Sunrooms and four-season rooms on a BC custom home: inside the envelope or outside it

Every sunroom conversation we have starts the same way, and it usually surprises the client. Before we talk about glass, roof or furniture, we ask whether the room is inside the house or outside it.

That is a real question with a technical answer. The BC Building Code defines conditioned space as any space within a building whose temperature is controlled, directly or indirectly, by heating or cooling over substantial portions of the year. A room that meets that definition is part of the house. A room that does not is a covered, glazed outdoor space with a door on it. Both are good rooms. They are different buildings, and the design has to know which one it is from the first sketch.

Three-season: outside the envelope

A three-season room sits outside the insulated shell of the house. It has no heating or cooling of its own. The wall between it and the house is built as an exterior wall, insulated and air-sealed, with an exterior-grade door. The glass in the room itself can be lighter, single or double glazed, because it is not being asked to hold heat in.

In Greater Vancouver's climate a three-season room is honest about its name. It works from spring into fall. In January it is cold, and the door to the house stays closed. For a lot near Burnaby Lake or along the Sperling side, where the value is the garden and the trees rather than the view, that trade is often the right one: a bright, screened, rain-covered room that costs less to build and asks nothing of the energy model.

The code treats it that way too. The energy efficiency requirements in Section 9.36 of the BC Building Code state that buildings or portions of buildings that are not conditioned spaces are exempted from the section. The room is outside the model.

Four-season: inside the envelope

A four-season room is conditioned. The moment it is heated and cooled through the year, it stops being an accessory and becomes a room of the house, and every part of it has to perform like one.

Section 9.36's building envelope requirements apply wherever the envelope separates conditioned space from unconditioned space, the exterior air or the ground. In a four-season room that means the glass walls, the roof and the floor are all the envelope. They go into the whole-building energy model that the BC Energy Step Code requires before the design is finalized, the same model that has to show the house meets Burnaby's requirement of Step 3 plus EL-4 of the Zero Carbon Step Code for permits submitted on or after January 1, 2025.

A room with glass on three sides is the hardest room in the house to get through that model. Glass, even good glass, loses more heat than an insulated wall. So a four-season room costs performance somewhere else: better windows elsewhere, more insulation in the roof, a tighter blower door result. The room can still be built. The way to build it is to put the sunroom into the energy advisor's hands during design, and to spend the glazing budget where it matters. Our comparison of triple and double glazed windows in BC applies here with more force than anywhere else in the house.

On floor area, the two versions also part ways. A conditioned room is finished floor area of the house on the permit drawings, and it counts in the zoning calculations that cap the size of the house on the lot the way any other enclosed room does. How the City counts a particular room depends on the zoning bylaw wording and the room's construction, so we confirm the treatment with the City at the pre-application stage rather than assume it. Either way, the City of Burnaby treats any change to a building's footprint or increase in floor area as an addition that needs a building permit, and its list names patio covers and decks alongside additions, so a sunroom of either kind is permit work.

A three-season room is a porch with windows. A four-season room is part of the house. Decide which you are building before the first drawing, because the wall between them is the whole design.

Icon Projects Team

Orientation and the August problem

Everyone plans a sunroom for the winter sun and forgets the summer one. On the Wet Coast the summer one is the problem.

The 2024 BC Building Code added a cooling requirement: cooling facilities capable of maintaining an indoor air temperature of not more than 26 degrees Celsius in at least one living space in each dwelling unit at the summer design temperature. A sunroom is the last room we would choose to be that space. Its job is to take in sun, and a room designed to take in sun in January will take in more of it in August, when the sun is higher and the days are longer.

