information on this issue, see "Choosing Rigid Foam."

  5. Jud Aley | | #5

    We have insulated 5 or 6 attics in the following manor:
    Rafter vents
    Fiberglass or Roxul between the rafters
    2” R-Max Polyiso with a white vinyl faced finish, all seams and screw heads taped with R-Max white tape.
    The 2” R-Max is not cheap but the home owners love the white sterile looking environment of their attic and no longer fear going up there to store things or change the filter on their air handler.
    We buy the R-Max and special screws and washers from IDI which has several locations around New England.
    I like this system better than spray foam as it requires no special equipment like a spray rig, can be installed by our employees and is easy to remove and put back if some kind of repair is needed in the future as opposed to a hatchet to chop out the closed cell foam.
    It looks a lot nicer than spray foam.
    It creates a thermal break across the rafters that closed cell job does not, unless you spray really deep and therefore costly.
    Its labor intensive but still comes in less costly than closed cell equivalent, least in my area with our own labor it does (Norwalk CT).
    As a remodeling contractor I like it because it is something my guys can do, keeping us on site while the subs are doing their work in the main house below, or on the day when we are waiting for the building inspector to show up....or not show up even when they are scheduled.....any body else have this experience with building inspectors?

  6. User avater GBA Editor
    Martin Holladay | | #6

    Rmax makes at least six kinds of polyiso for residential use, so any GBA readers interested in your approach have to be careful. As far as I know, only one of these products -- Rmax Thermasheath XP -- can be left exposed. (Building codes usually require rigid foam to be protected by 1/2-inch gypsum wallboard, which acts as a so-called "thermal barrier" to protect the rigid foam from the risk of fire.)

    Rmax Thermasheath XP has a special facing that has passed tests that allow it to remain exposed when installed on an interior surface.

  7. Jud Aley | | #7

    The Rmax Thermasheath XP is what we have been using because it can be left exposed and with the white vinyl side facing out it gives a fairly good finished look.
    - Jud

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