Both rooms are sunrooms. The differences are inside the walls.
A three-season and four-season sunroom look similar from the curb, but what's behind the aluminum and glass is fundamentally different. The three-season room is built for spring through fall use. The four-season room is built to serve as conditioned living space year-round, in Michigan winters and summers. Both have a place. The right choice depends on how you'll use the space, what your local building code requires, and how the construction differences affect cost over time.
This guide walks through the actual specs (frame thickness, glass technology, insulation, thermal break), Michigan code requirements, pricing context, and how to think about the value of each.
What's actually inside the frame?
The headline difference is frame thickness: a three-season frame is 2.25 inches, a four-season frame is 5 inches. But the thickness isn't the point. The point is what fits inside.
A three-season frame is essentially a hollow aluminum extrusion. It's sturdy, weather-tight, and supports single-pane sliding glass and removable screens. It does not contain insulation, and there's no thermal break separating the inside surface from the outside surface. That means in cold weather, the aluminum itself becomes a heat conductor, which is fine for spring-through-fall use but inefficient for year-round heating.
A four-season frame is engineered as a layered system. Inside the 5-inch profile is insulation, a thermal break, and provisions for double-pane Low-E glass with argon gas fill. Each layer addresses a specific heat-loss path. The insulation slows heat transfer through the wall cavity. The thermal break is a non-conductive material (usually a polyamide strip) that interrupts the aluminum's heat-conducting path from outside to inside. The double-pane glass with Low-E coating reflects radiant heat back into the room while letting visible light through.
The result: a four-season room can be heated and cooled efficiently, while a three-season room is best treated as semi-conditioned space.
Glass technology: where most of the heat moves
Glass area is the biggest variable in sunroom energy performance. According to InterNACHI, the best double-pane windows have a U-factor of 0.30 or lower (lower is better), compared to a standard clear insulated glass unit at U-0.49. That difference compounds across a sunroom's worth of glass area.
Three-season glass: Single-pane sliding glass. Optimized for ventilation and natural light, not insulation. Easy to remove for cleaning. Excellent for spring through fall, when the goal is to enjoy outdoor air with weather protection.
Four-season glass: Double-pane Low-E2 (low-emissivity) glass with argon gas fill between panes. The Low-E coating is a microscopic metal oxide layer that reflects infrared heat. In winter, it reflects heat back into the room. In summer, it reflects solar heat away. Argon gas is denser than air, so it slows convective heat transfer between the panes. Combined, this glass package can deliver U-factors around 0.30 and R-values up to 5 (compared to roughly R-2 for a standard insulated glass unit).
For a Michigan four-season sunroom that might have 200+ square feet of glass, the difference between standard and Low-E argon glass is significant. The U.S. Department of Energy notes that residential windows are responsible for 25-30% of heating and cooling energy use, and in a glass-heavy room that share climbs higher.
Thermal break: the detail most homeowners miss
This is the spec that separates a properly built four-season room from a four-season room that disappoints in February.
Aluminum is an excellent thermal conductor. That's a problem in cold climates because heat moves from the warm side of the frame to the cold side easily, creating cold surfaces inside (which cause condensation) and significant heat loss.
A thermal break solves this. It's a strip of non-conductive material (usually polyamide or polyurethane) inserted into the aluminum profile to interrupt the heat-conducting path. The inside surface of the frame stays closer to room temperature. The outside surface stays closer to outdoor temperature. Heat stops flowing through the frame.
Three-season frames don't have a thermal break. They don't need one for their intended use. Four-season frames must have one to perform in Michigan winters. A four-season room without a thermal break will have cold aluminum surfaces, condensation problems, and heating costs that climb well above what a properly engineered room would.
Insulation and R-values
Three-season rooms have no wall insulation by design. The kickplates (the panels below the windows) can optionally be upgraded to insulated poly for improved shoulder-season comfort, but the room is not engineered for year-round heating.
Four-season rooms are fully insulated through walls, roof, and kickplates. Insulation values vary by manufacturer and installation, but a properly built four-season sunroom should target wall R-values comparable to the rest of your home (typically R-13 to R-20 depending on the area of Michigan).
A note on glass and R-value: even the best double-pane Low-E argon glass tops out at around R-5. That's far less than R-13+ wall insulation. This is why glass area decisions matter so much in a four-season room — more glass means more heat loss, regardless of how good the glass is.
Comfort and temperature control
Three-season room: Comfortable spring through fall in Michigan, roughly April through October with shoulder-month help from supplemental heat. With removable windows and screens, it's especially well-suited to summer use, where ventilation matters more than insulation. Winter use is possible with portable heaters, but you'll be working against the room's design.
