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7 Ways to Improve Ducts in an Unconditioned Attic

Flex Ducts In An Unconditioned Attic Installed As Low As Possible

A lot of people have gotten the message that ducts for your HVAC system—and the system itself—should be in conditioned space. Researchers at the National Renewable Energy Lab (NREL) studied the effect of ducts in an unconditioned attic and found they add 25% to the cooling load in hot climates.  In their study, the NREL team assumed the ducts were sealed well.  New homes that have to pass duct tests are often sealed well, but the reality for many duct systems is that they’re leaky.  In addition to the losses associated with the leakage, unbalanced duct leakage creates other problems.  So, get those ducts out of an unconditioned attic if at all possible.

But what can you do if have to put ducts in an unconditioned attic?  Maybe you’re buying a tract home, and they won’t encapsulate the attic or move the ducts.  Maybe you’re replacing a system that’s in an unconditioned attic now and can’t afford to encapsulate the attic, too.  Maybe you’re worried about moving the building enclosure to the roofline.  Here are some ways you can make the most of a not-so-ideal situation.

Never, ever put ducts against the roof deck

The absolute hottest part of an attic on a summer day is the underside of the roof deck.  It can get so hot that you can’t even keep your hand against it.  Why some contractors think it’s OK strap a duct right up against the deck is beyond me.  In addition to the heat, the roofing nails coming through the decking can puncture the duct insulation jacket or the duct itself.

Duct against the roof deck in an unconditioned attic
Duct against the roof deck in an unconditioned attic

Keep the ducts low in the attic

Attics get very hot.  On a sunny summer day, the air temperature in an unconditioned attic can get up to 120° F or even a bit higher.  But what you may not know is that attics aren’t uniformly hot.  The hottest part of an attic is the highest part.  The coolest is down near the attic floor.

Ducts in an unconditioned attic installed too high
Ducts in an unconditioned attic installed too high

In the photo above, the installer didn’t put the ducts in the worst place, but they certainly didn’t try to minimize heat gain either.  The temperature is lowest at the attic floor, and that’s where those ducts should be, as they are in the lead photo above.  (Note that the lead photo shows the ducts before he attic was insulated.  After being insulated, those ducts were all buried in this dry climate house in California.)

Use a horizontal air handler

For the same reason you want the ducts to be low in an unconditioned attic, the air handler should also be low in the attic.  The furnace and air conditioner shown below are vertical, with the supply ducts at the top.  In summer, you’re putting cold air into those supply ducts higher in the attic because of the choice of vertical air handler.

Air handler in an unconditioned attic installed vertically
Air handler (furnace & AC) in an unconditioned attic installed vertically

A horizontal air handler in an unconditioned attic should be installed as low in the attic as possible, for the same reason the ducts should be low in the attic.  The photo in the insulation section below shows a horizontal air handler.

Seal the heck out of the ducts

Duct leakage is really bad when ducts are in an unconditioned attic.  Seal them with abundant quantities of mastic.

Seal the ducts with copious amounts of mastic
Seal the ducts with copious amounts of mastic

Insulate the heck out of the ducts and air handler

Attics get really hot.  Insulation helps reduce heat flow.  Use a lot of it.  Unfortunately, standard duct insulation is available only up to R-8.  In a dry climate, you can bury the ducts in blown attic insulation.  In a humid climate, you might be lucky and get away with that if you put only R-8 on the ducts.  But buried ducts in a humid climate are at risk for condensation.  The building code now allows it, but only with R-13 on the ducts before you bury them.

Air handler and plenums well insulated in an unconditioned attic [Photo by Mike MacFarland, used with permission]
Air handler and plenums well insulated in an unconditioned attic [Photo by Mike MacFarland, used with permission]
An alternative is to put closed-cell spray foam insulation on the ducts before burying them.  The photo below is from a Building America report presentation by Robb Aldrich on buried, encapsulated ducts.  Inside that mound of closed-cell spray foam insulation is a duct.  The whole thing was buried deeply in blown insulation after this photo was taken.

