GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Alternatives to HVAC System in an Unconditioned Attic

hung8582 | Posted in Energy Efficiency and Durability on

I have 2 floor, 3200sqft new construction in NJ, the contractor are currently considering putting the 2nd zone hvac mechanics and ductwork in the unconditioned attic.  Off the bat, i already know that is a bad idea. On top of that, the entrance to the attic is a pull down staircase. So what is the best approach to fix this issue? I really only have two idea in mind..
– The most cost-effective and sensible way, to me at least, is to relocate it to the basement and run the duct thru some closet.  Keeping everything in conditioned space. But the basement is only 7′ 9″ high and with the additional duct work, im afraid itll just make the basement even less desirable to use as living space. Secondly, would the system be strong enough to reach all the rooms on the second floor? I will also be giving up some sqftage to accommodate the ductwork.
-Condition the attic space.  Is this the best solution? accessing the attic is a pain but shorter duct work definitely will help.

So I’m wondering what you would do in this situation.  And is there an alternative that would make more sense?

Insulation –
I havent decide on insulation yet with the contractor, But i do want to spray foam to seal as much air leaks as possible. Given that my exterior studs are 2×4 , i dont have much choices between the studs.  do a flash and batt? use rockwool? or just fiberglass?
I spoke to the contractor about exterior foam, he’s saying itll cost way too much to add 2inch foamboard since i would have to alter the trims of every window.  The most he said he can do is 3/4 foamboard, which is better than nothing I guess.  So should i push to get the  2″ exterior foamboard or spray foam the interior?

appreciate anyone that can shred some light. Thanks.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Kiley Jacques | | #1

    I anticipate experts will advise you to bring the attic into the thermal envelope. Here is an article on how best to do that: Creating a Conditioned Envelope.

    1. JC72 | | #3

      ^^This. Encapsulate the attic. If possible do it from the outside with exterior rigid foam. I really don't like spray foam because it can cause some big issues when poorly mixed/sprayed.

      You can save money by using spray foam to seal around the stud bays and then batt insulation for the cavity itself. Or just tape the seams in the exterior sheathing.

      1. hung8582 | | #4

        Thanks. I wish I can find someone that can do exterior rigid foam in the attic , that would be ideal. It seems hard to find someone that does that in NJ. I talked to the insulation/sprayfoam installer. Based on his knowledge,reviews and years of experience, he gave me the impression that there wont be any issue with spray foam installation.

        The cheaper solution might work as well, but would it be enough to keep the attic in adequate condition for hvac/ductwork?

  2. hung8582 | | #2

    Thanks for the reply, Kiley! I did read the article,but the cost to condition the attic will probably blow my budget. Ill definitely look into it but hoping to see if there's an alternative.

  3. user-5946022 | | #5

    The alternatives include
    a) Use a horizontal system in the attic, build a box in the attic for your air handler/furnace, and drop the plenum box down into the conditioned space, hopefully down a central hallway or something (that would have a lower ceiling
    b) Use a horizontal system in the attic, build a box in the attic for both the air handler and the duct, and try to keep that towards the center of the house (so that the box is smaller)
    In either case make the box part of your conditioned space and air seal like crazy, and insulate between the box and unconditioned attic.

    1. hung8582 | | #7

      thanks! that is a solution i didnt think of. so its essentially a mechanical room. ill run this over with the contractor and see if this is a feasible option.

      do you guys think relocating it to the basement is a bad idea ?

  4. Expert Member
    Akos | | #6

    Around me, the furnace is almost always in the basement. There is no issues with conditioning a 3 story structure with this as long as the ducts are sized correctly. For new construction, that is the only spot I would go for.

    7 9" is quite tall by local basement standards (most older ones are only 7' if lucky, usually less), even if you need to drop the ceiling 10" in spots to clear ducting, it won't be low enough to bump most peoples heads.

