Three Season and Four Season Structures Explained

by | Feb 19, 2018 | Planning

Sunspace Design builds elegant glass structures, residential additions, and home renovation projects that can withstand the full force of a New England winter. And when it comes to regional winters, few are more ferocious than New England’s! Rain, sleet, snow, below-freezing temperatures, and other environmental factors common to Massachusetts, New Hampshire, and Maine pose unique construction challenges. Homeowners looking to add glass elements to their property must address these challenges seriously. With over 35 years of specialty glass design and construction experience, we can help.

Whether you’re installing a brand new custom skylight, introducing a functional workspace like a greenhouse, or directly annexing a glass conservatory addition to your home, it’s essential that the new construction achieves the desired performance across a range of expected conditions. We discuss some of the ways that we achieve this performance in our blog article on thermally regulating glass spaces, and we encourage you to check it out.

But what does “desired performance” really mean? That all depends on your needs. Although most of our projects are designed and engineered for constant, year-round use, not every Sunspace project is intended to be used throughout the year. You’re familiar with the concept if you’ve ever heard of a “three season” structure, enclosure, or addition.

Today we’ll be discussing some of the key differences between three and four season glass construction. The differences are numerous, so we encourage you to think carefully about your goals during your construction project’s planning phase. This guide should help.


What are the differences between three and four season enclosures?


The “three season” vs. “four season” distinction can apply to virtually any type of enclosed glass space, including but not limited to: sunrooms (also known as solariums), orangeries, conservatories, greenhouses, pool enclosures, patio enclosures, mudroom enclosures, and porch enclosures.

A three season enclosure is constructed for optimal use during the warmer period of the year—usually spring to fall—and not for typical winter weather. By comparison, a four season enclosure is constructed to be used throughout the year, and should be able to withstand whatever Mother Nature throws at it.

Remember that there can be a large difference in usability across a single season. It’s not uncommon for frosty winter temperatures to persist for five months or more. Thus it is essential that your material and hardware decisions (including glass, HVAC, insulation, and more) support your intended use of your new space.



4 season enclosures incorporate more, higher-performing insulation so that less heat is lost through the roof and walls. Most 3 season enclosures are insulated as well, although some simple porch enclosures have little to no insulation. When a 3 season enclosure is insulated, you can expect lower R-values as compared to 4 season enclosures.

Because most 3 season enclosures aren’t heated, they’re often not subject to existing insulation regulations. As a result, it’s important to be very careful when vetting a distributor that offers 3 season additions. A cheap, prefabricated room outfitted entirely in aluminum panels and vinyl windows will not offer much in the way of insulation. By comparison, a well-built 3 season enclosure from Sunspace Design will offer longer usable seasons and lasting durability, even if not quite meeting the thermal performance of our 4 season offerings.


Glass Choices

It goes without saying that glass selection is essential to the construction of a glass product, enclosure, addition, or renovation. Three season sunrooms and conservatories often use single pane tempered glass. This is adequate for warmer weather, but is a liability to your heating and electric bill if intended for use in the winter. By contrast, four season enclosures necessarily require high performance glass panes with better thermal ratings. The resulting windows, doors, and glass roof elements are energy efficient and insulated. You’ll lose less radiant heat during the frosty months and stay cooler during the scorching months. Insulated glass also prevents ultraviolet radiation from reaching your space. This protects your skin and your furniture alike.


Heating, Ventilation, Air Conditioning (HVAC)

Both three and four season enclosures can benefit from heating, ventilation, and air conditioning, but to different degrees and in different ways. For example, while a three season porch enclosure might be outfitted with a heating element to cut through the chill during the fall and spring, a lack of conventional and glass insulation values will prevent that element from efficiently warming the space through the winter. On the other hand, a cooling system will have to work harder to keep that same three season space cool during the hottest months. A four season conservatory with a comparable design but superior wall, roof, and glass insulation values will be better able to provide year-round comfort and enjoyment efficiently and cost-effectively.


What about “all season” additions?

Your research has probably revealed that “all season” represents another common glass enclosure label in addition to “three season” and “four season”. This term is generally used interchangeably with “four season” and should be understood as offering the same benefits.


Examples of Sunspace Design’s Three and Four Season Enclosures


A photograph of a brand new three season porch enclosure designed and built by Sunspace Design in York, Maine across the street from the Atlantic Ocean

Three Season Porch Enclosure

York, Maine

A coastal building was outfitted with a brand new three season porch enclosure designed and built from scratch. Blending seamlessly with the existing building, the new structure offers magnificent views of the Atlantic Ocean.
A photograph of a new three season porch conversion built residentially by Sunspace Design in York, Maine

Three Season Porch Conversion

York, Maine

Sunspace Design worked with the homeowner to plan a time and money-saving porch conversion located at the front of the property.  The resulting space is excellent for relaxation and entertainment. Total construction time was about one week.
A photograph of the interior of a gorgeous mahogany four season office conservatory located in North Hampton, New Hampshire

Four Season Glass Conservatory

North Hampton, New Hampshire

We worked with the homeowner to design and build a brand new glass conservatory to serve as an office. The project replaced an aging sunroom that had outlived its usefulness. The new office space is fully thermally regulated and connects directly to the home’s interior.
A photograph of the interior of a four season glass conservatory custom designed and built by Sunspace Design in Cambridge, Massachusetts

Four Season Home Conservatory

Cambridge, Massachusetts

This four season conservatory was introduced to a historic Harvard Square home in the heart of Cambridge. All materials used achieve top thermal performance and ensure comfort throughout the year. The new space flows into the home’s interior.

Should I choose a three season or four season enclosure?


All of our custom three season and four season additions benefit from the design and construction expertise that we’re known for. Both offer elegant looks, relaxing interiors, and have transformative effects on homes. Both allow the entry of incredible levels of natural sunlight. Both provide wonderful, accessible views of the surrounding environment, and both deliver incredible value. The question you should be asking is: what are my goals for my home improvement project?


Cost and Time

A three season enclosure typically costs significantly less than a four season enclosure. That’s because the latter requires more expensive and higher-performing materials. Related to this is construction time. You can expect that a four season enclosure will require more work; it’s not uncommon for new foundations to have to be laid, for example. A three season enclosure offers a much quicker turnaround. We can often modify your existing porch, patio, or mudroom in a fraction of the time it would take to build a totally new room on your property. A four season structure typically represents a price increase in the range of 20-25%.


Personal Expectations

The amount of time you spend in your home each year is a great way to start determining the best fit for you. Are you someone who travels every winter? If so, the additional thermal and solar performance offered by a four season space may not be necessary. On the other hand, you can expect a four season enclosure to be durable and high-performing for years to come, and it may be a better home investment. Also consider access: do you want the structure to open directly into the interior of your home? In that case, you’re unlikely to want a three season structure that isn’t properly insulated for winter conditions. Your final decision must be based on your priorities and immediate needs.


In Conclusion


We’ve been at this since 1981. In that time, we’ve helped our clients achieve virtually every home construction goal under the sun. Our reliable construction planning process means that both three and four season residential additions can be tailored to meet your specific needs, and designed to pair perfectly with the appearance of your existing architecture. If you’d like to learn more about what we can offer you, consider calling us at 1-800-350-2505, emailing us at, or contacting us via our website. Thanks for reading!

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