Spray foam encapsulated duct that will be buried deeply in insulation in an unconditioned attic
Spray foam encapsulated duct that will be buried deeply in insulation in an unconditioned attic [Photo from Building America report by Robb Aldrich]

Increase the air velocity in the ducts

This recommendation helps maintain comfort in the home.  With ducts in conditioned space, slower moving air reduces the resistance to air flow, which is a good thing.  When ducts are in an unconditioned attic, the air inside picks up heat in summer.  The more slowly the air moves through the duct, the more the temperature of the conditioned air will rise.  Moving the air faster spreads the heat gain across more air with less temperature rise in each cubic foot.  So design the duct system for faster air in this case.

Keep your attic cooler with a reflective roof or radiant barrier

Another way to reduce heat flow into your ducts from a hot, unconditioned attic is to keep the attic cooler.  The best way to cool your attic is by stopping the heat before it has a chance to enter by using a reflective roof.  A good second choice would be to install a radiant barrier beneath the roof deck.  A really bad choice would be to use powered attic ventilators.

An infrared image of a hot, dark roof
This infrared image of a dark roof shows lots of heat on the roof. Much of that heat makes it into the attic, but reflective roofing can reduce it tremendously. [Photo by Cameron Taylor, used with permission.]
Yeah, ducts in an unconditioned attic are often a big liability.  They require you to get a bigger air conditioner and spend more on your heating and cooling bills.  But follow the guidance above, and you can make that liability much smaller.


Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.


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The Best Way to Cool Your Attic


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This Post Has 23 Comments

  1. Allison, I got a slight issue with keep ducts low in the attic. I would clarify that unless they are buried under insulation, they should be kept off the insulation by at least a few inches & in some cases can argue that being higher up can be better.
    The first reason is this allows the radiant barrier to work properly & eliminates conductive transfer. The second is the second hottest point is generally the top of the insulation during the summer with the sun beating down / holds the heat longer than the air. The third is this also allows them to pull them tight and/or seal around them easier. A minor fourth as most attics don’t have proper ventilation and location matters is that by being higher up they will be exposed to more outside temperature currents as compared to just stagnant / downward driven heat.

    1. Hi Sean, a few rebuttals to your post:
      (a) A radiant barrier (in this case, an insulated duct’s outer liner) only needs an inch or so to work, and even if ducts are touching ceiling insulation, that only represents a fraction of the circumference. Also, a radiant barrier only reduces conductive heat transfer, not nearly eliminate it.
      (b) I’d like to see research that demonstrates that ducts supported just above ceiling insulation incur more heat gain than ducts located higher in the attic. Let’s just say I’m skeptical that the ceiling insulation is hotter than the air above it.
      (c) In my experience, your third point is moot since mechanical rough is typically completed before insulation. In new construction, I would never allow a duct installation crew to trample through pristine ceiling insulation to do their work!
      (d) My own measurements of attic temperature gradients (from top of insulation to ridge) don’t support your premise that attic air is warmer at the bottom! In particular, I don’t know what ‘stagnant / downward driven heat’ means.

      1. David – sorry I wasn’t clear enough & nice reply. You can have three or more temps in a space, roof sheathing, the air temp, & then the actual temp of the insulation. Air temp is the most variable & I will easily concede that in general air temps are higher towards the top. The catch is the top of the insulation (my issue) which if you take an infrared camera to might surprise you on how hot it really is & how long it holds the temps.

