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Q&A Spotlight

Dealing With Ductwork in an Unconditioned Attic

What’s a good solution for a 1960s brick ranch house that has the furnace and ductwork installed in the attic?

Problems in the attic: Not only is the furnace and ductwork in this Climate Zone 4 house located in an unconditioned attic, but the homeowner also is dealing with a powered attic ventilator.
Image Credit: Tedliu

Ted has more than a few cobwebs in his attic. The unconditioned space also houses his HVAC system.

The 1,800-square-foot brick ranch in Climate Zone 4 dates from the 1960s, but the previous owner installed both a furnace and ductwork in the attic just four years ago. Ted also has inherited a powered attic ventilator. Although both the attic floor and the ductwork are insulated, Ted recognizes the situation isn’t ideal.

Summer hasn’t even arrived, he notes in a post at GBA’s Q&A forum, and when the temperature outside climbs to more than 95°F, the temperature in the attic tops 120°F. That has kept the ventilator working, but Ted surmises the high temperatures are reducing the efficiency of his air conditioning equipment.

“Should I turn off the ventilator so it won’t suck cool air from the conditioned area?” Ted asks. “Should I put more insulation material on top of the attic floor and the ductwork? Will a radiant barrier under the rafters help?”

Those are the questions to get this Q&A Spotlight rolling.

Add more insulation

Dana Dorsett would have Ted get the level of insulation much closer to the R-49 recommended in the International Residential Code.

“In a DOE Climate Zone 7 (not to be confused with a USDA plant hardiness zone 7) a mere 8 inches of fluff is woefully inadequate,” Dorsett says. “IRC code-minimum for zone 7 is R-49, and you have barely half that. Air sealing all the ducts and air handler connections and seams, and air sealing all the duct boots to the ceiling gypsum (and any other penetrations) is a critical first step prior to adding any insulation.”

(As it turns out, Ted initially confused the two climate zone designations. His house is located in DOE Climate Zone 4, not Zone 7 as he had originally indicated. But the code recommendation for attic insulation is the same — it’s R-49 in both in Zone 4 and Zone 7.)

Dorsett suggests that a low-e silver-colored paint on the underside of the roof deck, or a perforated radiant barrier under the rafters, would make a difference during the cooling season (with only modest increases in heating costs during the winter). The first step, though, is more insulation.

“But you might start with over-blowing the floor-fluff with 6-8 inches of cellulose, to bring the combined depth to about 15-16 inches,” Dorsett says. “(As a DIY, do what the pros do — install multiple depth gauge strips to know when you’re there.)”

Turn off the fan

GBA senior editor Martin Holladay notes that the fan is problematic: “The best approach is to immediately disconnect the powered attic ventilator, and then to hire a contractor to transform your vented unconditioned attic into an unvented conditioned attic,” Holladay says.

No matter what the climate zone, a powered attic ventilator is not a good idea, he adds. (For more information on this issue, see Fans in the Attic: Do They Help or Do They Hurt?)

“Creating a conditioned attic to bring HVAC equipment inside the home’s thermal envelope is always good advice — it’s never a good idea to locate HVAC equipment or ducts outside of your home,” Holladay writes. “That said, the required work is expensive, so the investment often makes little sense from an economic payback perspective.”

Holladay continues, “Installing a radiant barrier under your rafters may make sense in your case. Moreover, it’s always a good idea to seal leaks at ductwork seams, to seal leaks in your home’s ceiling (the attic floor), and to improve the R-value of your attic insulation and duct insulation.”

The cooling effects of an attic ventilation fan are in most cases “negative,” Dorsett says. That is, Ted will end up using more energy, not less.

“Attic ventilation is really never about cooling,” he says. “It’s primarily about moisture control, to keep the wood in the attic from getting moldy or rotting. In more humid climates such as on the Gulf Coast, in air conditioned homes even passive attic ventilation puts more moisture into the attic than it removes.”

