· Michael Blair
The two are built differently, permitted differently and cost very differently. The honest question is how many months of the year you expect to use the room.
What each actually is
A three-season room is an enclosed porch: single glazing, uninsulated walls and roof, no conditioning. Comfortable in spring and autumn, hot in a Nashville July and cold in January.
A four-season room is conditioned living space with insulated walls, insulated glazing and heating and cooling. It is an addition rather than a porch enclosure.
The Middle Tennessee reality
Our comfortable outdoor months are roughly April to May and September to October. A three-season room extends those at both ends but does little for the hottest and coldest stretches.
That is genuinely useful for many households, and it costs far less. Just be clear it is not year-round space.
- Three-season: enclosed porch, no conditioning, spring and autumn
- Four-season: insulated, conditioned, counts as living space
- Four-season needs its own HVAC capacity or a mini-split
- Only four-season adds to heated square footage
Do not extend the existing HVAC into a three-season room
Ducting a porch enclosure into the house system is a common shortcut and a poor one — the system was sized for the house, and an uninsulated room is an enormous load it cannot carry.
The result is a room that never conditions and a house that is less comfortable everywhere else.
What it does for the house
A four-season room adds heated square footage and appears in an appraisal. A three-season room usually does not, though it still adds appeal.
Permitting differs accordingly, and a four-season room is a structural addition with foundation, insulation and energy code requirements.