        Missed A – a radiant barrier does not block conductive heat loss or gain, no space = no radiant blocking & conductive is allowed to pass right through. As for any other “R” properties that will help with conductive

        C – yes you are correct, but what happens when they fail the duct test & said builder…. The other problem by only leaving an inch or two is it is harder on subs to install nice & level or what happens if they want to add more. Hmmm just where are those balancing dampers at…

        D – Center of attic = very little air movement if any as most air movement happens in the space between the trusses from the soffit to ridge (yes this can be thrown off by duct leakage / over pressurizing the house forcing air up / or using a dreaded PAV thus pulling from house but we are talking properly built). Downward driven is summertime reverse stack effect & solar drive.

        Hope that clears up my thoughts more though of course my first thought is – yeah don’t do it & if you have to use a properly done Hot Roof system

    2. Sean & David, thanks for your comments. My recommendation for a radiant barrier was to install it on the underside of the roof, not just as the duct insulation jacket or lying on top of the insulated attic floor. They’re often not cost effective to install in efficient new homes, as Michael Blasnik showed in his Houston study, but if you have ducts in the attic, they may help a little bit. I almost left this recommendation out, but there are some cases where it could help.

    3. Sean,
      You say “insulation” generically. Blown fiberglass is different from blown cellulose. Fiberglass lacks the mass and thermal qualities to form an air seal and be an effective barrier between attic-installed ducts and the hot (or cold) ambient environment of unconditioned attic space. Cellulose is required to be installed to a specific depth to account for that warmer several few inches. That means installing the cellulose at a greater depth above the ducts. If the rest of the attic is R-49, above the ducts should be R-49. I do it when I have to but my best advice is never to install HVAC equipment in an unconditioned attic. Doing so is irresponsible…and just plain dumb.

  2. @Allison, regarding radiant barrier… I would argue that it’s never cost effective to install a radiant barrier on the roof in an existing home. Labor costs get rather crazy (especially for products associated with a free steak dinner ;-). Better to spend that money on duct tightening and/or additional insulation. OTOH, the additional cost foil-coated OSB is reasonable in new construction or when roof deck is being replaced.

    Regarding velocity… what Proctor has argued is that downsizing a replacement system (which typically means lower system airflow) can increase conducted losses since duct surface area is larger relative to the volume of air being moved. I’m not sure I buy that the converse is true: that increasing airflow in an existing system (unless airflow is already too low, which we know is a common problem with other implications). I know that in hydronics, increasing the fluid velocity through a heat exchanger will actually INCREASE total heat transfer somewhat. You see also this in expanded performance tables for AC’s and heat pumps — increasing the airflow increases total capacity! The trade-off is in blower energy, not in heat transfer through the heat exchanger. Just the opposite.

    In a new construction project where there’s no other option than to locate ducts in the attic, my target CFM and branch velocities are not be influenced by that. Instead, I reduce duct surface area by focusing on shortening the branches, NOT by using smaller ducts.

    1. David, those are all good points, as usual. On the radiant barrier issue, I’m with you on existing homes. It could be a DIY project, though. My article is aimed at both new construction and existing homes, and I was thinking more of new homes for radiant barriers.

      On the velocity issue, again, I didn’t make it clear but that recommendation was for new duct systems. Lower surface area is only part of the reason for increasing the velocity. The other result of smaller ducts and higher velocity is that the there’s less temperature increase in the conditioned air when it’s moving faster. Even if the heat gain in the duct were the same, faster moving air means the BTUs are spread over more air.

      1. I think we need to clean up some of the physics here. The overall thermal resistance between the air in a duct and the air outside of it depends partly on the convective heat transfer resistance between the moving air inside the duct and the interior duct wall. If you increase the airflow rate, and keep all else constant, you will reduce that internal wall resistance, thus reducing the overall resistance which will then increase the heat transfer through the duct. It is true for this case that the change in air temperature through the duct will decrease, but that is because of the higher airflow rate, not because of the higher velocity. It is the same situation as mentioned for water flowing through a coil. If you increase the water flow rate, you will increase the heat transfer rate even though the temperature change of the water will decrease. Remember that heat transfer rate equals fluid mass flow rate times fluid specific heat times fluid temperature change. Thus, you can have a higher heat transfer rate with lower temperature change of the fluid when the fluid mass flow rate increases.

