Shade Cloth for Greenhouse Roof: Inside or Outside?

Outside, inside, or partial roof shade

For most greenhouses, shade cloth over greenhouse glazing controls heat better. It intercepts sun before the cover, frame, benches, and air inside absorb the load. Greenhouse Management makes that same placement distinction: outside shade is strongest for reducing heat gain, while inside shade works only as part of a vented house.

Inside roof shade is still useful when exterior cloth would be hard to reach, unsafe in wind, or likely to rub the cover. UF/IFAS describes greenhouse tomato shade as either 30-40% cloth over the house or an inside cable setup that follows the ceiling contour. The inside version is easier to move during cloudy periods, but it must not create a hot pocket above the plants.

Partial greenhouse roof shade often beats full-roof shade. If the worst heat comes from the south or west roof edge, cover that side first and leave the cooler side brighter. Purdue Extension warns that shade reduces plant growth light, and its greenhouse guidance points to 30-50% shade for many conditions rather than a dark permanent roof layer.

Glazing already reduces light before the cloth is added. Greenhouse Management reports that single-layer glass may lose about 20-30% light transmission and double-poly houses may lose about 35-40%. Add a heavy full-roof cloth on top of that, and tomatoes, peppers, and other fruiting crops can lose the light they need for flowering, fruit set, and ripening.

Attach roof cloth without damaging the greenhouse

The attachment method matters as much as the shade percentage. Clips and rope can work for a small removable summer panel, but they do not prove the cloth can survive every windy roof. Greenhouse Megastore lists clip spacing from 6 inches to 2 feet depending on the material; treat that as a product detail, not a roof-wide wind rating.

Finished panels with grommets and lacing cord give cleaner edge control than random folds. They also make it easier to remove the cloth before a storm or winter. Install on a clear, cool, dry, non-windy day so the panel can be tensioned without fighting wind while you work.

Frame-mounted hardware is better where the greenhouse structure is built for it. Poly-Tex describes wiggle wire and U-channel as a greenhouse frame fastening method for polyethylene and shade material. Use it only where the channel can mount to actual greenhouse frame members, not to brittle trim, moving vent hardware, or unsupported glazing edges.

Protect the cover from abrasion. BC Greenhouse Builders warns not to place shade cloth directly on polycarbonate panels because damage can occur over time. Keep fabric off plastic film, panel ribs, roof peaks, glazing edges, vent corners, and automatic opener arms. A raised ridge strip, lower dowel, PVC edge, guide wire, or frame support is better than cloth rubbing on the roof in every gust.

Interior panels need clearance too. Penn State Extension warns that shade cloth should not touch plants because hot cloth and wind movement can damage foliage. Keep inside panels above the crop canopy, away from plant tops, and far enough from vents that opener arms can move normally.

Keep airflow, crop light, and seasonal removal working

Roof shade cloth is not greenhouse cooling by itself. Purdue Extension explains that natural ventilation depends on sidewall, end-wall, and roof openings, and that roof vents paired with sidewall vents improve temperature reduction. UMN Extension calls inadequate top ventilation a common high-tunnel design problem. If hot air has no high exit, darker cloth can leave the house dim and still too hot.

Leave every moving air part free: roof vent, ridge vent, side vent, roll-up side, door, fan shutter, intake opening, gutter, vent arm, and automatic opener. Purdue also notes that screens or obstructions can sharply reduce natural ventilation. A cloth edge that looks harmless in the morning can become the reason a vent sticks shut during the hottest hour.

Use seasonal cues, not a national calendar. UMN reports that shade cloth can lower high-tunnel temperatures by about 6-9 F in its context and suggests hanging it when tunnel air repeatedly reaches about 80-85 F. Utah State Extension says temporary cloth plus ventilation can offset extra heat and notes removal in early fall. Those are examples; crop response and local weather decide when the cloth should go up or come down.

Watch plants after the cloth goes up. Fruiting crops need more light than lettuce or tender starts. Purdue lists daily light integral targets of 10-12 mol/m2 for normal greenhouse growth and 15-18 mol/m2 for vegetable crops such as tomatoes and peppers. Stretching, pale growth, weak flowering, poor fruit set, and delayed ripening are signs to reduce coverage, lift the panel, or remove it sooner.

Avoid the common roof-shade mistakes: fastening cloth to moving vent hardware, letting fabric sit directly on polycarbonate panels, blocking fan shutters, hanging a side strip over a roll-up side, choosing darker cloth to hide poor ventilation, and leaving a full-roof panel over light-hungry crops through cool cloudy periods. When the greenhouse remains much hotter than outside after vents and doors are open, use the greenhouse cooling guide before buying more cloth.

• Do not block roof vents, ridge vents, fan shutters, doors, roll-up sides, gutters, opener arms, or crop walkways.
• Remove, roll back, or reduce shade when recurring heat passes, plants stretch, flowers slow, fruit set drops, or cool cloudy weather returns.
• Use the greenhouse shade-cloth buying guide for broad percentage, color, and product choices; use the percentage guide for crop-by-crop shade density.