Will look into tinting further. Unfortunately, adding another hvac for a second zone isn't in the budget.

Do you have access to the ductwork and if so can you adjust the dampers to send more air to the bedrooms? You'll have to adjust them ever spring and fall but at least you's have usable bedrooms.

Adding drapes over the blinds will definatly help. Foam lined drapes are inexpensive and accomplish 75% of what the most expensive will. Foam board cut to friction fit window opening is the most bang for the buck and can be covered in fabric or wallpaper for better appearance. I'm not versed on window tint/film but it's worth looking into. Insulating garage door doesn't help and venting isn't worthwhile if tempature differential is less than 20 deg. If exterior of garage door is dark colored,radiant reflective barrier inside door will help a little. If space between ceiling and roof has vents,make sure they aren't blocked by lint and dirt. If present,try and make sure other vents are also functional. On the hottest nights,does the ac cycle off every 30 minutes or less? It's not pratical messing with ac ducts unless one understand's what they are doing. The most improvement would likly come from additional intake/return air upstairs (preferably near center of loft ceiling or toward bedrooms). I can almost visualize returns on both floors where one is partly blocked in summer and other in winter. Put a jacket on the water heater. If there's no return on second floor,this is Rube Goldberg but may surprise you with how much it helps. Suspend a box fan in one or both 2 story openings at ceiling level so it blows air from first to second floor. Probably no comfort but this is a very common complaint.

This is a very common problem with very few pleasing choices, because no matter what you do a two story home with one HVAC system and one thermostat there will always be some place uncomfortable in the home.

You keyed in on the problem when you said... "while keeping the first floor at a reasonable temperature (usually 72 to 74)"

So currently in order to make the upstairs cooler, you make the first floor cold. Right?

The reason is you have one thermostat for two areas with much different comfort requirements. The downstairs doesn't require as much cooling as the upstairs does, but in order to cool the upstairs the unit must run while sending unneeded cooling to the down stairs...

Obviously you are on a somewhat right path in that you first look at reducing the load of the upstairs area. Basically cutting down on sun infiltration thru windows by either using curtains or some kind of window treatment, is the most cost efficient way forward.

However, the comfort you feel on the first floor of 72-74 could be as high as 84 or more on the second floor, regardless what you do to treat the windows, curtains or whatever. Heat rises. The upstairs will always be warmer. Don't forget people are loads too. The more people, the warmer it will be.

So what can you do?

The best way to fix this is,install what is called a HVAC zoned system. The system is controlled via special controls that allow you to cool the upstairs separately from the downstairs. With this system installed you would have 2 or more thermostats depending on the number of zones you want to create.

There are problems and pitfalls with zone systems. If the structure doesn't have the room to zone properly, this leads to a system that doesn't function well, more costly repairs and the list goes on and on.

Many zone system manufacturers make cheap controls that don't stand up... either that or the system was designed or installed improperly.

Another more costly way would be to put in two separate HVAC systems. This is very costly because you will not be able to reuse your current system. It would be grossly oversized. So to go this route you would need two new properly sized HVAC systems to handle the different zones. This plan may not work either due to amount of room necessary to pull it off.

This is virtually EVERY two story "open" home in the USA on a single zone. Buy a high quality portable AC for each upstairs bedroom ( unseen from exterior) , add room darkening/blackout honeycomb cellular shades versus blinds to the upstairs windows ( bedroom) . Those have a hidden Mylar within, can't be seen from outside at all, and are far more energy efficient insulation than blinds of any type. Turn the portable ac on when needed in those rooms. You will have spent 2 to 3k, all in but you will have FOUND the solution. It will be perfectly cool and comfortable for sleeping in both bedrooms, Bonus is you run it only when you need it, and most cool so well, you can pre cool, and turn off at bed time

We have a walkout bungalow with a south-facing wall that is all windows, so we have the same issue. Our solution is to strategically close floor register covers, seasonally (the dampers are too hard to reach). In the summer, we close most of the downstairs vents and in the winter we close most of the upstairs vents. My HVAC guy advised me not to close all of the vents and to only close them 90% of the way, to ensure some air still flows through them. We also used this “system” in our former, two storey home, which had an area open to the first floor and, in both places, it works/worked quite well. It does take some experimenting to get the right combination of opened/closed vents and, you should discuss this with your HVAC technician first, just in case your system can’t handle it. In addition, we run our furnace fan 24-7/365. This not only helps clean the air but greatly reduces cold/hot areas. Finally, we have electrically-controlled awnings but they are pricey and not really suitable on a second storey. Since I can’t use them in the winter, I’ve installed thermally-lined drapes in our bedroom, which definitely help.

