A passive solar greenhouse heats and cools itself without propane, electric heaters, or fans running all winter. It does this through orientation, insulation, glazing selection, and thermal mass -- storing daytime heat and releasing it overnight. On coastal BC, where winters are mild but overcast, the design choices differ from what works in Alberta or the BC interior. After 10 years of building these on Vancouver Island, I can tell you that a greenhouse designed for Kamloops will underperform badly in Campbell River. The problems are different here.
This guide covers the shed-roof design we build, the two standard sizes we offer, glazing tradeoffs, thermal mass requirements, foundation, ventilation, and snow load considerations for coastal BC.
Why Passive Solar Works on the Coast
Campbell River sits at 50 degrees north latitude and receives approximately 1,800 sun hours per year. That is more than most of coastal BC. Winter lows rarely drop below -12 to -15 degrees Celsius -- far milder than interior locations. The frost-free season outdoors runs about 193 days (mid-April to mid-October).
A well-built passive solar greenhouse adds roughly 42 days to that growing season in a 3-season configuration. With a climate battery and sufficient thermal mass, you get 365 frost-free days -- year-round production of cold-hardy crops like kale, spinach, lettuce, carrots, and herbs.
The challenge on the coast is not cold. It is cloud cover. November through February delivers only 1.5 to 3.0 peak sun hours per day. That means your glazing angle and material choice matter more here than in clear-sky climates. Shallower angles (40 to 55 degrees from horizontal) capture more diffuse light from an overcast sky. Polycarbonate diffuses light better than glass -- spreading it evenly across all plants rather than creating hot spots and shadows.
The Shed-Roof Design
We build a simple shed-roof greenhouse. Single slope from a tall south wall down to a shorter north wall. Not a peaked ridge greenhouse. Here is why:
- Simpler framing -- no complex ridge beam or dual-pitch trusses
- Fewer joints -- fewer potential leak points in a wet climate
- The tall south wall is fully glazed with polycarbonate and captures low winter sun effectively
- The low-pitched roof (12 to 15 degrees) sheds rain and snow reliably
- The insulated north wall stores heat and blocks prevailing north/northwest winds
How the Profile Works
The long axis runs east-west. The short dimension (width) runs north-south -- this is the rafter span. The south wall is tall and fully glazed. The roof slopes from south (high) down to north (low). The north, east, and west walls are insulated.
At 50 degrees north latitude:
- Winter solstice noon sun angle: 16.5 degrees above the horizon
- Equinox noon: 40 degrees
- Summer solstice noon: 63.5 degrees
A vertical south wall catches low winter sun at a steep angle (good penetration into the greenhouse) while naturally rejecting high summer sun (reduces overheating). This is actually better for Vancouver Island's overcast climate than a steep angled glazing face.
Two Standard Sizes
We build two standard greenhouse sizes. Both use proven cut lists and local materials. No custom engineering delays.
9x12 Backyard Greenhouse (108 Square Feet)
| Dimension | 9 ft wide (N-S) x 12 ft long (E-W) |
|---|---|
| Floor area | 108 sq ft (10.0 m²) |
| South wall height | 9 ft (2.7 m) |
| North wall height | 7 ft (2.1 m) |
| Roof pitch | ~12-14 degrees |
| South glazing area | ~10 m² |
| Good for | 1-2 person household, herbs, greens, tomatoes, season extension |
| Permit | Generally exempt (under 10 m² threshold) |
This size is designed specifically to sit under the 108 square foot permit exemption in the BC Building Code. No building permit required in most municipalities. You still need to meet setback requirements -- typically 1.5 to 3 metres from property lines.
14x20 Production Unit (280 Square Feet)
| Dimension | 14 ft wide (N-S) x 20 ft long (E-W) |
|---|---|
| Floor area | 280 sq ft (26 m²) |
| South wall height | 10 ft (3.0 m) |
| North wall height | 7.5 ft (2.3 m) |
| Roof pitch | ~10-12 degrees |
| South glazing area | ~19 m² |
| Good for | Family of 4, year-round salads, fruiting crops, seedling starts |
| Permit | Required (building permit needed) |
| Optional | Climate battery (subterranean heating) |
The production unit is where a climate battery starts making sense. The floor area provides enough volume for the underground pipe network to function efficiently, and the glazing area generates enough excess heat in shoulder seasons to charge the soil beneath the greenhouse.
