Quick Answer: Improved paint technology lets you push down as low as 35 degrees F (as long as you buy that type of paint). In Northern states, stop around November. In Southern states, stop around December. For a more nuanced answer, read on.
Longer Answer: Optimal painting season for house exteriors is usually thought of as Summer. The weather is warm, rain scarce, and in general, it's a weary task made more fun by the inclusion of sun and warmth--not to mention those steaks on the barbecue at the end of the day.
But if you can't paint during that perfect season, when can you? How far can you stretch the painting season? Can you paint during the dead of Winter?
Pay attention to two factors when considering lengthening your painting season: temperatures and moisture (Note: all temperatures here are referred to in Fahrenheit):
1. TemperaturesPaint manufacturers recommend a minimum outside temperature for painting. As paint quality has improved over the years, so too has the minimum temperature point. For a long time, it was recommended that you never paint a house when the ambient temperature is 50 degrees. Today, this tends to hover around 40 degrees, but some premium paints--namely Sherwin Williams' Duration, Resilience, and SuperPaint--can be applied down to as low as 35 degrees. As long as you stay above that minimum temperature, you should be fine. Just as important are temperature fluctuations. Paint isn't happy when temps during the day are 105 degrees and then suddenly drop down to 43 degrees in the evening and night.
For that reason, early Fall can be a good time of the year in your location to paint. Daytime and nighttime temperatures are often closer than during other times of the year. Using New York City as an example, average low temperatures are 47 degrees in October and 38 degrees in November. So, in New York, November is the cut-off month for exterior painting season, at least based on averages. In Columbia, South Carolina, you can push your painting season one more month. Average lows in November are 45 degrees. It isn't until December that averages drop down to 39 degrees--still within the range of premium paint specs
2. MoisturePaint should only be applied to a dry surface. Your painting surface can become moist either by direct contact from rain or snow or by humidity in the air. After a hard rain, wait at least one full day before painting. Check the surface to be painted. Does it feel wet? Even if it does not feel wet to the touch, it may be wet within, especially for porous surfaces like bare wood and masonry. Remember, moisture happens indirectly, too. Dew forming over night or in early evenings can quite easily mess up exterior paint--even though it was a dry, balmy 70 degrees just six hours earlier.
Tip: What Happens After You Punch Out? Homeowners tend for forget that paint is still drying after they stop painting for the day. Look at yesterday's evening low and today's predicted low. Even though lows tend not to occur until deep into the evening, it's prudent to assume that they might happen right after sunset.
What Do Professional Painters Say About Painting Seasons?Some painters are like the U.S. Postal Service: "Neither snow nor rain nor heat nor gloom of night" will prevent them from painting your house. If I heard a professional painter talk like that, I would pass on him. It's just not possible to paint during all seasons. So what do they say?
In the cold, snowy clime of Minnesota, you will not (or should not) find painters working at all during the winter. It's just too cold for the paint to cure. And if you do try, you may find that the paint flakes off. Instead, this is the time for interior painting.
Pro painters in Ohio and Illinois conservatively block off the entire mid-October to end-of-March period as verboten for exterior painting. They say that even the low-temperature premium paints don't make a difference. A professional in Eastern Massachusetts even backs off from exterior painting around October 1st.