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Community and Q&A

Return ducts in attic; Exposed ducts in conditioned space

onacurve | Posted in General Questions on

I’ve read a bunch of articles here and on BSC’s site concerning ducts in the attic. My 1948 ranch house has supply ducts in the crawlspace, return ducts in the attic. I’ve replaced the supply ducts and have insulated and encapsulated my crawlspace according to the best methods presented on this site.

I have a couple questions…

A. Return ducts in my attic.
My next my project is to deal with the return ducts in my attic. After reading BSC’s article, “Duct Dynasty” (BSI-074), covering the ducts with closed cell spray foam seems to be the second best method, next to relocating them in the conditioned space, which I don’t have the time or money for. A 600 board foot kit of closed cell spray foam is about $600 at the distributor here, but that is also out of my budget.

So I was looking for a cheaper alternative. From my crawlspace project, I have some leftover recycled polyiso that I bought on Craigslist for $5/sheet that is in pretty good shape. Montana Energy Services has a youtube video which shows them building a box around the ducts and using can foam to fill in the gaps — cut-n-cobble style. Is that a good approach for a low budget solution?

There’s two parts to the video. In part two, at 1:17, and 3:47, that they show what they are doing (1).

B. What’s wrong with exposed ducts?
The BSC article shows how to build soffit boxes for locating the ducts in the conditioned space. Why do all that? Aesthetics? If you google search “exposed ductwork” you get a lot of images that look appealing to me. There are quite a few that come up on the Houzz website and one on Pinterest (2). Do most people find them ugly to look at? I think they’re better looking than the soffit boxes and they accomplish the same thing. Maybe they’re too “industrial” looking. They appear to be much simpler than the plenum trusses of Mr Bailles recent article, but I’ve seen no mention of them on GBA, BSC, or on the Energy Vanguard blog.

Wouldn’t boxing the ducts in the way Montana Energy Service suggests be essentially the same thing as locating them in the conditioned space with a plenum truss or some other method?

Thanks in advance for your response.


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  1. GBA Editor
    Martin Holladay | | #1

    If you want to install cut-and-cobble insulation on your attic ducts:
    1. First, seal the duct seams with mastic.
    2. Make sure that the rigid foam is secure. (I would use foil-faced polyiso, which is the easiest type of foam to tape, and tape it securely.)
    3. Come up with a plan for insulating the underside of the ducts.
    4. Be aware that some building codes require foam to be protected by a layer of 1/2-inch drywall. You may want to use Thermax insulation, which may not need drywall protection.

    Concerning exposed ducts: Allison Bailes illustrated a recent blog on the topic with a photo of exposed ductwork. Here is the link: How to Get Your Ducts Inside the Building Enclosure.

  2. onacurve | | #2

    Thanks for that, Martin.

    So you don't think what MES did in the youtube video was a good technique? Maybe you didn't have time to check it out.

    On your point 3 - I don't really see how leaving the space under the duct uninsulated is substantially different from the dropped soffit in the living space approach. You're just moving the insulation boundary. Of course you'd want to make sure the R-value above the duct box was equal to the rest of the attic floor. I was just trying to come up with the least cost method of achieving the same results.

    I remember that article Allison Bailes wrote, and checked it out again. The last photo was quite comical. There's a serious way to put an exposed duct in a residential living space, though I don't know why he didn't include one. That's why I included a link to the Pinterest page. I see exposed ducts in commercial spaces quite a bit, but not often in residential spaces. I was just wondering why. I guess because most people think they're ugly.

    Thanks again.

  3. GBA Editor
    Martin Holladay | | #3

    Concerning Allison's blog: I was referring to the first photo at the top of the page, not the joke photo at the bottom of the blog.

    If you don't plan to install any insulation under the ducts, you need to carefully air-seal the gap between the rigid insulation installed on the sides of the ductwork and the drywall below. It's doable, but it's tricky, fussy work.

  4. KeithH | | #4

    I have another comment to add to your considerations. Do you have any rodents in your attic (ever? might have in the future?)? The brain trust that remodeled and did an addition on to my home in 87 built supply ducts for the new room over the garage in the garage ceiling out of polyiso. In the category of no sh* sherlock, the mice chewed through that polyiso. Now, I realize that is a different situation for you (the mice could literally get into the duct in my case), I though it worth mentioning that you might do a beautiful job of air sealing and boxing in those ducts with foam board only to have the first squirrel in the attic make really short work of it.

    I don't really have a solution other than to say rodent proofing should probably be part of the attic work ...

    Good luck.

  5. GBA Editor
    Martin Holladay | | #5

    You wrote, "I don't really have a solution other than to say rodent proofing should probably be part of the attic work."

    One other point to mention: if you want to keep rodents out of your ducts, install galvanized ducts, securing all seams with sheet-metal screws and mastic.

  6. onacurve | | #6

    Thanks, Keith and Martin.

    I haven't had trouble with attic creatures in this house. That's not to say I never will.

    I'm not yet sure how I'm going to go forward. I need to keep costs at a minimum so I might use this recycled polyiso I have left, which has a black fiberglass facing that tape won't adhere to, and glue a sheet of foil-faced 1/2" polyiso that Menard's has for $9/sheet. I don't have much ductwork up there so I think 5 sheets will be enough. I can tape to that.


    After looking it over more this weekend, I think I could bring the return ducts inside without a huge amount of trouble. My mechanical closet is in the center of my 1000 sq ft house, so I can get to each of my 3 bedrooms, kitchen and family room pretty easily with short runs. I might just get a few sections of spiral duct and make that work. Then I can have a more consistent insulation layer in my attic.

    I don't think leaving the return ducts exposed in the living space will look too hideous. I like the look of ones I've seen on the web but they are all in high ceilings, not 8 ft ceilings like mine.

    A tip to whoever is managing the rebuilding of this website -- use Disqus for the discussion boards. It's easier and works much better.


  7. user-626934 | | #7

    An even easier solution would be to put a single return (properly sized, of course) in the wall near your mech. closet and use transfer grilles for the rooms that get supply air and have doors that close.

  8. onacurve | | #8

    Thanks John.

    I wonder if Martin would agree.

    That sounds like an option worth considering, but from what I've read, a return in each room is better than a large central return. Are transfer grilles an equitable substitute? My sister has a system in her house with a single large return - horribly noisy.

    Also, where would you put the transfer grilles? I saw an advertisement that showed them installed high in the walls between bedrooms. That seemed like a great idea until I thought about the potential for certain sounds to come from a parent's bedroom that may not be appropriate for younger audiences.

  9. GBA Editor
    Martin Holladay | | #9

    Transfer grilles or jumper ducts work well -- much better than undercutting doors -- but there is (as you note) the potential for sound transmission. You'll have to weigh the advantages and disadvantages of all the suggested approaches.

    For more information on transfer grilles and jumper ducts, see Return-Air Problems.

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