South-Facing Glazing: 8mm vs 16mm Polycarbonate
Glazing is the most important material decision in a passive solar greenhouse. It determines how much light reaches your plants, how much heat escapes at night, and how long the structure lasts.
| Property | 8mm Twin-Wall | 16mm Triple-Wall |
|---|---|---|
| R-value | 1.8 | 2.6 |
| Light transmission | 90% | 72% |
| U-value (W/m²K) | 3.6 | 2.5 |
| SHGC | 0.58 | 0.48 |
| Snow load | ~25 lbs/sq ft | ~40 lbs/sq ft |
| Weight (4x8 sheet) | 9.6 lbs | 14.5 lbs |
| Best for | 3-season (can freeze) | Year-round (no freeze) |
When to Use 8mm
If you are building a 3-season greenhouse and accept that it may freeze on the coldest January nights, 8mm twin-wall polycarbonate is the cost-effective choice. It transmits 90% of available light -- important during the dark months. The lower R-value (1.8) means more heat loss overnight, but in mild coastal BC where extreme lows are -12 to -15 degrees Celsius and happen only a few nights per year, cold-hardy crops (kale, spinach, chard, carrots) survive fine.
When to Use 16mm
If your goal is year-round production with no freezing allowed -- Mediterranean perennials, aquaponics, or frost-sensitive crops through winter -- use 16mm triple-wall minimum. The R2.6 insulation value reduces heat loss by 30% compared to 8mm. Pair it with insulated blinds (doubles the effective R-value to ~R5) for best performance.
The tradeoff is light: 72% transmission vs 90%. In Campbell River's dark November-December (1.5 peak sun hours/day), that 18% difference matters. Plants get slightly leggier. But polycarbonate diffuses light so well that every leaf gets even coverage, which partially compensates.
Thermal Mass: 100 Litres Per Square Metre of Glazing
Thermal mass stores daytime heat and releases it overnight. Without it, the greenhouse overheats during the day and drops to outdoor temperature by midnight. With it, you hold 4 to 8 degrees Celsius above outdoor overnight minimums.
Sizing Rule
100 litres of water per square metre of south-facing glazing
Water is the best affordable thermal mass material. It holds 4.18 kJ/kg per degree Celsius -- roughly 5 times better than rock or concrete per unit weight.
What That Looks Like
| Greenhouse Size | South Glazing Area | Water Needed | 200L Barrels |
|---|---|---|---|
| 9x12 (108 sq ft) | ~10 m² | 1,000 litres | 5 barrels |
| 14x20 (280 sq ft) | ~19 m² | 1,900 litres | 10 barrels |
Placement
Line barrels along the north wall. Paint the sun-facing side black (absorbs heat). Paint the north-facing side white (reflects light back toward plants). This placement does not shade any growing area and puts the mass where it is most effective -- absorbing direct sunlight through the south glazing all day.
Do not fill barrels completely if your greenhouse may freeze -- leave 10% air space for ice expansion.
Insulated Blinds: the Highest-ROI Add-On
Insulated blinds installed inside the glazing effectively double the R-value (R1.8 becomes R3.6, R2.6 becomes R5.2). They reduce total heat loss by approximately 30% and extend thermal mass effectiveness from 12 hours to 16 hours overnight. Close them at sunset, open at sunrise. Manual operation is fine -- no electronics needed.
Insulated North Wall
The north wall takes no useful winter sun. Insulate it heavily. Our standard is 2x6 framing with rigid foam or spray foam to achieve R20 minimum. R30 is better if budget allows.
Target R-values for each surface:
- North wall: R20-R36
- East and west walls: R20-R36
- Roof (insulated portion): R30-R40
- Knee wall (below glazing): R20 minimum
Common insulation materials and their R-value per inch: spray polyurethane (6.9), XPS rigid foam (5.0), rock wool (4.3), fibreglass batt (3.9), EPS (3.8). A 2x6 wall with spray foam hits R33. Same wall with fibreglass batt hits R20.
Soil Floor: Grow Directly in the Ground
Do not pour a concrete slab inside a greenhouse. Concrete disconnects your plants from the subsoil micronutrient web, forces raised beds, and requires constant amendments.
Our greenhouses have a soil floor. Plants grow directly in the ground -- their roots access subsoil nutrients that raised beds cannot reach. The living soil also contributes thermal mass (wet soil holds 1.5 kJ/kg per degree Celsius) and decomposing organic matter releases CO2 that feeds photosynthesis.
If you want clean pathways, use gravel paths between growing beds. A gravel floor at $3 to $6 per square foot gives you drainage and clean footing without the problems of a slab.