So we design the sunroom to gain what it should and shed what it should not. The rules we follow are the same ones in our post on window placement and passive solar:

• South glass with an overhang sized for the site. The overhang shades the high summer sun and lets the low winter sun reach the floor. On a south-facing rear yard in Government Road or Buckingham Heights this is the easy case.
• East glass for a morning room. The sun arrives early, warms the room for coffee, and is gone by the time the day heats up.
• West glass with caution. The long, low afternoon sun of a Greater Vancouver August comes in under any overhang and keeps coming until late evening. A west-facing glass wall overheats in a way that interior blinds cannot fix, because the heat is already inside. We limit west glazing, shade it with the deck roof or a planted screen, or move the room.
• North glass for view rooms on the north slopes of Burnaby Heights and Capitol Hill, kept in proportion so the room does not lose more heat in winter than it gains in daylight.

Exterior shading beats interior shading every time. A blind inside the glass stops light after the heat has entered the room. An overhang, a pergola or a deciduous tree stops it outside.

Roof glazing or a solid roof with clerestory windows

The glass roof is what most clients picture, and on a four-season room it is the first thing we talk them out of.

Roof glazing counts as glazing in the energy model. It faces the sky, so it loses the most heat in winter and takes the most direct sun in summer of any surface in the room. Every joint in it is a roof penetration on a coast that gets rain for months at a time, which is the whole subject of our post on skylights and roof glazing. And it is the hardest surface in the house to shade, because the sun that hits it is overhead.

The alternative gives up almost nothing. A solid, insulated roof, detailed like the rest of the house, with a band of clerestory windows set high on the wall just under it. The clerestory glass brings light from above the eye line, washes the ceiling, and makes the room read as bright and tall. The insulated roof above it holds the temperature. Add a deep overhang at the clerestory and the high summer sun is blocked while the low winter sun still reaches in.

On a three-season room the calculation changes, because the room is outside the envelope and the roof does not have to hold heat. A glass or polycarbonate roof over an unconditioned room is a fair choice there, as long as it is detailed to drain and the client understands the room will be hot under it on a July afternoon.

Connecting to the heat pump

A four-season room needs heating and cooling, and in a new Burnaby house under EL-4 that means the heat pump. The question is how to connect it.

The sunroom gets its own zone. The two rooms have different heating and cooling curves. The sunroom gains fast when the sun is on it and loses fast when it is not, while the living room behind its insulated walls barely moves. Put them on one thermostat and one of them is always wrong.

So the sunroom gets its own indoor unit on a multi-split system. Natural Resources Canada describes a multi-split as multiple indoor units in the home served by a single outdoor unit, with ductless units heating or cooling the air in their space directly. The sunroom head runs on its own thermostat, cools the room in August without freezing the kitchen, and warms it on a January morning without running the whole house.

Sizing matters more here than in any other room. The same NRCan page says heating and cooling loads should be determined by a recognized sizing method such as CSA F280-12, and warns that an oversized heat pump may not deliver the expected savings because it runs inefficiently in milder conditions. A glass room tempts an installer to round up. We have the mechanical designer run the sunroom as its own zone in the load calculation and size the head to that number.

The ventilation side comes along with it. The heat recovery ventilator that serves the house gets a supply into a conditioned sunroom like any other habitable room, so the air in it turns over and condensation on cold glass in winter stays under control.

Where it sits: house, sunroom, deck, garden

The last decision is placement, and it belongs on the floor plan before any glass is chosen.

The sunroom works best as the step between the main living space and the deck, on the same floor level as both, with wide doors on each side. From the kitchen the layers read in order: room, glass room, deck, garden. The sunroom is the covered, sheltered layer, the deck is the open one, and the garden is the far one. The deck stays a deck rather than being swallowed by the addition.

Two placements we avoid. A sunroom that replaces the deck entirely, so the house has no uncovered outdoor floor left, gets used less than either would have on its own. And a sunroom set at a different level from the living room, reached by steps, becomes a room people look at rather than walk into.

On a lot that slopes to the rear, common across Coquitlam's hillside streets and the north side of Burnaby, we keep the sunroom on the main floor level, where the kitchen is, and let the deck outside it step down toward grade. The room keeps its view over the garden and its direct connection to the house, and the stair belongs to the deck rather than to the room.

