A three-season room or sunroom is a porch that has been modified to keep the weather out. While a three-season room protects you from the wind, rain, and snow, it is not heated or cooled. A sunroom not only keeps the elements out, it offers a heated and cooled environment like any other room in your house. Consider a three-season room if you want a protected spot to get some fresh air but don't mind what the temperature is. Rather than braving the cold or oppressive heat to enjoy the sun, consider an all-season sunroom as opposed to a three-season room.
The most cost-effective way of converting a porch or a three-season room is to install glass and screen panels and a storm door between posts or columns. Alternatively, you can build a three-season room with floor-to-ceiling windows and screen panels.
Due to the fact that a three-season room is not insulated and not heated, it will not get much use throughout the winter months in cold climates. Then again, on those chilly days, late in the fall or early in the spring, when a porch would be uncomfortably chilly, a three-season room can warm nicely as radiant heat from the sun is captured inside. In some cases, the off-season may be the summer, when the uninsulated, unaired space tends to get too hot during those months.
As a form of outdoor living, the three-season porch has declined in popularity over the years. Thanks to low-cost insulated glass windows and doors, as well as the relative simplicity of adding insulation on walls and ceilings, sunrooms are more affordable and practical than ever.