Insulating and heating my garage sounds like a pain. What are the consequences of simply allowing water-based latex paint to freeze in the winter? Is it ruined forever or does it just need tons of extra mixing in the spring?
I have an idea for you. Instead of insulating and heating the entire garage to save a few gallons of paint, perhaps you could build a small insulated paint locker. Build a box or cabinet large enough to store all your paints. Insulate the inside with some 2 inch rigid foam or R-11 blanket insulation and put some foam weather stripping around the insulated door or cover to make it fairly air tight. You could then install a low wattage light bulb as a source of heat. Honeywell makes an inexpensive plug=in/plug outlet feed thermostat that would work fantastic in this application. I use one for running three 40 watt clamp-on light fixtures in the engine compartment of my boat. The thermostat can be set to turn on the lights when the temp lowers to 35 to 40F and turn off at 40 to 50F. You set it to keep the locker above freezing. I wish I was good at drawing pics with the puter, but a good design would be to mount your lights and thermostat at the very bottom of the locker with a venting or wire shelf just above, so the heat rises in the locker and the bulbs are protected from getting bumped by paint cans. I'm sure you could build the whole project for under $100. The bonus is that something like this would be very cheap to run all winter.
Latex paint is composed of a mixture of components that when frozen can cause the solids to expand and separate from the mixture. Effects of using damaged latex paint can result in degraded performance such as un-even coat, less gloss, cracking and peeling of paint much sooner when exposed to the environments (sun light), and etc.
It is not recommended to expose latex paint to extreme temperatures. You probably can get away with it once. If it's frozen, thaw (expose to room temperature) and once thawed, stir the paint. If the paint is smooth, you should be okay; however, if the paint is lumpy (the solids have separated from the mixture because it was damaged by being frozen) then you have a bad mixture.
Some mixtures may include propylene glycol, which is used to protect the paint from freezing temperature and damaging the mixture.
I accidentally froze a 5 gallon bucket of paint. I didn't want to throw it away, so I thawed it out and used a paint mixer on my drill to blend it. It was still lumpy, so I used an old screen and strained all the lumps out of it. I used it to paint my son's living room. By the way it looks just like the paint that was not frozen. I lost some paint in the straining process, but salvaged most of the frozen paint and used all the remaining paint. It was well worth the try, since that brand of paint is about 32 bucks a gallon.
For a latex paint (water soluble) I added water and actually used a hand blender for 3-4 minutes, the result was an even, smooth paint. The paint was completely granulated before.
Whether or not it stays on the wood will be determined in a couple of years...
EDIT: The paint had frozen completely at least twice!
the easiest way to store paint in the winter is in an old refrigerator in the workshop, has insulation, etc just leave the unit as is and install a incandescent light bulb of about 40w connected to an old wall thermostat in the bottom. When the temperature inside the unit drops below what you have set it to the light comes on. If you are really worried, use two bulbs when/if one fails another there for backup da da, and during the rest of the year, keep you beer in it!! :-)