Get those four things right, the envelope decision, the orientation, the roof, and the heat pump zone, and the sunroom becomes the room the family uses most. Get the first one wrong and every other decision is being made for the wrong building.

Frequently asked questions

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

A three-season room is unconditioned: it sits outside the insulated shell of the house, has no heating or cooling of its own, and the wall and door between it and the house are built as exterior wall. A four-season room is conditioned, meaning its temperature is controlled by heating and cooling through the year, so its walls, roof and glass are part of the building envelope and it is designed to the same standard as any other room. The BC Building Code defines conditioned space as space whose temperature is controlled by heating or cooling over substantial portions of the year, and that definition is the line between the two.

Does a four-season sunroom count toward the BC Energy Step Code on a Burnaby custom home?

Yes. Section 9.36 of the BC Building Code applies its building envelope requirements wherever the envelope separates conditioned space from unconditioned space, outside air or the ground, and it exempts only spaces that are not conditioned. A conditioned sunroom goes into the whole-building energy model that the Energy Step Code requires, so its glazing area and performance have to be balanced against the rest of the house to hit Burnaby's Step 3 and EL-4 requirement. An unconditioned three-season room stays outside the model.

Do I need a building permit to add a sunroom to a house in Burnaby?

Yes. The City of Burnaby treats any change to a building's footprint or any increase in floor area as an addition, and additions need a building permit, with patio covers and decks named on the same list. A sunroom on a new custom home is simply part of the permit drawings for the house. On an existing house it is a home improvement permit, applied for through the City's online portal.

Can a sunroom be the room that meets the 26-degree cooling requirement in a new BC home?

It can, but it is the hardest room in the house to make that work. The 2024 BC Building Code requires cooling facilities capable of holding at least one living space in each dwelling unit at not more than 26 degrees Celsius at the summer design temperature. A room with glass on three sides gains the most heat in summer, so we usually designate a different living space to meet the requirement and design the sunroom so its heat does not spill into that space.

Should a sunroom in Greater Vancouver have a glass roof or a solid roof?

For a four-season room we recommend a solid, insulated roof with clerestory windows set high on the wall. Roof glazing counts as glazing in the energy model, loses the most heat in winter and takes the most direct sun in summer, and every roof penetration on the Wet Coast is a place to detail carefully. Clerestory glass gives light from overhead without the heat load. On an unconditioned three-season room a glass or polycarbonate roof is a reasonable choice, because the room is outside the envelope.

How do you heat and cool a four-season room with the house heat pump?

We give the room its own indoor unit on a multi-split heat pump system, which Natural Resources Canada describes as multiple indoor units in the home served by a single outdoor unit. The sunroom then runs on its own thermostat, because its heating and cooling pattern differs from the rest of the house. The load for each zone is calculated with the CSA F280-12 method that Natural Resources Canada points to for sizing, and the whole system is sized to that calculation rather than rounded up, since an oversized heat pump runs inefficiently in mild weather.

Which direction should a sunroom face on a Burnaby lot?

South, with an overhang sized to shade the glass at the high summer sun and admit it at the low winter sun, is the orientation we plan for first. East works for a morning room. West-facing glass takes the long, low afternoon sun of a Greater Vancouver summer and overheats in a way that blinds cannot fix, so we limit west glazing or shade it with the deck roof or a planted screen. On a north-sloping lot in Burnaby Heights or Capitol Hill, a north-facing room can still work as a view room if the glass area is kept in proportion.

Where should a sunroom sit relative to the deck and garden on a Coquitlam or Burnaby custom home?

Between the main living space and the deck, at the same floor level as both, with wide doors on each side. That way the sunroom is the covered step between inside and out, the deck stays a deck, and the garden reads as the far layer from the kitchen. On a Coquitlam or Burnaby lot that slopes to the rear, we set the sunroom on the upper floor level and let the deck stair down to grade so the room keeps its view and its connection to the house.