        If you lower the air velocity in the duct by keeping the airflow rate constant but reducing the cross-sectional area of the duct, that is a different story since you are changing the surface area of the duct. But keep in mind that pressure drop will also increase as well as fan power and duct leakage.

      2. Roy wrote: “…heat transfer rate equals fluid mass flow rate times fluid specific heat times fluid temperature change. Thus, you can have a higher heat transfer rate with lower temperature change of the fluid when the fluid mass flow rate increases.”

        Thanks for explaining this better than I did. My point was that it’s not a zero sum trade-off. Ignoring the pump (or blower) energy trade-off, if you increase flow rate, the reduction in delta-T is not exactly proportional to the increase in mass flow, thus total heat transfer slightly increases with flow rate. The incremental increase in heat transfer diminishes as flow rate increases.

        BTW, in your last paragraph, I think you meant “If you *increase* the air velocity…”

        1. David, thanks for catching that. The last paragraph should have been:

          If you increase the air velocity in the duct by reducing the cross-sectional area of the duct while keeping the airflow rate constant, that is a different story since you are also changing the surface area of the duct. But keep in mind that pressure drop will also increase as well as fan power and duct leakage.

          1. RoyC,
            What is “fan power?” The load on a fan motor is measured in amperes. A heavier load draws more current.

  3. Hi Allison,

    Another great post.

    A question as it relates to “seal the heck out of the ducts” and the general principle of “do no harm.”

    Should you always measure total external static pressure as a “test-out” procedure when sealing ducts? I think the answer is yes. But I’m curious if there are reasons or situations why measuring the impact of duct sealing on a system’s static pressure may not be necessary.

    1. @Casey, great question! As you surmise, duct sealing naturally and necessarily increases external static pressure. The question is to what extent (the leakier the ducts, the bigger the increase), and whether there’s a meaningful reduction to system airflow. I would say the only exception where you don’t always need to check external static pressure is if it’s a variable speed blower with constant CFM. But even then, if pre-sealing static is on the margin, sealing the ducts may push it past the blower’s limit. With other types of blowers, a reduction in airflow may impair AC performance. Worst case, the evap coil turns into a block of ice long after the duct sealing crew is gone. (Yes, that has happened!)

      The problem we run into is that many duct sealing crews don’t have the technical training & experience to assess system airflow. Also, if test-out static reveals airflow has dropped too low, the technician needs to know how to adjust the blower or in some cases, modify the duct system in order to reduce external static (e.g., add another return, increase filter surface area, etc).

      1. David, of what benefit is it for a laborer who seals sheet metal ducts to have a working knowledge of how to assess system airflow? The assessment was done by the designer.

        And, exactly how does duct sealing increase external static pressure? Upon what is the external air pressure increased?

        Why do you say “pre-sealing static may push the blower past its limit?”

        And how does a variable speed blower maintain a constant volume?

        1. Hi Larry,

          Lots of good questions. As a poor substitute for David, I’ll take a crack at answering a few.

          As a contractor who is selling the benefits of duct sealing, you (hopefully) want to deliver those benefits to the customer. You don’t want to sell the benefits of duct sealing (comfort, IAQ, reduce energy use) and then have your customer experience reduced airflow, higher energy bills, and/or (worst case) a cracked a heat exchanger. Whether the person sealing the ducts is a laborer or not, I hope the builder (in new construction) or owner of the HVAC or home performance company (in existing homes) wants to deliver the results that they promise to their customers.

          As far as your other questions, they might be too detailed for a comment section. But static pressure increases when there are obstacles to airflow (kinks, bends, filters, etc.) and generally has an inverse relationship to velocity pressure and airflow. Higher static pressure means either reduced airflow or more energy is needed to move the same amount of air. PSC motors faced with higher static pressure will move less air. ECM motors will ramp up energy use in order to try maintain airflow.