Foundation: Augered Sono Tubes
The standard foundation for both greenhouse sizes is augered sono tube piers. Here is why:
- Fast installation -- done in a day
- Handles rocky ground (common on Vancouver Island)
- Clears the frost line (2 to 3 feet on the coast)
- No full excavation required
- Keeps the structure off the ground (moisture management in a wet climate)
- Does not seal off the soil floor -- plants can still grow in the ground between piers
For the 9x12 greenhouse: 6 piers (3 per long side). For the 14x20 production unit: 8 to 10 piers depending on site conditions. Piers are set to the same elevation using a laser level, then connected with pressure-treated 2x6 sill plates.
Other Foundation Options
| Type | Best For | Notes |
|---|---|---|
| Screw piles / helical piers | Rocky ground, fast install | No excavation needed |
| Concrete perimeter footing | Permanent, best frost protection | Most expensive |
| Rubble trench | DIY-friendly, good drainage | Long-lasting, low cost |
| Floating PT deck | Budget, relocatable | Adequate for mild frost |
Ventilation Sizing
Under-ventilating is the most common greenhouse mistake I see. A sealed greenhouse on a sunny March day hits 35+ degrees Celsius by noon -- well above the 29 degree maximum for healthy plant growth. Plants get heat stress, powdery mildew, and blossom end rot.
Vent Area Rules
- Low vents: 2-foot-high louvres along the full length of the knee wall (south side). These are the fresh air intake.
- High vents: At the peak of the north wall or roof. Area should be 15% MORE than low vent area (air expands when heated and needs more space to exit).
- Minimum 1 vent per 10 m² of floor area
What That Means for Each Size
| Greenhouse | Low Vent Area | High Vent Area | Vent Count |
|---|---|---|---|
| 9x12 (10 m²) | 2.2 m² (3.7m length x 0.6m) | 2.5 m² | 1 low, 1 high minimum |
| 14x20 (26 m²) | 3.7 m² (6.1m length x 0.6m) | 4.2 m² | 3 vents minimum |
Automation
Solar-powered wax actuators open louvres automatically when temperature rises above a set point -- no electricity required. They cost more than manual vents but prevent crop loss when you are not home on a sunny day. We recommend them for any greenhouse where the owner cannot check temperatures daily.
Condensation Management
Vancouver Island gets 1,500 mm of rain annually. Moisture management inside a greenhouse is critical. Tilt all flat surfaces to drain. Slope the floor slightly toward a drain point. Use pressure-treated framing where moisture exposure is likely. Ensure adequate air circulation even in winter -- crack the high vent on mild overcast days.
Snow Load Considerations
Vancouver Island coastal areas carry a design snow load of 1.0 to 1.5 kPa. That is light compared to the interior (3.0+ kPa), but it still needs to be accounted for in the framing.
Our standard framing -- 2x6 Douglas fir rafters at 24 inches on centre -- handles this without issue at the shed-roof pitch of 12 to 15 degrees. The pitch is enough to shed wet coastal snow before it accumulates.
If you are building at elevation (above 400 metres) or in a known heavy-snow microclimate, upgrade to 2x8 rafters or tighten spacing to 16 inches on centre. The 8mm twin-wall polycarbonate is rated to 25 lbs/sq ft snow load. The 16mm handles 40 lbs/sq ft.
Campbell River Solar Data
Here are the average daily peak sun hours by month for Campbell River (50.0 degrees north):
| Month | Peak Sun Hours/Day | Growing Notes |
|---|---|---|
| January | 1.8 | Maintenance mode -- harvest overwintered greens |
| February | 3.0 | Early starts under supplemental light |
| March | 4.8 | Main seeding begins |
| April | 6.5 | Full production ramp-up |
| May | 7.9 | Fruiting crops transplanted |
| June | 8.8 | Peak production -- manage overheating |
| July | 9.5 | Peak -- shade cloth may be needed |
| August | 8.6 | Late summer crops, fall seeding starts |
| September | 6.7 | Fall harvest, overwintering crops seeded |
| October | 4.3 | Season winds down outdoors, greenhouse extends |
| November | 2.1 | Harvest only -- little growth |
| December | 1.5 | Lowest light -- cold-hardy crops hold |
Total: approximately 1,800 peak sun hours per year. The shoulder seasons (March-April and September-October) are where a greenhouse delivers the most value -- extending productive growing into months when outdoor gardens are dormant or just starting.