    If you want to keep the basement clear, the best way is to run the supply and return trunk along one of the outside wall perpendicular to the joists, not down the middle of the house. You loose a bit of sqft of the basement, but now all your local feeds can run up from this trunk and through the joist bays and take up no height.

    Since you are in an area that will need a lot of AC, make sure to install a large return near the ceiling on the 2nd floor. This is important for cooling, will take up a fair bit of space, but don't skimp on it.

    P.S. Make sure somebody competent (havc engineer not installer) runs your Man J, most installers would probably put something like a 5 ton unit, which is ridiculously oversized for any reasonable new construction.

    1. hung8582 | | #8

      appreciate your thoughts! I am definitely going to spend a little extra to have an 3rd party engineer to run a man j for me. I dont trust the installer's calculation.

      i still think basement is the best solution. but would you then seal the rafter or jsut do the attic floor?

      1. Expert Member
        Akos | | #9

        If the attic is not living space, the simplest and cheapest insulation is the attic floor.

        If you want to use the attic for storage and the bottom of the trusses are not deep enough for the amount of insulation you need, you can lumber on edge perpendicular to the trusses. This can be 2x6/2x8/2x10 depending on how much extra height you need. You can than loose fill the floor with cheap insulation and install the plywood deck on top for storage.

        Pull down stairs are usually very leaky. I would spend some extra effort on gasketing and sealing this up properly.

        1. hung8582 | | #10

          thank you for the suggestion. This can possibly work, ill discuss this to my contractor as a viable solution

  5. kyle_r | | #11

    What is the ceiling height of the second floor? If you can’t find room to get the ductwork to the basement another option is putting the air handler in a closet on the second floor and running ductwork in soffits below the trusses.

    1. hung8582 | | #12

      the second floor ceiling is 8' high.
      So I spoke to a hvacr contractor, he recommends I leave the hvac in the attic and just insulate and seal the attic floor. He is going to do a manual j,s,d for me. Im a little iffy about this recommendation mainly because it still doesnt solve the issue with the mechanic in a unconditioned attic.

      1. Expert Member
        Akos | | #13

        HVAC in the attic generally means an extra ton of cooling capacity. It can work, but you'll be paying for running that larger unit.

        Ducting and air handler should be inside the house for a new build. This is not hard to design in and install. Fixing leaky air handlers and sweating ducts down the road will cost you more than doing the job right in the first place.

        Post the Man J (full one, not just summery) from the contractor when you get, can take a quick look to see if their finger is on the scales to hit those magic rules of thumb.

  6. hung8582 | | #14

    We had a talk today and i think we both agree to keep it in conditioned space. And we decided to put it in our laundry room on the second floor. It is next to a bedroom, so would insulate the walls with rockwool work well enough to block the noise? We will run ducts thru the attic and insulate it as much as possible. Keep the attic vented , but air seal and insulate the attic floor. enclosed the location of our laundry room. think this is a good idea?

    1. Expert Member
      Akos | | #15

      If you are putting the air handler close a bedroom, make sure to go for a modulating unit with an ECM blower. These are significantly quieter and since they are modulating, it will be running at some load most of the time instead of cycling.

      No amount of sound insulation will make a budget air handler bearable. I used to live with one before, despite my best effort, I could never get it quiet enough.

      Ducts in the attic are still not the best but can be done efficiently if the ducts and the register are encapsulated in closed cell spray foam.

  7. hung8582 | | #16

    ooh yes ! he did mention a modulating unit with ecm. this was my first time hearing that term and learn what it is. hopefully with a combination of this and insulation will help. maybe even insulate the door to further dampen the sound.

    and yea , it isnt the best solution with duct still in an unconditioned attic, but i think this is a good compromise.
    and like you suggested, i will probably use close cell foam to insulate the ducts.

  8. hung8582 | | #17

    Hey All, So I finally got a Manual J,S,D produced. So Do you guys think the hvacr did a good job at calculating the load and duct size? This is my first time doing this , so expert opinion and advice are much appreciated. Thanks!

