I've struggled with this problem since moving into our 40 year old
home 3 years ago.
Here is what I've learned through trial-and-error:
1. Seal the attic to prevent air infiltration from the upper floor
into the attic. This will prevent the loss of conditioned air into
the attic space.
2. Make sure the attic has lots of ventilation - soffit vents,
channels in the rafter spaces where rafter meets header, ridge vent or
passive vents high-up in the roof. Make sure that all of the vents,
etc, are unobstructed. Good passive ventilation will help to reduce
the heat in your attic space. A cooler attic will mean that your
upper storey ceiling will act less as a giant radiant heater!
3. Install an electric powered attic exhaust fan that is
thermostatically controlled, and set it for about 100 degrees.
These three steps made a huge difference in our house in terms of
keeping the upper storey cooler without the a/c working so hard.
The advice in earlier posts is great about closing vents in basement
and 1st floor and opening them upstairs. Just be careful--if the
airflow becomes too restricted the pipe going from the outside
condensor to the furnace may ice up. It that happens, change the air
filter first. If it continues to ice up with a new filter, then
you've restricted the airflow too much.
I have used a small portable box fan laying on top of the upstairs
floor vents to help boost airflow. You can also buy special booster
fans or in-line fans to do the same task. It helps a bit.
The second part of the problem is the basement overcooling. This is
caused by three factors:
1. Cool air sinks. Try closing the basement door, if you can. I
tried that, but my dog doesn't like it.
2. Regular HVAC vents do not fully restrict airflow. If you're
lucky, your system may have individual shutoff valves for each
register. Find them and close the valves for the basement vents. If
you can't find a valve, they stuff a plastic bag full of crumpled
paper and shove it into the ductwork until the airflow is entirely
restricted (remember to take it out come winter when the furnace kicks
in). I would also close off any return air ducts in the basement--you
want the hot air returned, not the freezing cold air from your
3. HVAC ducts often leak both air and temperature. Sealing all of
the ductwork carefully will ensure better airflow in the HVAC system
(cooler air upstairs) and less conditioned air leaking into the
basement. Insulating any exposed ductwork will also prevent the
radiant heating/ cooling effect. Before I sealed and insulated my
ductwork in the utility room in the basement, I would be able to close
the door, and within 15 minutes the adjoining basement family room
would be 5 degrees warmer.
Working through these steps have helped me....good luck.
Mr Fixit eh