Attic conversions can be expensive

As if to underscore Holladay’s point, Brian Gray, writing from Chicago, says that he faces a nearly identical situation as Ted does, and the fix is looking to be anything but cheap.

“I’ve been debating my situation for what seems like forever,” he says. It comes down to three options, the first of which is to convert his attic into a conditioned space by installing 4 1/2 inches of polyiso insulation above the roof deck and then adding a new standing-seam metal roof. He’d add open-cell foam below the roof deck to give the assembly the required R-value, but the bids range from $40,000 to $60,000.

His other options are cheaper, but they have some disadvantages.

“$40K+ is a lot of money to solve a problem, but I get a new roof, better curb appeal, and surely a higher resale,” he says. “That said, it’s a lot of money.”

Encapsulating ducts in spray foam

One option Gray is considering is to encase the ducts in his attic with a 3-inch layer of closed-cell foam, a job he believes would be possible with one or two DIY spray kits and a total cost of between $600 and $1,200. Would this idea have any value in Ted’s situation?

Because closed-cell foam can only be safely applied in layers, or “lifts,” of 2 inches at a time, Dorsett replies, that’s probably the limit of what should be considered for ductwork.

Using different versions of closed-cell foam, applied with different blowing agents, would be slightly more expensive but also allow thicker layers to be applied in a single lift. “At 2 inches, most [hydrofluoroolefin] blown 2-pound foam would be north of R-14, and north of R-20 at 3 inches,” Dorsett says. “It still may not be financially rational to go more than 2 inches.”

Open-cell foam, he adds, is another option. Cheaper and less of a threat to the environment, open-cell foam is relatively vapor-permeable, meaning some moisture will collect during long periods of heavy air conditioning use, he says, “but unlike fiber insulation, open-cell foam isn’t air-permeable, and the rate of accumulation is still quite slow, probably slow enough to be OK in a Zone 5A climate.”

Dorsett says that most spray foam would have to be coated with an intumescent paint to be fully code-compliant.

How much insulation do ducts really need?

Ted notes that flexible ducts sold in retail stores seems to be insulated to R-4 or R-8, but by his own measurements conditioned air still shows a 10° to 15° drop from one end of the supply trunk to the other. Should there be such a steep drop in temperature in a run of only about 10 feet?

“You are correct that the drop in temperature indicates that the duct is poorly insulated,” Holladay replies. “R-4 or R-8 duct insulation is certainly better than nothing, but for those of us who care about insulation, even R-8 doesn’t make much sense for ducts that are outside of the home’s thermal envelope.

“After all, most new homes have R-49 insulation on the attic floor,” Holladay continues. “When the attic is 20°F, that insulation is addressing a delta-T of 50 F°. But the air in the ducts is facing a delta-T of maybe 100 F° or 120 F° — a greater delta-T. So (logically) the ducts deserve more, not less, insulation. Obviously, it’s hard to install R-60 duct insulation. But I raise the delta-T discussion to demonstrate why it’s so important to bring your ducts inside your home’s thermal envelope.”

Dorsett adds this thought to the discussion about duct work insulation: “The reason ducts don’t need R-60 insulation despite the higher temperature difference is that total square footage of the duct surface area is much less than the attic floor, and the duty cycle on most systems isn’t anywhere near 100%. R-8 is usually fine for supply ducts, and less is OK for return ducts (due to the smaller temperature difference).”

This may be true, Holladay says, but even R-8 insulation doesn’t solve the problem when ducts are located in the wrong place. “Ducts need to be indoors,” he says.

Our expert’s opinion

Peter Yost, GBA’s technical director, adds this:

No organism we know of puts its lungs, unsheltered, outside its body; so putting HVAC systems outside a conditioned space is hardly biomimicry. But there we are, with ducts and HVAC units in the hostile environment of an unconditioned attic.