    1. Expert Member
      Akos | | #18

      It looks like it is in the ballpark, although there is some fudging as there is 9000BTU duct loss included. With the ducts inside conditioned space, your duct losses are 0.

      Also looks like the ventillation system is exhaust only, not the best form energy efficiency and indoor air quality. An ERV/HRV is best for a reasonably well sealed structure.

      If you add and ERV and fix the duct losses, your cooling and heating load should be under 3 tons for the whole house.

      Both furnace and AC units are oversized. There is no easy answer for a furnace as 40000BTU is typically the smallest readily available.

      A greener option that would be also better for comfort is to go for a heat pump instead of a furnace. The material cost of a decent cold climate heat pump is not much more than the cost of the furnce+budget AC and install should be less as there is no gas piping or venting required. Quality heat pumps also will also be modulating and would be significantly quieter than the single stage AC included.

      1. hung8582 | | #22

        Thank you for your awesome reply.

        as for ducts being inside conditioned space, that isnt exactly true. I am only insulation the attic floor. Spray foaming the attic cost way too much, it wouldve blown my budget. So i opted for batts/blown in insulation for the attic floor, which total to be around r49.

        Ill definitely look into heatpump upstairs. thats is great suggestion. But now that the duct is not in conditioned space. would you still consider heat pump on the 2nd level? Does that now justify his calculation for the 9000btu duct loss?

        1. Expert Member
          Akos | | #24

          Only ducts in the attic is better than air handler+ducts but still not the best idea for a new build. If you have truss floor joists (doable but harder with I-joists), it is pretty simple to run the ducting through there. There is no reason to have ducting in the attic for a new build.

          If there is really no way around attic ducting, have the spray foam guy encapsulate the ducting and all the registers with foam before the blown in insulation goes on. This is easy to do on a new build and should not add that much extra cost. Once the ducts are sealed with SPF and buried in insulation, your duct losses should be low enough to be ignored.

          As for the heat pump, Rheem units are so-so when it comes to cold weather performance but with a 10F design, it should still work. Something like this is a decent fit for the 2nd floor:


          A better unit if you can get would be either this:


          The 2nd one is a re-badged Midea unit which has better modulation range and would be a better fit for your load.

  9. kyle_r | | #19

    If you must have a natural gas furnace, you could see if someone installs Dettson furnaces in your area. They have a 15k and 30k modulating unit.

    Most of your heat will likely be served by the downstairs unit. You could stick with natural gas unit there and install a heat pump for upstairs.

    1. hung8582 | | #23

      Thanks! but this installer the contractor hired only carries Rheem. But i am definitely consider installing heat pumps upstairs. thank you for your suggestions!

  10. walta100 | | #20

    I have to ask why, is the basement so shallow? Most every new build I see has a 9 or 10 foot basement pour for small money makes the basemen a much nicer place.

    If your electric rates are reasonable eliminating the gas line saves you the $30 a month in connection fees.


    1. hung8582 | | #21

      ah, good question. original plan was for a 9ft basement. but town did not allow us to dig so deep because of the high water table in the area. According to the contractor, the best they can do is 7'9". I guess ill have to live with it. its not the end of the world, but i wish i had the 9ft ceiling

  11. walta100 | | #25

    If the basement can’t go lower can the roof go higher?

    Do you have enough slope on your property so the basement could drain to daylight?

    Personally I would not want a basement if I could not drain to daylight. Without a daylight drain you will be reliant on a sub pump to keep you basement dry and sooner or later it will rain when the electricity is out and the basement will flood.

    “but town did not allow us to dig so deep because of the high water table in the area”

    Something about this statement sounds wrong to me. It seems very unlikely they can track the water table from property to property.

    I doubt there is anything in the code that allows them to limit basement excavation depth.

    I think it a game if the basement happens to be less than 8 feet or some other number then you can never legally finish the basement as living space.

    Have you priced flood insurance for this property?

    If the water table is that high do you really want to build on this property?


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |