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How to deal with ductwork that is installed in an unconditioned attic and a powered attic ventilator?

TedLiu | Posted in Energy Efficiency and Durability on

I am a homeowner of a brick ranch built in 1960s in Climate Zone 7, and it is about 1,800 sq. ft. [Editor’s note: It turns out that Ted is really in Climate Zone 4.]

The previous owner had the furnace and ductwork were installed in the attic and a powered attic ventilator (PAV) too in 2013. The attic floor does have some fiberglass as insulation material and it is about 8 inches. The ductwork is insulated flexible duct (supply duct is black from the outside and the return duct is silver from the outside).

In hot days last week (about 95F outdoor), the attic was over 120F and the PAV was running (thermostat of PAV set at 120F). All the ductwork were warm from the outside so I think the hot attic reduced the efficiency of the A/C (also newly installed in 2013).

In hot days, the indoor temperature was about 76F but not able to go blow it when attic was hot. I read lots of useful articles on this website and in general, it seems the PAV and the radiant barrier are not recommended. I am not sure if the general talk also applied to my situation.

Should I turn off the PAV so it won’t suck cool air from the conditioned area? I see vent of the gable and I can see daylight come in on two edges of the roof so I think they are for outdoor air to come in to the attic.

Should I put more insulation material on top of the attic floor and the ductwork? Will radiant barrier under the rafters help?

Thanks in advanced for your comments and recommendations.


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  1. Expert Member
    Dana Dorsett | | #1

    The attic ventilator almost certainly raises the total amount of cooling power used.

    Both the supply & return ducts would be better off if they were silvery, but as long as they are both insulated it doesn't make a huge difference.

    Low-E silvery paint on the underside of the roof deck AND a perforated metalized fabric radiant barrier will make a difference during the cooling season, at the cost of a modest increase in heating season energy use. But you might start with over-blowing the floor-fluff with 6-8" of cellulose, to bring the combined depth of ~15-16". (As a DIY, do what the pros so- install multiple depth gauge strips to know when you're there.)

    In a DOE climate zone 7 (not to be confused with a USDA plant hardiness zone 7) a mere 8" of fluff is woefully inadequate. IRC code-minimum for zone 7 is R49, and you have barely half that. Air sealing all the ducts & air handler connections & 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.

  2. GBA Editor
    Martin Holladay | | #2

    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.

    For more information on this topic, see these articles:

    Keeping Ducts Indoors

    Creating a Conditioned Attic

    Fans in the Attic: Do They Help or Do They Hurt?

  3. Anon3 | | #3

    For a house in the 60s, if you don't suck air from your house into the attic you'll get air from the attic coming into your house, the smell and dust will be terrible. This is probably the reason the powered ventilator was installed.

    Try to turn it off and let us know of the results.

  4. Expert Member

    You have given similar advice several times recently. Can you explain the mechanism that would cause attic air to make its way into the conditioned area of a house from the attic, especially in older, leaky houses?

  5. TedLiu | | #5

    Dona, thanks for mentioning the DOE climate zone is different to USDA. I did mess up the climate zone. I am actually located in Climate zone 4 (according to

    Martin, does the climate zone 4 (instead of 7) change your recommendations? Will it make sense to install radiant barrier because the duct is in the attic? Thank you

    ANON3, unless the attic is over 120F, the PAV is not running. I don't actually smell anything when the fan is not running (the fan is barely running in early spring). For the dust, my wife does complaint about it. Do you have any thoughts to reduce the dust? thanks

    This HVAC issue is not expected and I wonder if there is any DIY task I can do to improve the situation before we get budget for this? I think I will need to seal all gaps and cracks on the attic floor (to the ceiling) as first step. The attic floor does have about 8" fiberglass so I wonder if I should add more insulation?

  6. GBA Editor
    Martin Holladay | | #6

    My advice on powered attic ventilators applies to all climate zones. The reasons that you don't want a powered attic ventilator are explained in my article, Fans in the Attic: Do They Help or Do They Hurt?

    The advice on 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. That said, the required work is expensive, so the investment often makes little sense from an economic payback perspective.

    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.