Thank you all for your comments. I do have a "master" setting on the main duct leading from the furnace that has a summer and a winter setting - this makes some difference - and there is a return air vent upstairs as well as on the first floor. The energy audit representative discouraged me from completely closing some air vents (like on the first floor). He said the system was designed for air flow through all ducts. Two zones/separate a/c units would be optimal but not sure I want to put that kind of money into solving the problem. But the heat pumps in my development seems to starting to fail so, when mines goes and has to be replaced, I will investigate potential solutions with two zones. At this point, for the bedrooms, I'll go with good insulated drapes that I will keep closed in the summer. For the window above the door, I finally found someone knowledgeable about shades (at my local Lowe's), He showed me a double cellular shade that will be included in a 40% off sale in February. A motorized shade will be in the $250 range plus installation. I used Lowe's for all the windows in this house and my last one and been very satisfied. The only issue now is that the motorized mechanism has batteries housed in a unit in the shade header. This will be 16' above the floor, so replacing batteries is going to entail my handyman coming over (I'm 70 and don't do ladders!) with at least a 12' ladder when this is needed. Thanks again for all the good thoughts.

I agree with window tint/film. I live in a loft with high ceilings and have clerestory windows that are too high up that I didn't want to deal with roller shades. Instead I installed heat control window film, and it's worked out great. It didn't impact the amount of natural light coming in, yet blocks the heat and UV light to protect the furniture/rugs etc. Installation was DIY and the film achieved exactly what I was looking for.

https://www.gilafilms.com/for-home/heat-control-window-film

https://www.amazon.com/gp/product/B000KKM8EG/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

I use solar shades for this problem with good luck and they allow you to see the view and give privacy during the day but not at night so you still need drapery if privacy at night is na issue. I would in vest in a couple of portable air conditioners for the guest room and any other bedrooms.

I think you need where the heat is coming from - through the windows, up from below, or down from above (do you have an attic or crawl space above the second floor?). You say the garage is very hot. Try leaving the garage door open over night and see if that helps. If it does, maybe your heat is mostly coming from there. How good is the insulation above the garage? Is replacing your windows within your budget? Just some ideas to chew on.

mtvhike - Thanks for ideas. I suspect heat is coming from several place, especially from above, but the area is not accessible without removing drywall on ceiling. Windows are relatively new (10 years) but will check into evaluating them. Insulation above the garage is excellent and cavity to floor of bedroom is full. Yes, remembering to open the garage door for a couple of hours after dark helped when I thought to do that the end of last summer. Need to be better about doing that every night and early in morning. Don't feel comfortable for security reasons to leave it open all night. Patrica Colwell - Great minds run in the same track! Was just thinking today about possibility of portable a/c that I could put in bathroom window (so it does not show from street), which opens to loft, when guests are here. Thanks. Meanwhile, I have started shopping for insulated drapes for bedrooms.

I block off about 1/2 of the lower floor air return so as to increase the amount of upstairs hot air being drawn into the A/C. I also close about 3/4 of the lower floor vents and leave the circulation fan on. It makes several degrees difference in the upstairs temperature.

Oliviag55 - townhouse is only 10 years old and heat works fine in the winter. I had the ducts cleaned when I moved in last year. How does one check for leaks in ducts?

Jan Moyer, thank you for mentioning the blackout cellular shades, which are quite energy efficient. I have installed them in my upstairs bedrooms which face south. Because of the mylar interior lining, which you do not see unless you peer into the cells from the side, the rooms are considerably cooler than when I had regular cellular shades. Plus the room darkening helps my guest sleep in a bit later in the morning when the sun is up. Blinds companies are always running specials if you keep tabs on several vendors. I usually save at least 40% off MFRP.