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I have a finished basement, ground floor, and upstairs. The ground floor and basement cool nicely with the AC. The upstairs doesn't cool very well.

The returns are really just holes in the drywall- there are no ducts behind them. The ground floor has a 30"x6" return vent. The upstairs has that plus 2 15"x7" return vents. I checked last night and there does seem to be decent movement of air into the big upstairs return (one or both of the other two seemed significantly weaker).

My primary question is if it makes sense to block air behind the wall above the upstairs ducts? I'm thinking of using expanding spray foam. My reasoning is it would help seal the inside and create better suction.

I'm open to other ideas too. Since I moved in 4 years ago, I've:

  • had the attic insulated
  • had the ducts cleaned and sealed
  • put supply register boosters on 2 upstairs bedrooms
  • increased the HVAC blower fan speed
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  • How many SF upstairs? How many return ducts/supply ducts? – HerrBag Jul 10 '13 at 16:14
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All the things you've done should have helped some, except the duct boosters. They can't add much flow to the system unless the return ducts were increased proportionally (or similarly boosted). You get increased flow by increased pressure or larger ducting. Keeping upstairs doors closed will help by slowing the 'fall' of cooler air downstairs. Try leaving fan "ON" for a 24 hr period to see if your house will equalize better.

You can force the system to concentrate on the upstairs by reducing both the supply and return vents in the main floor and basement. You may have to do some makeshift blocking (reduction ) of the returns with plastic sheeting and 'no residue' duct tape. If successful, I believe magnetic covers are available.

AFA blocking above the upstairs ducts, by all means. Save yourself some expense (and trouble, that much foam is hard to scaffold in a vertical shaft) and make 2x4 blocking (essentially fire blocking) to go above the vents. If you have typical framing 16 inches on center, a 14 1/2 inch piece will span stud-to-stud. Use a pair of pocket screws on each end to secure the blocking in place. Use fire caulk/foam for any gaps.

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    Depending on how the ducts are laid out, dampers on the ducts coming off the AC, to redirect the air either upstairs or downstairs, are useful with a single zone setup. They'll do a lot more than altering the returns. – BMitch♦ Jul 10 '13 at 15:59
  • Thanks. These give me some ideas. I should have clarified that only one upstairs bedroom has a return inside and I don't care about keeping that room cool. So closing the doors probably won't help? Also, I can get away with cooling the house at night and leaving AC off most of the day with the windows closed. The problem is at night when I need the AC to cool my bedroom fast. Once it's cool I can get away with using a window fan. I live in Colorado where we get large daily temperature swings. – davidkovsky Jul 10 '13 at 16:05
  • @BMitch somewhat true, they will simplify limiting the supply side. I have suggested limiting BOTH supply and return. If you don't reduce the main floor returns, the upstairs will be 'return starved' by the main floor returns. – HerrBag Jul 10 '13 at 16:06
  • Update: I took HerrBag's recommendation to block the space above the returns with 2x4s and pocket screws, plus caulking. That was definitely the way to go but unfortunately did not solve my problem, nor did keeping the fan on all the time or adjusting duct flows. – davidkovsky Nov 14 '13 at 0:31

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