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Sunroom Types and Categories: Three-Season vs. Four-Season

In this blog, we break down the key differences between three-season and four-season sunrooms, and explore the five essential categories every homeowner should know.

Table of Contents

Without a doubt, sunrooms are one of the most effective ways to add value, utility, and happiness to any home. It might seem like there is little variability from sunroom-to-sunroom, but they’re not all created equal. This isn’t to say every sunroom is completely different than the next, rather that there are key distinctions and categories of sunrooms that any homeowner should understand before installing their own. In this article, we’ll go over what a sunroom is defined as, what constitutes as a four-season sunroom, and the five different categories of sunroom categories.

Regular Sunroom vs. Four-Season Sunroom


Defining a Sunroom
The International Residential Code defines a sunroom as single-story structure attached to a residence that has a glazing area that makes up at least forty percent of the total area of the structures outside walls and roof. In case you didn’t know, the glazing area is essentially the transparent portion of a glass panel, excluding the framing. However, this doesn’t mean that sunrooms can’t be two stories, which you can see some examples of here.

Three-Season Sunrooms
A three-season sunroom provides the basics for homeowners looking to only use their space for a smaller portion of the year, hence the name “three-season.” These rooms don’t have any sort of climate control, use materials with little to no thermal insulation, and aren’t enclosed with anything more substantial than plastic or single-pane glass. Three-season sunrooms aren’t fully integrated into the home and are instead separated by a wall.

Four-Season Sunrooms
Also known as an all-season room, four-season sunrooms are fully integrated into the home. These rooms are constructed from higher quality, fully insulated materials as well as thicker glass to ensure that the sunroom meets, if not exceeds, the quality of the home. There is no separation between the home and the sunroom, with the home’s thermal controls used by the sunroom as well. As a result of this, the room is suitable every single day of the year, since the weather no longer has any bearing on the usability of the room.

Sunroom Type Comparison Chart

Type? Insulation? Climate Controlled? Integrated Into Home? Enclosure Material
Three-Season Little to none No No Plastic or single-pane glass
Four-Season Yes Yes Yes Double-pane glass, wood

Different Categories of Sunrooms

The five sunroom categories are legal designations that every sunroom has been assigned to during design and construction. There are three main designations you must know to understand these codes. The first of these is a thermally isolated sunroom, which means a sunroom that isn’t connected to a home’s temperature controls or equipment. Next, a conditioned sunroom is one that has temperature control. Finally, a habitable sunroom is one used for living, sleeping, eating or cooking.
  • Category I: A thermally isolated, unconditioned sunroom that is either open or enclosed with very thin film or screen.
  • Category II: A thermally isolated, unconditioned sunroom enclosed with translucent or transparent plastic or glass.
  • Category III: A thermally isolated, unconditioned sunroom enclosed with translucent or transparent plastic or glass, meeting additional requirements for air and water resistance.
  • Category IV: A thermally isolated, conditioned sunroom with enclosed walls, designed to be thermally controlled independently of the home.
  • Category V: A habitable and conditioned sunroom with enclosed walls, open to and designed to be thermally controlled by the home.

Sunroom Category Comparison Chart

Responsive Table
Category? Thermal Isolation? Conditioned? Habitable? The Difference?
Category I Thermally Isolated Unconditioned Nonhabitable Open or enclosed with film/screen
Category II Thermally Isolated Unconditioned Nonhabitable Enclosed with plastic/glass
Category III Thermally Isolated Unconditioned Nonhabitable Meets additional air/water resistance requirements
Category IV Thermally Isolated Conditioned Nonhabitable Enclosed with regular walls, thermally controlled separately of home
Category V Not Thermally Isolated Conditioned Habitable Open to and thermally controlled by the home

With this in mind, we can understand the connection between a three or four-season sunroom and the five categories. As defined earlier, three-season sunrooms don’t use the higher quality, more insulating materials found in four-season sunrooms. This means the rooms aren’t conditioned, labeling Category I, II, and III sunrooms as three-season. However, a key part of the definition of a four-season sunroom is the room being open to the rest of the home, using the home’s thermal controls. Only a Category V sunroom is a true four-season room equipped for year-round usage.

Category V Sunroom: Benefits

Simply put, a real four-season, Category V sunroom is built with better materials than any other kind of sunroom. For most homeowners, the lower category sunrooms become a liability. Due to the lower quality materials used, these sunrooms tend to age at a much faster rate than the rest of a home, meaning a replacement will be needed after ten to fifteen years. In comparison to that, four-season sunrooms are built from similar materials to the home itself, aging at a similar rate to the home as well.

As you can imagine, there is an increased cost to building a sunroom to these specifications rather than those of a three-season sunroom. However, the extra money spent on a four-season sunroom makes the purchase an investment rather than an expense. Lower category sunrooms tend to either make no difference on or lower the value of a home. Even worse, because these rooms aren’t considered habitable, they don’t count towards the square footage of the home. On the other hand, Category V sunrooms are habitable and increase the value of a home.

Arguably the most important factor in any sunroom purchase should be how usable the sunroom is. When it comes to usability, the name “three-season” is misleading since there’s no guarantee the sunroom is even usable for all three seasons. Unpredictable weather can make the room too hot or cold, in some cases even too unpleasant because it’s raining outside. With a four-season, climate-controlled room that is attached to the house, there won’t be a single day that goes by where the sunroom is unusable.

"We love our new room. We moved the kitchen table and now enjoy all our meals there. We are fortunate enough to live in a nice neighborhood and have a backyard with some trees to enjoy. If you're having second thoughts, don't, you'll never regret it."
Norman B.
Satisfied Sunroom Customer

This testimonial perfectly encapsulates the joys of having a four-season sunroom. As highlighted in our article, four-season sunrooms are designed to be fully integrated into your home, providing year-round comfort and usability. Unlike three-season sunrooms, which can only be enjoyed part of the year, a four-season sunroom offers a seamless extension of your living space.

Request a free catalog today to see how a Brady-Built sunroom can transform your home!