  7. Expert Member
    Dana Dorsett | | #7

    The IRC 2015 code minimum for zone 4 is also R49, so air-sealing the ceiling plane & ducts/air-handler and adding an overblow of cellulose is still going to be more worthwhile than a radiant barrier. It's likely that you don't have sufficient vertical space to hit R49 over the exterior walls at the eaves to hit R49 and still have the necessary 1" clearance between insulation & roof deck at that point. Installing chutes to guarantee the gap to the roof deck doing what you can is still worth it.

    If installing a radiant barrier under rafters it's worth applying low-E paint to the roof deck as well. In fact, just start with the low-E paint, see if it makes enough of a comfort difference. Radiant barrier on the under side of the rafters needs to be vapor permeable (or incomplete coverage at both the eaves & ridge to allow convection) to mitigate the risk of wintertime condensation in the attic. Aluminized fabric radiant barriers with fine grids of small perforations tends to run about 5 perms, which is permeable enough to keep major moisture accumulations at bay. When installed by contractors radiant barrier is almost never cost-effective on an energy savings basis when you have at least R19 in the attic and the ducts are insulated, and sometimes not even with DIY, but in cases where it makes the difference in whether the AC can keep up or not it can be "worth it" on a comfort basis.

    Get rid of the powered attic ventilation fan completely, including the wiring if you can. The cooling energy "benefits" of powered attic ventilation are in most cases negative (you end up using more energy.) Attic ventilation is really never about cooling- it's primarily about moisture control, to keep the wood in the attic from getting moldy or rotting. In more humid climates on such as the gulf coast, in air conditioned homes even passive attic ventilation puts more moisture into the attic than it removes.

  8. Bdgray | | #8

    I'm going to piggybag on Ted's question as I'm in an almost identical situation (main difference is that I'm in Chicago which is zone 5).

    I've been debating my situation for what seems like forever. Where I've gotten is this:
    Option 1: Convert attic into conditioned space: Install 4.5" of recycled Polyiso above the roof deck and add new standing seam metal roof. Blow in OC sprayfoam beneath to get needed R-value. I've received bids (excluding the OC foam) for this ranging from $40K and $60K. It's my first choice but struggling with cost.
    Option 2: CC sprayfoam the underside of the roof. I've pretty much discounted this option because it will cost ~$15K and I'll still need a new roof at some point (another $15K). Plus the GWP of CC sprayfoam is insanely high.
    Option 3: CC sprayfoam just the ductwork (with extra attention to the ceiling penetrations). I can probably do this myself for the cost of 1-2 CC spray canisters ($600-$1200). My goal would be to encase the ducts with at least 3" of CC (or whatever is recommended). If needed I could blow in more insulation but I'm already in the R40s (or better).

    Currently I'm very torn between options 1 & 3. $40K+ is a lot of money to solve a problem but I get a new roof, better curb appeal and surely a higher resale. That said, its a lot of money.

    To the experts on the GBA, I would be curious your thoughts on option 3. Do you think this would "solve" the attic ductwork problem or just make a bad situation a little less bad?

  9. Expert Member
    Dana Dorsett | | #9

    Brian- this really should have been a new thread, even though it's a nearly identical issue.

    If the ducts are currently insulated to R8 or better, encapsulating them with foam isn't financially rationale.

    If the ducts are currently NOT insulated, a couple inches of closed cell foam on the ducts could be "worth it". HFC blown foam can only be safely installed in lifts of 2" with a cooling off period between, so the most that makes any sense at all would be 2".

    Using HFO or water blown closed cell would be greener than HFC blown goods, and it can be installed in thicker layers, but it's slightly more expensive but also slightly higher R. At 2" most HFO blown foam would be north of R14, and north of R20 @ 3". It still may not be financially rational to go more than 2".

