In the DMV, we experience every season—from humid July afternoons to freezing January nights. While most people just say “sunroom,” the International Residential Code (IRC) actually breaks them into five technical categories. Knowing which one you’re building is vital because it determines your foundation, insulation, and even your tax assessment.
Category I: The Screened Porch
The simplest starting point. This is a roofed structure enclosed only by insect screening. It is “thermally isolated” (separated by a door) and completely unheated. It’s great for summer breezes, but strictly a seasonal escape.
Category II: The Basic Enclosed Room
This is a step up from a screen porch. It uses translucent or transparent plastic or glass to keep out the rain and wind. Like Category I, it is unheated and separated from the house. This is your classic, entry-level “three-season room.”
Category III: The High-Performance Three-Season Room
To the naked eye, this looks like Category II. However, the building code requires Category III rooms to meet much higher standards for air and water infiltration. This means the windows and walls are tighter and more durable against the DMV’s occasional heavy storms and snow loads.
Category IV: The Thermally Isolated Four-Season Room
This is a “conditioned” space, meaning it has its own dedicated heating and cooling (like a mini-split). It remains thermally isolated from the main house by a door, but the walls and glass must meet higher energy-efficiency standards so you can use it year-round.
Category V: The True Room Addition
A Category V sunroom is not a “separate” space; it is a full extension of your home. The wall between the house and the sunroom is removed, and the room is open to your main HVAC system. Because there is no separation, the entire room—despite being mostly glass—must meet the exact same rigorous energy and structural codes as a standard bedroom or family room addition.
For further reading, the “Specifications For Sunrooms” publication by FGIA and NSA is the bible on this topic.
