What is the difference between a three-season and four-season sunroom in Ottawa?

The difference between a three-season and a four-season sunroom comes down to whether the room is built as conditioned living space. In Ottawa, with winters that regularly reach well below -20°C and hot summers, the choice affects how you use the room, how it is built and what it costs.

A three-season sunroom is meant for spring, summer and fall. It is usually a lighter structure, often aluminum or vinyl framing with less insulating glass, a floor with little or no insulation, and no permanent heating. Think of it as a well-built enclosed porch. It is cheaper to build, but it will be cold and unusable through much of the winter, and you should not expect to heat it comfortably.

A four-season sunroom is a heated addition that happens to have a lot of glass. It needs insulated walls, floor and roof, a proper air and vapour control layer, high-performance insulated glass, a foundation designed for frost, and a heating system sized by a heat-loss calculation. Under the Ontario Building Code, an addition must meet an energy compliance package or the addition table; your designer will show how the large glass area is handled within that.

The foundation is a major difference. A heated room needs footings protected from frost heave; under the Building Code, footings in clay or soils that are not clearly defined must be at least 1.2 m deep and not less than the depth of frost penetration. A three-season room may sit on piers or a lighter foundation, but ask your designer how it will handle frost movement, since even an unheated room can be damaged by heaving.

Glass is the other big difference. A four-season room needs high-performance insulated glass to limit heat loss and keep the inside surfaces warm enough to avoid condensation. Heating options include a cold-climate heat pump and in-floor heating; ask for quotes on each.

As perspective rather than a quote, a three-season room typically costs much less than a four-season room of the same size, and a four-season room is priced more like a conventional addition. Size, glazing, foundation and finishes drive the difference, so ask for itemized quotes on both.

On permits, the City of Ottawa lists additions among permit projects. Whether your particular sunroom design needs a permit, and what the City will review, is something to confirm with Building Code Services at buildingpermits@ottawa.ca. Fees are $12.75 per $1,000 of construction value. Compare itemized quotes for both options from contractors in the Ottawa Construction Network directory.

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Looking for experienced contractors? The Ottawa Construction Network connects homeowners with qualified professionals:

• Homeupgraders
• RenoMotion Inc.
• Grunt Work 4 Grunts
• The Fixer
• L.L. Renovation