First thing: Kill the attic ventilator. Unless you have a perfect air barrier at the ceiling line — and why would you if you are already in a building where someone chose to put the space conditioning outside? — you will be pulling makeup air along the paths of least resistance, including from the conditioned space below. You can’t vent your way out of this predicament.

The best option is to move the conditioned boundary to the roofline. Insulate and air seal to pull the attic inside the building. This could and should have been done in the first place, but now you have another chance to make it right.

The next best option is to encapsulate the ducts. See recommendations for accomplishing this at the Building America Solution Center. Note all the tabs for various items of key information.

For more information, see two other suggested solutions: Ducts Buried in Attic Insulation and Ducts Buried in Attic Insulation & Encapsulated. Note that the former approach is not recommended in moist and marine climates while the latter approach is OK for all climates.

18 Comments

  1. Malcolm Taylor | | #1

    Biomimicry
    "No organism we know of puts its lungs, unsheltered, outside its body"

    Great analogy.

  2. epom47 | | #2

    Alternative strategy
    A much cheaper diy alternative might be to place the ducts along the top side of the ceiling then build air tight chases around them with sheating. Follow this with any kind of insulation you like. The air handler could be treated the same way. This would, in essence, be like bringing the ducts inside the home. This might be a lot of work but the expense would be minimal for a diy'er.

  3. Malcolm Taylor | | #3

    Epom,
    Sounds like an absolute nightmare to me. Above the ceiling you have the roof structure to be navigated. At best ceiling joists, more commonly the bottom chords and diagonal members of trusses. How you would frame, air-seal and then cover it with insulation is hard to imagine.

  4. User avater
    Elden Lindamood | | #4

    You can lead a horse to water
    My mother recently bought a 1980's walk-out ranch house in western ND, climate zone 6. I visited for the first time last winter and it was raining in the house. She said, with alarmingly little urgency, "that happens when it is sunny out".
    She asked me to go into the attic to find the roof leaks or "where the snow is blowing in through the soffits". I found, instead, a vintage 3 ton AC unit with 1" rigid insulation taped all over it and "insulated" flex duct snaking all over the poorly insulated attic. The rigid insulation was saturated from condensation. The flex ducts were literally filled shut with ice in the low spots, compressing the scant insulation layer beneath them. The sun would come out, melt some ice, and it would find it's way to the nearest hole into the house. There was no evidence of roof leaks, even around the attic fan.
    I sat her down at the dining room table and told her she heeded to get a new AC system installed, INSIDE THE THERMAL ENVELOPE, and remove the old one from the attic. Then air seal and insulate the attic much better (incidentally she also used 1200 gallons of propane last winter).
    Her response was "The water will evaporate from the ducts by the time I need the AC on". She is also still convinced that snow is blowing in through the soffits despite my evidence that is is not and in fact can't because there are no vent chutes to connect the soffits to the attic.
    Lesson: Let's not forget we are up against the general public. If one contractor tells them it is a $40,000 fix, and another tells them it is a $400 fix, the majority of people will believe the latter, science and common sense be damned. Things that make us cringe are the norm, and what concerns us seems inconsequential to far too many.
    Nevertheless, I will persist.

  5. User avater
    Dana Dorsett | | #5

    Try a $4000 fix @ Elden Lindamood
    "If one contractor tells them it is a $40,000 fix, and another tells them it is a $400 fix, the majority of people will believe the latter, science and common sense be damned."

    Try to investigate the ~$4000 fix:

    In more cases than not a 3 ton AC with the ducts in the attic has a ton or so of load that's just from the location of the ducts in the attic and the associated duct gains, leakage, etc.. The remaining 2 tons of capacity is at least 1.5x oversized for the actual 1% load. So the real cooling load for the HOUSE is likely to be no more than 1-1.3 tons, but it could even be on the order of 3/4 tons with the air leaks to the attic & sagging attic insulation are corrected. Of course a Manual-J is better than a WAG, but the WAG probably isn't that far off.

