I'm hoping some of you can offer some advice on how best to add a second AC in our home -- or on whether we should look for a different solution. I should say upfront that I've done a lot of reading on this topic and feel more confused than ever. I'm not crazy about the idea of an attic AC (leaks, cost, efficiency). But I also want my pregnant wife and soon-to-be newborn to be comfortable. Some of the more popular alternatives won't work for us, whether they're minisplits (wife hates the look) or a whole house fan (wife has bad allergies and we try not to open our windows because of friction / encapsulated lead paint).

Anyway! We live in central Indiana in a 1920s brick colonial -- no insulation in the walls, a few inches of cellulose in the unfinished attic. The house is about 1700 square feet and currently has a newish Lennox 2.5 ton AC, but while the house has plenty of supplies and returns downstairs there are no returns upstairs. (There is a supply in each room upstairs -- I'll put a copy of the upstairs floor plan, about 750 square feet, at the end of this post.)

In the summer there's a noticeable difference in temperature and mugginess between the two levels, and I really hate the idea of making the downstairs freezing during the night just so the upstairs is bearable. We're considering putting a second AC in in our unfinished attic with the idea that it'd allow us more comfort and more control.

We've received three bids -- the two I'm still considering are in the $4800 range for the labor, second condenser, air handler, and a low-powered electric heater if we ever need to supplement in the winter.

A couple questions in case we go with the second AC. Based on the floor plan below, would you put a supply and a return in each room (one bid's proposal) or just put a supply in each bedroom and a bigger return in the hallway, near the stairs (another bid's proposal)? I imagine we'll be shutting the doors more with kids, but there's a big gap between each door and the floor since the previous owners had thick carpet and tile, which we removed.

Second question: how much do we need to worry about the ductwork and unit being in the very warm attic, outside the house's "envelope"? I've asked both vendors about this, based on my reading online, and they both said it wouldn't be an issue with our smaller house -- and that the AC units will have insulated cabs and that the ductwork will be insulated, too.

Of course, this second AC may be a terrible idea in the first place given the possibility of water leaks and inefficiency and whatever else. So third question: would finding an unobtrusive way to run one return up the second floor hallway and sealing / insulating the heck out the attic be worth trying first? (I don't want to insulate the walls because that's very tricky with brick.) Maybe one return plus a better attic solution and possibly a portable AC unit will be enough.

Or maybe there's a way to do a permanent attic AC smartly (and to make my pregnant wife happy). Curious to hear your thoughts. Sorry for the novel length post! If I've forgotten to add any details please let me know.

Best, Craig

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  • A return an supply in each room will likely help, especially if you typically close doors. Instead of a second unit, using dampers to balance the system might be a better, cheaper solution. Without seeing the house, or being able to run diagnostics on the system, it's quite difficult to provide an accurate solution.
    – Tester101
    Commented May 29, 2016 at 18:08
  • If you put in a new unit for the second floor, you should resize the first floor unit. Otherwise the first floor unit could short-cycle, as it will be oversized.
    – Tester101
    Commented May 29, 2016 at 18:13
  • appreciate the advice, tester101!
    – ctf9
    Commented May 30, 2016 at 1:59
  • Adding a good quality ceiling fan at the highest point of the stairwell can also help a lot.
    – Trevor_G
    Commented Mar 9, 2017 at 15:40

4 Answers 4


I've been the HVAC industry for more than three decades. To answer both of your questions: The ideal installation would involve installing a return and a supply to each room. This allows you to accurately and more precisely control the climate in each room. The answer to your second question: Yes, you will need to insulate the ducting outside the insulated envelope of your home. If you do not insulate the ducting it will sweat, causing water damage to your home. Home this helps. Hope I answered all of your questions. P.S. Shop around and hire a legitimate HVAC contractor who will size your equipment properly or you might get stuck with the wrong size HVAC unit. Good luck to you and congratulations on your growing family!


Before undertaking expensive options, you might try some inexpensive options.

You can buy vent booster fans that fits in the rectangular openings. This increases the flow of cold air. Search on "booster vent fan" and check the user ratings. For example: https://www.amazon.com/Tjernlund-RB12-Register-Booster-Fits/dp/B005FNL0SS/ref=sr_1_5?s=hi&ie=UTF8&qid=1472475877&sr=1-5&keywords=booster+vent+fan

Closing the AC vents on the lower floors also helps.

Also, some HVAC units have adjustable circulation volume. If you have one of those, see if your unit is set to the highest setting.

Also, you can try to keep the hot sun out of the windows, with reflective coatings or shaded.

Finally, the heat may be coming down from the attic. How well insulated is the floor of the attic? Insulating the floor will keep the top floor warmer in winter as well as cooler in summer.

These inexpensive options can bring down the temp a good 10 degrees with no ongoing costs.


a few inches of cellulose in the unfinished attic.

I had a new two-story home and the upper floor would get too warm when the outside temperature went to the upper 80s. The whole problem went away when I installed an appropriate R-value of insulation in the attic, nearly double what the contractor had put in. Far less expensive and far less trouble than considering anything else.


I grew up in a similar house so I feel your pain. Here are your best bets, in terms of cost-effectiveness and non-intrusiveness:

  1. Seal up the holes and air leaks in the attic floor. Go nuts with caulk and canned spray foam. Every little bit helps, because your attic turns into a furnace in the summer and you don't want that hot air seeping into the bedrooms upstairs. This should cost maybe $50.

  2. Add a radiant barrier in the attic, stapled to the bottom of the rafters--provided you have soffit and ridge ventilation. You probably have a steep-ish attic, so this job will be easy to do yourself, and therefore cheap (ca. $200).

  3. Add more insulation to the attic. A lot more. Like, 15 inches or more. You can again do this yourself pretty easily if the access is good. Total outlay: likely under $500.

  4. Have an HVAC contractor add returns to each bedroom if none of the above fully solve the problem. This will be fairly intrusive and expensive, but will definitely make a difference. Budget $2,000 or more.

  5. Get Low-E storm windows if your house doesn't already have them or modern Low-E replacement windows. Modern window coatings can keep out more than 80% of the sun's heat, which may be an issue if a lot of sunlight streams through the windows. Window replacement is often not cost-effective ($400+ per window), so if you don't have storm windows, Low-E storms can often be added for $100-150 per window--less if you do the work yourself.

Also you should really look into having the walls insulated, too. The least intrusive way is to have dense-packed cellulose blown into the walls through little holes. This is also a pro job, and might cost you $3-5,000, but there can often be utility or state government rebates for this kind of work.

Finally, the next time you need to redo the roof, look into light-colored shingles or even light-colored metal, if it wouldn't seem too architecturally out of place.

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