          For more information you can probably use search terms like “total external static pressure” and “duct sealing.” There are lots of good analogies using garden hoses to make these ideas slightly more intuitive.

        2. @Larry, I should clarify that my comment referred to duct sealing work done AFTER the system has been initially commissioned (i.e., in new construction, the installer is responsible for verifying proper airflow as per the design after the ducts are initially sealed during construction).

          The problem is with existing homes with leaky ducts. If the homeowner decided to have the ducts sealed, this will change system performance. In that case, the duct sealing crew is responsible for ensuring proper airflow after the sealing work is done. Of course, the duct sealing crew may not be aware of the initial design CFM, but there are well established guidelines for airflow based on AC tonnage.

          > exactly how does duct sealing increase external static pressure?
          This is sorta hard to explain without a whiteboard, but leakage paths in a duct system reduce static pressure in the same way that adding an additional return or supply reduces static pressure. Conversely, sealing those leakage paths will have the same affect as a closed vent, clogged filter or crushed duct.

          Manufacturers publish blower tables that correlate external static to airflow. This is typically how a designer or field tech assesses airflow.

          > Why do you say “pre-sealing static may push the blower past its limit?
          What I said was that duct sealing may push a constant CFM blower past its limit if it was already on the margin to begin with. For example, if a constant CFM blower is rated for a maximum external static of 0.7 IWC (inches of water column), and it is already close to that limit before sealing, then sealing the ducts may push external static past its limit. This can cause the blower to buffet or stall. Best case, airflow will drop off past 0.7 IWC.

          Variable speed blowers that are constant CFM (note that not all ECM or variable ECM blowers have this feature!) use electronics to monitor the motor’s torque and RPM’s and adjust as necessary.

          I’m happy to provide this explanation, but as Casey said, your questions are a bit too detailed for a blog comment string, not to mention way off topic. In any case, these concepts should already be familiar to anyone who oversees duct sealing work or otherwise modifies duct systems.

  4. Roy C,
    You are assuming a delta T great enough to provide heat to transfer. Again, a well-insulated, but unwisely placed duct will survive in an unconditioned attic. It will never achieve the energy conservation possible if installed in conditioned space, but the loss won’t break the bank. The major “crime” is locating mechanical HVAC equipment in an attic. The industry seems to do everything it can to insure equipment needs replaced sooner rather than later. Shame on them!

  5. Allison, Several years ago I knew some folks who had bought a home that was well maintained and over the years had some improvements including a newer HVAC system with insulated ducts in the attic. During the summer (think South Louisiana) the one story house simply would not cool. After having two HVAC companies evaluate it and tell them the system was too small, they planned to have the 2nd company increase the size of the equipment. I asked them to let me go in the attic and take a look. I bought about $250 worth of hard fittings and sealant at Lowe’s and a friend helped me install them at every turn and stretch out the ducts so they were tight. I removed more than 25% of the duct work. The house cooled fine after that.

    1. Bob,

      It’s great the HVAC system functioned sufficiently after sealing all the duct leaks. But what is the insulation level (R-value) above the ducts. “Cooling fine” does not mean “cooling efficiently.”

  6. Larry,
    In loose fill applications in spite of all their benefits, neither fiberglass nor cellulose insulation provides an adequate air seal. While some research suggests that cellulose resists and/or blocks air movement more effectively than fiberglass, these materials are and should only be installed to reduce the passage of heat – not the passage of air. Only foam insulation and certain caulks can block air movement in a way that truly improves comfort and boosts energy efficiency.
    In dense pack wall applications, the Pink L77 product is the equivalent of cellulose in infiltration performance. source:NAHB research study 10/09

  7. Larry: Fan power is the electrical power supplied to the fan motor, typically measured in watts. It is what your electric meter measures and determines how much you pay the electric company.

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