    Depending on the floor plan a single 1 ton or 1.25 ton cold climate mini-split may be able to cool the whole shebang more comfortably & cheaply than the noisy Medusa of moldy flex in the attic, and also heat most of the place 75% of the time at about half the cost of heating with propane (or less). If the mini-split keeps up with the cooling load sealing up the registers to the ice-maker in the attic (lowering both the heating & cooling load) would be easier to argue, without having to start the 5th grade science lesson all over again.

    The napkin calculation on heating costs goes like this:

    ND retail propane is running about $1.40/gallon in recent years, so she probably spent ~$1700 on heating this season. It's been as high as $2/gallon back when WTI crude was priced closer to $100/bbl, but there was also an extreme spike in 2014 when it went to $4.50/gallon. See:

    https://www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=W_EPLLPA_PRS_SND_DPG&f=W

    ND residential retail electricity has been reliably under 10 cents, ( https://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_6_a )and with the high penetration of zero marginal cost wind power in that state there is growing downward pressure on wholesale electricity pricing- it's not likely to go over 10 cents any time soon. Even assuming an as-used COP of 3 (HPSF 10.2, well below the nameplate 13-ish HSPFs of current cold climate mini-splits ), a dollar of electricity buys her 102,000 BTU of heat delivered into the house.

    That's more than just the 91,600 source fuel BTU content of a gallon of propane. At $1.40/gallon dollar only buys 0.71 gallons of propane. Burned in a 95% efficiency furnace or boiler that delivers 91,600 x 0.71 x 0.95= ~62,000 BTU per dollar, without even correcting for distribution losses, or the cost of the electricity used.

    When it's sub-zero outside the as-used efficiency of the mini-split will be less than that, but when it's above 30F it'll probably be higher, but even at -10F but if the EIA electricity & propane pricing is true in her case, the cost of the heat coming out of the mini-split will still be on-par with that of condensing propane at sub-zero temps. The total capacity of the mini-split probably isn't enough to cover the load at -10F, and the heat distribution would likely be terrible at -10F, so the propane burner is going to stay, but the mini-split would cut the propane use by half or more, replacing the $800-900 of propane burned with $400-500 of electricity use, for a net savings of at least $300/year, but it could be as high as $500/year. The thing eventually pays for itself just on heating season savings within the anticipated life cycle, but it'll also save during the cooling season. It'll probably pay for itself in COMFORT right away.

  6. Dennis Heidner | | #6

    RE:You can lead a horse to water
    I would also gently remind your mother that stagnant water in ducts - are great breeding places for legionnaire disease... and the air moving over the water when the ducts unfreeze are a wonderful means to spread the Legionella bacteria in the house. Visit a local pharmacy and buy about 100 face masks for her to wear while the air conditioner is in use... it is a some what subtle reminder of how dangerous the situation can be...

  7. Malcolm Taylor | | #7

    Dennis
    If Elden's mother is anything like mine, he would probably be better off saving his breath to cool his pie.

  8. Joe Duchek | | #8

    Attic Ducts Buried in Cellulose - CZ5A / CZ6A
    I realize that burying insulated attic ductwork with cellulose isn't recommended in the eastern (moist) half of the US, but is anyone aware of actual condensation issues from "properly" buried (but not spray foamed) insulated ducts in the upper half of CZ 5A or lower half of CZ 6A (Great Lakes region, etc.)?

    Most of the cautionary stories seem reference humid, southern locations or the use of uninsulated, poorly sealed duct work, and therefore I'd be interested to hear about anyone's "up north" experiences when the guidance linked in this article was generally followed. BPI certified contractors here in the Chicago region have suggested / quoted me the loose-fill over ducts solution and therefore I suspect that they regularly do it without issues. Obviously not ideal, but I am juggling the near-term and long-term options for a poorly insulated attic with ductwork.

