Shade a greenhouse in summer with limewash brushed thinly on the glazing, roll-up mats or netting over the roof, or a deciduous tree on the south side that drops its leaves in winter. Shading cuts the sun that must be vented away, so it works with ventilation rather than replacing it, and it must be removable or fade by autumn.
Greenhouse shading reduces the sunlight reaching the glazing or the crop during the hottest months, lowering the heat that has to be removed.
Shade That Stays On In Winter Costs Yield
Summer sun is the overheating problem; winter sun is the point of the greenhouse. A permanent shade that blocks both turns a heating asset into a gloomy room. The strategies below are chosen because they can be adjusted, removed or washed off with the seasons.
What You Need
- Limewash (slaked lime in water) and a broad brush — see Lime Wash for making it
- Mats, hurdles, reed screens or open-weave netting that can be rolled up or taken down
- A deciduous tree planted well south of the house, if the site allows
- Cord and poles to hold roll-up shades above the glazing, not touching it, so air can pass between
Method: Choose And Apply Shade
Step 1: Start with the south and west faces. These take the strongest sun in the hottest part of the afternoon.
Step 2: Apply limewash thinly. A thin coat brushed or sprayed on the outside of the glazing scatters part of the sunlight. Thin coats let more light through than thick ones. Rain wears it off, so it fades by itself toward autumn, and you can wash off any remaining limewash with water and a brush.
Step 3: Hang adjustable shades above the roof. Mats or netting held a hand’s width above the glazing shade the surface and leave an air gap that carries heat away, which works better than a covering laid directly on the film or glass.
Step 4: Use trees to shape the season. A deciduous tree to the south (north in the Southern Hemisphere) shades in summer and drops its leaves for winter sun. A tree close to the glazing also drops debris and may hold moisture on it, so keep it clear of the roof.
Step 5: Remove shade when the crop needs light. Remove or open the shade on dull days and at the end of summer. Watch the plants: leggy growth and pale leaves mean too little light.
Worked Example
Shade and vent area work together. If a shade halves the sun entering the house, the heat to be vented falls by half. In the heat balance in Ventilation & Overheating Prevention, 900 W per square meter of floor becomes 450 W, and the airflow needed (0.149 m³/s per m² of floor at a 5 K rise) falls to 0.149 / 2 = 0.075 m³/s per m², so the vent area needed in the same wind halves as well.
Common Mistakes
- Painting thick and leaving it — the house is dim all winter.
- Laying a shade directly on the glazing — no air gap, so the heat stays in.
- Planting the tree on the south side too close — roots and shade arrive together, and leaf litter blocks gutters.
Safety
Lime burns and rooftop falls
Limewash is caustic and burns eyes and skin; wear eye protection and gloves. Do not climb on glass or film to apply shade; use a pole or a ladder placed on firm ground, and never put weight on glazing.
What’s Next
- Learn next: Ventilation & Overheating Prevention — the sum behind why shade helps
- Combine with: Shade Tolerance — how much shade the crop itself can accept