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.
I would guess that modifying the duct work in a 1920s masonry construction home will be prohibitively destructive and involving removing or damaging irreplaceable trim that contributes to the building's character.
Mini split heat pumps:
An alternative, which will not be inexpensive but may be significantly less destructive, would be investigating installing a multi head mini split ac unit with one head in each bedroom. Such a unit costs perhaps $2500 to $5000 for the unit, installation extra and depends on the difficulty of install. If you live in a heating climate, especially one with cheap electricity in the winter or might install solar and have sunny winters, you should likely invest in the heat pump versions. Models are available that operate down to -13 F now (Mitsubishi M-Series) in single head. Supposedly multi-heads that operate that low are coming out this spring.
Advs:
Individual temperature control for each bedroom
- Really high efficiency available for both heating and cooling
- Avoid destruction of interior trim
Disads:
- Potentially a lot of $$$
- Still a newer technology in the US (used all over Europe and Japan). If you plan to sell soon, recovering your investment at resale could be a challenge
- Some people hate the look.
Other than minisplits:
Air leakage:
I'd suggest you have a blower door test performed. All the insulation in the world doesn't matter if you are changing over the air in the house too quickly. If you didn't air seal the attic before filling it with cellulose, you could have this problem. An infrared gun on a cold night can help show these leaks too. Insulation installers will say things like seals in air but cellulose, fiberglass insulation, and mineral wool insulation are actually very air permeable. If you find that is the problem, you could move insulation out of the way in the attic to use canned foam (get a commercial gun, much nicer job to do than with home depot cans). If you can't move the insulation, you could install a vapor permeable sheet membrane like Tyvek or a smart vapor retarder like IntelloPlus on top of the insulation to block air movement. Do not use a vapor impermeable barrier like black plastic or visqueen (clear plastic); you could trap moisture and cause a mold problem. Attic access hatches are also causes of large volume air leaks. There are special hatch covers you can buy or build to address this problem.
Duct work in unconditioned space:
Do you have any ducts in unconditioned (uninsulated) space like an open crawl space or an unconditioned attic? If so, you could be losing huge amounts of heat to the crawl space or attic. If your ducts are older, they likely aren't air tight either. You could use specialized foil tape or duct mastic to seal the joints and you could insulate this duct work, though it won't be fun work. If your ducts are really leaky, you could get big gains in system performance from this work.
Humidity control:
Have you measured your humidity when your system is running? If your humidity is high in the winter or summer, you will feel colder or hotter. Your humidity could be high in the house for many reasons (improper shower venting) but dirt crawl spaces are a big source in the summer (not in the winter so much). Seal off the dirt crawl space to eliminate humidity, mold spores, radon, etc. from getting in the house. A radon membrane company, in areas with radon, might be a cost effective contractor to use.
Best Answer
A tale of misplaced insulation, leaky ductopi, and lazy builders
The problems you are seeing here are due to a combination of three problems:
Fixing the leak will stop the hot air from coming into the house, but it won't cure anything else that's wrong with this setup, as the unstretched flex hampers airflow through the ducts and the R8 or so duct insulation is nowhere near enough to keep attic heat from warming the cold air in the ductwork, either. Considering that there's no ice dams in SoCal, hence no reason to vent the roof deck otherwise, I would go to an unvented conditioned attic, or "hot roof", configuration.
You can either have a couple thick layers of rigid insulation board laid down with the gaps staggered and an overdeck laid over that next time you replace your shingles, spray foam applied to the underside of the existing roof deck if you want to do the retrofit now, or a mix of the two, provided you achieve the R-value you're after. If you are doing spray foam on the underside of the existing roof deck, you'll also want to put some 1/2" drywall on the underside of the rafters to provide a thermal barrier between the foam and any sort of flare-up that might happen in the attic (some folks use a sort of paint-on coating instead, but the drywall is easier to get right and maintain I reckon). Of course, the attic venting will have to be removed and replaced with solid materials instead, and supply and return grilles added to the attic so that the air conditioner can cool it.
While you're at it, you'll want to have that "ductopus" of flex replaced with a rigid trunk-and-branch duct system, reducing the amount of work your air handler has to do to push conditioned air through the system. Between these two upgrades, you'll be able to take a good 10-25% off of the energy usage of your air conditioner, and may even be able to downsize the A/C next time you are shopping for one, since your house no longer wastes effort trying to cool outside air that's just going to get vented anyway, and your air handler no longer has to push as hard to get cold air to you, either.
(As a sidebar: it is possible to "bury" the ducts in attic insulation, but that's trickier than cathedralizing the attic due to condensation issues; it requires careful spray foaming of the ducts in order to keep the dewpoint on the outside of the vapor-impermeable section of the system high enough that humidity in the outside air doesn't go off and condense on the outside of the ducts, creating a moisture problem.)