    Thanks in advance.

    Joe

  9. User avater
    Elden Lindamood | | #9

    Thanks all
    Thanks for the napkin calc Dana. I'll add it to my arsenal, but when I hoisted the mini-split flag it was pulled back down with the typical "I don't want to look at that ugly 'box' on the wall". Malcolm's quip is apt.
    I had thought of the legionnaire's argument too, but hadn't plied it. That may, in fact, speak more to her anxieties than spending $1500/yr on propane.
    Joe, I thought of covering the ducts too. They are hung from trusses, draped on top of the insulation, crossing each other, and generally poorly done. That wouldn't solve the AHU issue either, so I think the better solution is to make them go away.

  10. Bruce Thompson | | #10

    Fiberglass batts under the roof?
    What happens if one simply places insulation between the roof rafters?

  11. User avater GBA Editor
    Martin Holladay | | #11

    Response to Bruce Thompson
    Bruce,
    Assuming that there isn't any rigid foam above the roof sheathing -- and we know that Ted's roof doesn't have any exterior foam -- you can only install fiberglass batts between the rafters if you also create a ventilation channel between the top of the fiberglass batts and the underside of the roof sheathing.

    Creating this type of ventilation channel will only work if (a) there are soffit vents at the base of each rafter bay, (b) there is a ridge vent at the top of each rafter bay, and (c) there are no dormers, skylights, valleys, or hips that interfere with the ventilation channels.

    To learn more about all of the different ways to create an insulated sloped roof assembly, see this article: How to Build an Insulated Cathedral Ceiling.

    Now, to your question:

    Q. "What happens if one simply places insulation between the roof rafters?"

    A. One sets up a risky situation that encourages moisture accumulation in the sheathing, mold, and possible sheathing rot. And one is guilty of a code violation.

  12. Phillip Norman | | #12

    Please see my contribution to this conversation, in blog posts:
    https://energyconservationhowto.blogspot.com/search/label/Attic%20HVAC
    I freely share lessons from work I do by-myself as a weatherization contractor with engineering and technical writing experience. I find much fault with treatment of HVAC air flow with ignorance of energy involved that must be correctly examined with Bernoulli principle. Fast-moving fluid is unlike squeezed toothpaste. Design to keep the air moving at steady velocity, never dead-headed. Never to leak from flimsy and ragged fittings, hacked-in on the job. HVAC ducts should be buried under insulation, hard-covered too. Putting a furnace in an attic is not ideal, but will happen. Let's look for constantly-improving methods, through sharing.

  13. Mat R | | #13

    Would it be feasible or advisable to encase a short run of ducting in the attic (about 4 feet) in rigid foam, with airtight seams? If not, what would be the problems with that approach?

  14. LorieWhitaker | | #14

    Hi all, I hope you can help. 1965, two story house, located in DOE 3A (hot/humid), Dallas, TX. I've got a 21 seer machine cooling my upstairs (1000 sq ft) and some old Carrier cooling the downstairs (1000 sq ft). 3 years ago, I had new siding put on the top part of the house (bottom is brick). This required the vinyl siding to be removed, the layer of styrofoam insulation removed, the original wooden siding taken off and the tar paper the house was wrapped in removed. Then we put in new insulation (R15), covered the walls in plywood, applied vapor barrier and new LP siding.

    Ever since then I have one vent that does nothing but condensate. It is coming right off of the AC plenum, so I was thinking it was the difference of the temps (keep it around 74 during the day, down to 64 at night). I'm running a dehumidifier 24 hrs upstairs.

    The AC inside units are both located within the house, however the upstairs unit doesn't have a proper ceiling on it. There is a piece of wood on top, but I know it's not stopping any hot air as I can feel it when I open the closet it's in.