Growing Season Extension
Here is what a passive solar greenhouse actually delivers in Campbell River:
| Crop | Outdoor Season | Greenhouse Season | Extension |
|---|---|---|---|
| Lettuce / spinach | Apr-Oct | Year-round | +5 months |
| Kale / chard | Mar-Nov | Year-round | +4 months |
| Tomatoes | Jun-Sep | Apr-Nov | +6 weeks each end |
| Cucumbers | Jun-Sep | May-Oct | +4 weeks each end |
| Peppers | Jun-Sep | May-Oct | +4 weeks each end |
| Basil | Jun-Sep | Apr-Oct | +8 weeks |
| Carrots | Apr-Oct | Feb-Dec | +2 months |
| Hardy herbs | Apr-Oct | Year-round | +5 months |
Framing and Materials
All framing is local Douglas fir from Vancouver Island mills. Pressure-treated 2x6 sill plates at grade level. Hot-dipped galvanized or stainless steel fasteners -- the coastal environment eats standard zinc-plated hardware within 5 years.
9x12 Framing Summary
- South wall studs: 2x6 at 24" OC, 9 ft long -- 6 required
- North wall studs: 2x4 at 24" OC, 7 ft long -- 6 required
- Rafters: 2x6 at 24" OC -- 6 required
- Door rough opening: 36" x 80" with double 2x6 header
- Add 20% for blocking and hardware
14x20 Framing Summary
- South wall studs: 2x6 at 24" OC, 10 ft long -- 10 required
- North wall studs: 2x4 at 24" OC, 7.5 ft long -- 10 required
- Rafters: 2x8 at 24" OC -- 10 required
- Ridge beam: 2x10 or LVL, 20 ft
- Door rough opening: 36" x 80" with double 2x6 header
- Add 20% for blocking and hardware
Site Selection Checklist
Before committing to a greenhouse location, confirm these:
- Orientation: Long axis within 20 degrees of true south. Slightly southeast is ideal -- captures morning sun, avoids afternoon overheating.
- Solar access: No obstacles south of the site taller than 1/3 of their distance away. A 6-metre tree needs to be at least 18 metres south of the greenhouse.
- Wind: Sheltered from prevailing north/northwest winds. A treeline or building as windbreak on the north side helps significantly.
- Drainage: Not in a low spot that collects water. Slight slope is fine -- level is fine -- a depression is not.
- Access to water: Within reach of a hose bib or rain barrel for irrigation.
- Setbacks: 1.5 to 3 metres from property lines depending on municipality.
- Aspect ratio: Length-to-width ratio of 2:1 to 3:1 for optimal solar gain.
Climate Battery Integration (14x20 Only)
The 14x20 production unit has enough floor area to incorporate a climate battery -- a network of buried pipes that circulates hot greenhouse air through the soil beneath the floor. In summer, it cools the greenhouse. In winter, the stored ground heat warms the space. This can increase growing degree days by roughly one-third.
The 9x12 backyard greenhouse does not have enough floor area for an effective climate battery. The pipe network needs a minimum soil volume to function, and the smaller footprint cannot provide it. For the 9x12, thermal mass (water barrels) and insulated blinds are your heat retention tools.
Read the full technical breakdown: Climate Batteries Explained: Heating a Greenhouse with Soil
What We Build (Shell Scope)
Swell Farms builds the greenhouse shell: foundation (augered sono tubes), framing (Douglas fir), polycarbonate glazing (8mm standard), knee wall, doors, and vents. The structure goes up in days.
Interior fit-out -- thermal mass barrels, growing beds, irrigation, insulated blinds, climate battery -- is either a separate scope or owner-installed. This keeps the shell affordable and lets you phase the interior investment over time.
Use our free Passive Solar Greenhouse Planner to compare sizes, estimate solar gain by month, and see glazing options side by side.
Sources and References
- Rob Avis, P.Eng. -- Verge Permaculture (passive solar greenhouse design principles)
- Environment Canada -- Campbell River sun hour data
- BC Building Code -- accessory structure requirements and 108 sq ft exemption
- BC Building Code -- snow load tables for Vancouver Island coastal areas
- GreenhousePlastics.ca -- polycarbonate specifications (Vancouver Island supplier)
Want a greenhouse quote for your property?
Swell Farms builds passive solar greenhouses across Vancouver Island and the Gulf Islands. We handle site assessment, permits (where needed), and the full shell build. Contact us for a quote.
Get a Quote →