Backyard greenhouse — sizing and orientation

beds · advanced · ~8 min read

A backyard greenhouse extends Utah's growing season by 4 months on each end and lets you start your own transplants instead of buying them. The right size and orientation make it pay back in 2–3 years; the wrong setup becomes an oversized cold frame that's too hot in summer and too cold in winter.

Size: 8×10 minimum, 10×12 is the sweet spot

8×10 is the smallest size that fits a person plus benches plus walking room. 10×12 is the sweet spot for serious propagation — fits 2 propagation benches, a potting bench, and overwintering tender perennials. Anything smaller becomes "stand outside and reach in," which limits use in the cold months when you most want to be inside.

Orientation

Long axis east-west to maximize south-facing wall surface. South wall captures the most winter sun. Pitch the roof at a steeper angle (25–35°) than for a humid climate — sheds snow load, sheds intense summer sun. North wall can be insulated solid (less light needed from that side). Plant deciduous trees to the south at 30+ feet — provide summer shade, drop leaves for winter sun.

Glazing: polycarbonate vs glass

Twin-wall polycarbonate (8mm) — diffuses light evenly (no scorched leaves), insulates better than single-pane glass, lasts 15–20 years. Glass — looks better, transmits more light, lasts 30+ years, hail liability, single-pane is a thermal disaster. For Utah, polycarbonate wins on practicality. Single-pane glass is a money pit on heating costs.

Ventilation is non-negotiable

A sealed greenhouse on a sunny May day hits 110°F by noon and cooks every transplant. You need: roof vent (passive thermostatic opener like the Bayliss), side vents at bench level, and an exhaust fan for hot summer days. Aim for 1 air change per minute on the hottest day — that's a 10×12 greenhouse needs ~1,200 CFM fan capacity.

Heat for shoulder seasons

A small electric heater (Ouellet 1500W) carries an 8×10 through Utah winters down to about 25°F outside, holding interior at 45°F. Below 25°F outside you need a propane heater rated for greenhouses. Heat mats under propagation trays let you grow at 50°F ambient and 75°F root zone — a huge energy saving over heating the air.

Foundation

Skip slab concrete — water pools, freezes, cracks. Build on a raised wood frame on concrete piers, or on a 4-inch gravel pad. Both drain freely. Floor: gravel or pavers spaced over sand. Plumbing: spigot inside is worth every dollar in February.