    My attic is very hard to get around it - you can't stand up in it. The ducts for the upstairs unit are in this attic. There is no thermal foam/reflector barrier at all, although I do have 2 whirly birds and 2 new passive vents to help move the air out of the attic (the 2 passive vents have resulted in a dramatic cool down in the attic after the sunsets, which is nice - i know this based on my power consumption). When I bought the house there was about 1/2" of fluff up there (original construction?) and we added up to 12" additional fluff (there's not much room to add much more or the entire attic would be full LOL).

    The attic ductwork is flex duct work. It was new when the AC was put in 7 years ago.

    Recently I had another HVAC company come out and put in a new bathroom vent and they told me they believed the duct work was full of water. I called out the original HVAC company and they took a look and said they saw no water. Now, I know I should get up there myself, but I haven't.

    The new HVAC company recommended that I get metal ducts put in instead of the flex ones. They believed that would stop the moisture.

    After reading this thread, I'm honestly not sure what to do. The temps in Dallas can get over 105 in the summer, so I'm not sure if bringing the attic into the conditioned air is feasible. When I bought the house I was told the pitch of the roof is so steep they can't get in there to apply the spray foam insulation...has that changed do you think? Are there other duct types I should be looking to replace the flex ones with? Would that even change anything? Is it the fact that we sealed up the house much more than it had been before and now it can't properly breathe?

    For what it's worth, I'm not having any issues with the unit downstairs, but those ducts are within the AC part of the house. Prior to this, the downstairs AC unit cooled the entire house (not too well). Upstairs never got below 75 at night and the ducts were those box metal ones from god knows when and never had moisture issues.

    Any suggestions are appreciated. If it's something I need to save up for to do it right, I will.

    1. User avater
      Jon R | | #15

      If I understand correctly, you are getting condensation on AC ducts in unconditioned space. I'd push harder on getting someone to apply spray foam to the ducts.

      1. LorieWhitaker | | #16

        You understand correctly! So the ducts are that flex duct...would that withstand the spray foam? And how much should they apply to get the R factor I need?

        1. User avater
          Jon R | | #17

          This appears to be a good reference:
          https://basc.pnnl.gov/resource-guides/encapsulated-ducts#quicktabs-guides=1

    2. User avater
      Dana Dorsett | | #18

      >"I do have 2 whirly birds and 2 new passive vents to help move the air out of the attic (the 2 passive vents have resulted in a dramatic cool down in the attic after the sunsets, which is nice - i know this based on my power consumption). "

      Turbine vents usually depressurize the attic sufficiently to pull conditioned air up through the air=-leaky ceiling into the attic, which increases the cooling energy use. The passive vents probably reduced that negative effect by allowing more air to flow from the the outdoors, but it's still almost always better from an energy use point of view to NOT use turbine vents or powered attic vents of any significant size.

      In your location it may be worth removing/sealing all roof/attic vents entirely, and air sealing all leak between the attic & outdoors for both energy use and moisture control reasons. In humid climates like the Gulf Coast (including up to Dallas) vented attics usually bring more moisture into the attic than they purge. Even if the insulation layer is at the attic floor you'll probably be better off making the roof the pressure boundary of the house, in which case the absolute moisture levels (but not the temperature) of the attic will track that of the conditioned space below, which should reduce the condensation potential dramatically.

      Peak attic temperatures and most of the moisture can be reduced by installing perforated alumized fabric radiant barrier the underside of the rafters over the entire attic ceiling. The perforated fabric type is preferred, since that guarantees a drying path toward the interior, whereas the overpriced bubble pack or the unperforated aluminized fabric types are true vapor barriers, which could create moisture traps. At about 5 perms the perforated goods still limits the rate of moisture flow, so even if there is some minor air leakage at the soffits or imperfectly sealed vents the main volume of the attic won't be subjected to the high dew points of summertime air freely flowing through the attic, which is what you have right now. Air leakage from the outdoors is the primary enemy you're fighting here.

      Pictures of the roof, vents, turbine vents and attic from both the interior and exterior might be useful.

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