Four-season room: Comfortable 12 months a year. Heating and cooling can come from extending your home's HVAC, a dedicated PTAC unit (packaged terminal air conditioner), a ductless mini-split, or a baseboard system. Most Wayne Craft four-season installations use systems sized to handle Michigan's full temperature range, from negative single digits in January to 90°F+ in July.
Michigan permit and code requirements
Both rooms require building permits in Michigan, but the code requirements are different.
Three-season rooms are classified as enclosed but unheated additions. They require a building permit and must meet structural requirements (proper foundation, load-bearing connections, snow loading). They are not subject to the full energy code because the space isn't conditioned. Per Michigan municipal guidance, three-season rooms function more like a covered patio for code purposes.
Four-season rooms are classified as room additions under the Michigan Residential Code. This triggers full code compliance including the Michigan energy code, which requires insulated walls, high-performance windows, and integration with the home's heating system. Four-season rooms typically require electrical and HVAC permits in addition to the building permit.
The energy code requirements add cost to a four-season project, but they also ensure the room actually performs through Michigan winters. A four-season room built to code is a room you'll genuinely use in February.
Permit review timelines in Southeast Michigan typically run two to six weeks depending on the municipality. Wayne Craft handles permitting, engineering documentation, and inspection coordination as part of every installation.
Cost ranges in Michigan
Pricing varies based on size, site conditions, foundation requirements, electrical and HVAC integration, and material choices. General ranges for Southeast Michigan projects in 2025-2026:
|
Room type |
Typical cost range |
What you're paying for |
|
$25,000 to $60,000 |
Aluminum frame, single-pane glass, screens, foundation, basic electrical |
|
|
$50,000 to $120,000+ |
Insulated frame with thermal break, Low-E argon glass, full wall and roof insulation, HVAC integration, energy code compliance |
Note: Pricing reflects general 2026 ranges for Southeast Michigan installations. Your actual quote depends on size, site conditions, foundation requirements, existing structures you're tying into, electrical and HVAC work, and material choices. Material costs (aluminum, glass, insulation) shift year to year based on supply, so these ranges are reviewed annually. For project-specific pricing, request a free estimate. Wayne Craft also works with GreenSky financing partners for projects in this range. Details are on the financing page.
How to think about the value of each
The cost difference between three-season and four-season is significant, but it doesn't tell the whole story. Both rooms deliver value. They just deliver it differently.
Three-season delivers value through: lower upfront cost, ventilation and outdoor connection that a four-season room can't match, shorter permit timeline, and an excellent return when paired with summer-heavy lifestyle use. For homeowners who already have plenty of winter indoor space and want a backyard living area for spring through fall, the three-season room is often the better fit.
Four-season delivers value through: year-round use, addition to your home's livable square footage at resale (covered this in our sunroom property value guide), and integration with your home's heating and cooling system that makes it feel like an extension of the house rather than an addition. For homeowners who want true year-round living space, the four-season room is the only choice that delivers.
Neither room is the "better" room. They solve different problems. The honest question is which problem you actually have.
Choosing between them
A few questions that usually clarify the decision:
How many months a year will you actually use the space? If the answer is 6-8 months, three-season is sized correctly for the use. If the answer is 12 months, four-season is the only choice that delivers.
Do you want the room to count as livable square footage at resale? Four-season rooms typically do. Three-season rooms typically don't, though they still add value as a desirable feature.
Is heating cost a factor? Four-season rooms cost more to heat than the equivalent square footage of your existing home because of the glass area. Three-season rooms aren't designed for heating, so winter heating cost isn't really a question for them.
Does your existing HVAC system have capacity? Four-season rooms either extend your home's HVAC or get their own system. Three-season rooms don't need either.
What's your timeline? Four-season permitting is longer due to energy code review. If you need the room finished by a specific summer, a three-season room is faster.
The bottom line
A three-season sunroom is the right answer when you want a beautiful, well-built outdoor living space for the months Michigan actually allows outdoor living, at a lower cost. A four-season sunroom is the right answer when you want a true year-round room that becomes part of your home's livable square footage and you're willing to invest in the insulation, glass, and HVAC that make that possible.
Both rooms are built on the same aluminum manufacturing platform Wayne Craft has been refining in Southeast Michigan since 1943. Request a free estimate or call (734) 421-8800, and we'll help you talk through which fits your home, your use case, and your budget.
For related reading: sunroom vs screen room: which fits your lifestyle, do sunrooms add home value, and how to winterize a sunroom.