    Open cell is a lot cheaper (and greener) per R, and comparatively vapor permeable- some moisture will collect during long periods of heavy air conditioning use, 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. If the ducts don't drip condensation now, they won't drip after insulating either, and any moisture accumulation dries fairly quickly through 3" of open cell foam. Mind you, the higher vapor permeance itself becomes a code violation nonetheless, a regulation that makes a lot of sense in more humid climates with longer cooling seasons such as the gulf coast states, but perhaps less so in your area, depending on the duty cycle of your AC system:

    Most spray foam would need to be coated with an intumescent paint to be fully code compliant in your application, but some have better fire ratings than others. Discuss that with the foam installer ahead of time, and make sure anything required for code compliance is covered in the contract:

    The time to consider moving the insulation and air-boundary out to the roof deck is when it's time to re-roof.

  10. TedLiu | | #10

    Martin and Dana,
    Thank you very much for your answers.
    I Shut off the attic fan and will start with sealing air leaks and increasing the R-value. The current insulation is under the wood panels. Can I just put another layer of insulation on top of the wood panels? (Pictures attached)

    I have another question. The current supply and return are all running in the attic and the register and vent are both on the ceiling. However, there are old vent/register on the walls and they are not in use. What should I do with those old ductwork? Can I put fiberglass in and seal them?

    Thanks again your help and comments.

  11. Expert Member
    Dana Dorsett | | #11

    The large fan in the first picture is a "whole house fan", which is typically used to pull massive quantities of air from the house once the outdoor air is cooler than the conditioned space air. Cracking windows in overheated rooms allows the cooler outdoor air to come into the house, while the house air is being forced into the attic, and out the attic's roof vents. Is that what you have been referring to as powered attic ventilator?

    Attic ventilators are typically designed to pull hot air out of the attic, and doesn't intentionally draw air from the conditioned space air, but unless the attic floor is air tight, the depressurized attic parasitically draws air from the house.

    While it's possible to have both central air and a whole house fan, it requires some user training on how to use them both effectively (and it will never be at the same time.) Most older whole-house fans are a HUGE air leak for the house, as well as a large hole in the attic insulation. In your case pulling the whole thing out, installing ceiling gypsum air barrier where the louvered hatch had been and insulating over it makes sense.

    There is likely to be quite a bit of air sealing detailing to be done first (blower door directed, if you can), but it's then fine to just blow cellulose over the existing insulation until the combined depth reaches ~14-15" (which will end up around R49 after settling.) The flex ducts are already insulated, and there is no financial rationale for encapsulating them with foam. The fact that the ducts are insulated also means there's really little or no point to radiant barrier schemes, but fattening up the floor insulation is worth it. It's fine to heap install the insulation over the existing floor boards, as long as there are no air gaps under those floor boards. It may be worth pre-building a catwalk above the final insulation depth to provide service access to the air handler.

  12. TedLiu | | #12

    I knew it is whole house fan and noticed it is not working when we moved in. Currently, I use 3M window insulator to prevent air leak. The powered attic ventilator is close to the furnace.

    The flexible black duct is the supply air and in winter time, I checked the temperature difference between the supply register A (closest to furnace) and the supply register B (at the end of same supply duct). The difference could be as high as 10-15F. That's one of the reason I am concerning about the insulation of the duct.

    It seems the flexible duct sold in the retailer store is around R-4 to R-8. Does it make sense to have a 10-15F temperature dropping in the duct (hot traveled about 10ft)?

    All comments are greatly appreciated

  13. GBA Editor
    Martin Holladay | | #13

    You are correct that the drop in temperature indicates that the duct is poorly insulated. 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. When the attic is 20 degrees F, that insulation is addressing a delta-T of 50 F degrees. But the air in the ducts is facing a delta-T of maybe 100 or 120 F degrees -- 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.

  14. Expert Member
    Dana Dorsett | | #14

    The reason ducts don't need R60 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%.. R8 is usually fine for supply ducts, and less is OK for return ducts (due to the smaller temperature difference.).

    While there are marginal improvements to be had by going higher than R8, it's possible than even R20 would be uneconomic. As the diameter of the exterior of the duct insulation grows with more insulation, so does the surface area, so the improvements with increasing R isn't nearly as good as with planes such as attic floors.

  15. GBA Editor
    Martin Holladay | | #15

    I agree -- but even R-8 insulation doesn't really solve the problem of ducts located in the wrong place.

    Ducts need to be indoors.

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