Three-Season Versus Four-Season Sunrooms in Virginia

Begin with how you will use the room through the year

Picture the room in January and in August. Will it be a place for morning coffee from April to October, or a home office, playroom or dining area used every day? Will plants, furniture or electronics stay there all year? Do you want screens to open the room to breezes, or a room that feels like the rest of the house?

Those answers set the type. Seasonal use with fresh air points to a three-season room. Year-round daily use, especially as part of the main living area, points to a four-season room that is heated, cooled and built like the rest of the house.

Screened porch, three-season room or four-season room?

Many Northern Virginia homeowners weigh these three as steps on one ladder. A screened porch keeps insects out while staying open to the air, which suits summer evenings but offers no protection from cold or blowing rain. A three-season room replaces screens with windows, often operable ones, so the room is usable in cooler shoulder seasons and on rainy days, while still opening up to breezes. A four-season room is a true interior room with glass walls.

If you are converting an existing porch or deck, the existing structure becomes part of the decision. A deck built for outdoor loads may not be suitable to carry walls, windows and a roof, and an open porch roof may need changes to support an enclosure. Have the existing structure assessed before choosing the room type, because the answer may change the budget more than the choice of glass.

Understand what the labels describe

Three-season and four-season are product categories, not legal definitions, and each manufacturer defines them for its own systems. Patio Enclosures, for example, says its three-season rooms are available with either single-pane glass or double-pane insulated glass depending on planned use, and that unlike its four-season rooms they are not designed to be heated or cooled on an ongoing basis. It describes its four-season rooms as using structural thermal breaks in the frame and double-pane insulated glass, with heating and cooling added for climate control.

So the familiar rule of one pane for three-season and two panes for four-season is not universal. Compare the actual specifications of the systems you are offered rather than the label.

Source: Patio Enclosures: Sunroom types and options

Compare the entire enclosure, not just the glass

A sunroom's comfort depends on the whole envelope: glass, frames, roof, floor and how they are sealed. For glass, look at the NFRC label. ENERGY STAR explains that U-factor measures the rate of heat transfer and how well the window insulates, with lower values insulating better; Solar Heat Gain Coefficient measures how much solar heat comes through; and Visible Transmittance measures how much light passes. Air leakage and condensation resistance are rated too.

In a room that is mostly glass, Solar Heat Gain matters as much as U-factor. A sunroom facing south or west can overheat in summer if the glass admits a lot of solar heat, so ask how the glass, roof type and any shading were chosen for your orientation. A solid, insulated roof generally makes a room easier to keep comfortable than an all-glass roof, at the cost of some daylight.

Source: ENERGY STAR: Independently tested and certified energy performance (NFRC ratings)

Coordinate heating, cooling and ventilation

A four-season room needs a heating and cooling plan from the start. Extending the home's existing ductwork only works if the system has spare capacity, and a glass-heavy room has a larger load than a typical room of the same size. Many sunrooms use a separate system, such as a ductless mini-split. Graves used a ductless mini-split to condition a laundry room in an addition in Falls Church, an example of serving added space without relying on the original system.

Ask for a load calculation for the room, an explanation of how it will be heated and cooled, and how the room will be ventilated. For a three-season room, decide whether you will ever add portable heating or cooling, and understand that it was not designed for continuous conditioning.

See a conditioned addition: Laundry room in a Falls Church addition

Review foundation, structure and approvals

Fairfax County lists sunrooms, screen porches, decks and other additions among the residential projects that require a building permit, so the season label does not remove the approval. A sunroom built on an existing deck or patio needs that base checked to see whether it can carry the new structure; often a new foundation is required.

Site rules apply as well. Fairfax's planning guidance notes that zoning setbacks, prior property approvals, floodplains, Resource Protection Areas and soils can limit what is built, and that new structures or exterior changes require a site-related plan or survey in addition to building permits. If you belong to a homeowners association, its approval is separate and should be obtained before ordering.

Source: Fairfax County: Does my project require a permit?

Source: Fairfax County: Plan ahead (zoning, floodplains, RPAs, soils, site plans)

Compare scope and continuing costs

Compare proposals for the whole room: foundation or base work, structure, roof, glass and frames, insulation, flooring, electrical, heating and cooling, the connection to the house, interior finishes, permits and site restoration. A three-season room usually costs less to build because it skips much of the insulation and conditioning, while a four-season room adds those but becomes usable living space.

Think about running costs too. A four-season room adds conditioned area to heat and cool, and a glass-heavy room can be expensive to keep comfortable if the glass and orientation are poorly matched. The NFRC ratings and the heating and cooling plan are the best predictors you have before the room is built.

For budget drivers: Sunroom cost drivers and scope planning

Choose using documented specifications

Make the decision on paper. Ask each provider for the glass specification and NFRC ratings, the frame and roof construction, the insulation values, the heating and cooling plan, the foundation plan and the permit responsibilities. Then pick the type that matches how you will use the room.

• Describe how and when the room will be used through the year.
• Note which direction the room will face.
• Ask for NFRC U-factor, SHGC and visible transmittance for the glass.
• Confirm frame, roof and floor insulation.
• Get a heating and cooling plan with a load calculation.
• Confirm the foundation plan and who handles permits and site review.
• Obtain homeowners association approval if required.

Next step: Talk with Graves about planning a sunroom