Shading a greenhouse is not optional in summer. A house in full sun gains heat faster than almost any other structure of its size, and the crop has more light than it can use anyway, so removing some of it costs nothing horticulturally and removes a large fraction of the heat load.
The two traditional methods are a physical cloth over the outside, and a temporary white coating painted or sprayed onto the glazing. They achieve the same thing by different means and differ mostly in precision and reversibility.
The comparison
| Measure | Shade cloth | Whitewash or shading paint |
|---|---|---|
| Shade level | Specified, from 30 to 90 percent | Approximate, depends on dilution and application |
| Evenness | Uniform across the panel | Variable, streaks and thin patches are normal |
| Removal | Minutes, and it can be partial | Gradual, washes off with weather, or scrubbed off |
| Reapplication | Not needed within a season | Often needed once or twice through the season |
| Cost on a small house | Modest, and reusable for years | Very low per application |
| Cost on a large glasshouse | Substantial | The reason it is still used commercially |
| Effect on the glazing | None | Residue can persist, and it needs cleaning off polycarbonate carefully |
| Wind behaviour | It is a sail and must be removed before a storm | No wind exposure at all |
| Air gap and radiant heat | Sits clear of the glazing, so heated cloth is cooled by outside air | Bonded to the glazing, so absorbed heat is conducted inward |
| Suits polycarbonate | Yes | Less well, because removal from a textured or twin-wall surface is harder |
Why cloth usually wins at hobby scale
It is a specified quantity. A 50 percent cloth blocks about 50 percent, and a 70 percent cloth blocks about 70. That matters because different crops want very different shade: fruiting vegetables 30 to 50 percent, leafy greens and seedlings 50 to 60, and orchids and ferns 70 to 80. Whitewash gives you one approximate level for the whole house.
It comes off on demand. A dull week in the middle of summer is a week the crop wants full light back, and cloth can be pulled off for it. Whitewash is committed until weather removes it.
It can be partial. Cloth over the roof only, leaving the walls clear, is a common and effective arrangement that whitewash cannot replicate easily. So is cloth over a propagation bench only, at a heavier grade than the rest of the house.
It works on polycarbonate. Whitewash on a twin-wall or textured surface is harder to apply evenly and considerably harder to remove without abrasive scrubbing, which is exactly what a polycarbonate surface should not receive.
A BELLEBILL 50 Percent Shade Cloth, 10 x 12 ft ($15.90) covers a small house at the general-purpose grade, and SUNNY GUARD HDPE Shade Cloth, 10 x 20 ft ($30.99) covers a larger roof. For a propagation bench or a shade-crop section, Agfabric 70 Percent Sunblock Shade Cloth with Grommets, 10 x 12 ft ($29.99) at a heavier grade is the right tool, and 90 Percent Shade Cloth, 20 x 40 ft, with Grommets ($138.58) exists for the specific case of a heavily shaded structure rather than a growing house.
Where whitewash still makes sense
Two cases, and both are real.
Large glass structures. Cloth for a big glasshouse is expensive, awkward to fit over a complex roof and awkward to store. A sprayed coating covers any area cheaply and needs no fixings, which is why commercial glasshouses have used it for a century and many still do.
Exposed sites. Because a coating cannot catch wind, it removes the storm problem entirely. On a site where cloth would need taking off repeatedly through the summer, a coating that stays put has a genuine practical advantage.
Where whitewash is used, apply it thinly and build up rather than applying a heavy coat, because heavy application is what produces the uneven patches and the difficult removal. And use a product formulated for greenhouse shading rather than an improvised mixture, following the label for application and removal.
Fitting cloth so it works
- Fit it on the outside. Shade fitted inside has already let the radiation into the house and only stops it reaching the plants. Outside, the energy never enters at all.
- Leave an air gap. Cloth held a few inches clear of the glazing is cooled by outside air moving between the two. Cloth lying flat on the glazing conducts a share of its absorbed heat inward.
- Match the grade to the crop, not to how hot it feels. Run the shade cloth calculator with the crop, season, latitude and glazing.
- Tension it and secure the edges. A flapping cloth wears at every fixing and abrades the glazing beneath it.
- Take it off for winter, entirely. In the shortest weeks the light inside a greenhouse is frequently already below what the crop needs, and removing another half of it turns a marginal situation into a hopeless one.
- Use grommets and ties rather than staples. Grommeted cloth can be removed and refitted many times; cloth stapled to a frame is single-use.
The third option: interior screens
Commercial growers increasingly use retractable interior screens, drawn across on bright days and pulled back on dull ones, and drawn at night for insulation. That combination of daytime shading and night insulation is genuinely superior to both cloth and whitewash, and it is what a motorised thermal screen does.
At hobby scale, a hand-drawn version is achievable: a length of cloth or fleece on a wire, pulled across in the morning and back in the evening. It gives the flexibility of cloth with none of the wind exposure, at the cost of doing it daily, and the night version of the same idea is covered in insulating a greenhouse.
The catch remains that interior shading is thermally inferior to exterior shading, because the radiation has already entered the structure. For pure temperature control, outside wins; for flexibility and safety in wind, inside wins.
Shade cloth by grade
Whichever method you use, shade is stage one of four in summer temperature control, and it does more for peak temperature than the stages after it. The sequence is in cooling a greenhouse in summer.