Protected Cropping
The gaps do the damage. Five per cent of the roof left open carries a quarter of the heat the screen was supposed to save.
— %
Reduction in night-time heat loss with the screen deployed
Losses are computed per square metre of floor, so a house whose glazed area exceeds its footprint — most of them, once the gables and walls are counted — loses more than this shows. The model is steady-state conduction only: it ignores infiltration, which a screen also reduces, and it ignores radiative loss to a clear night sky, which is exactly what an aluminised screen is best at blocking, so the saving here is conservative on a cold clear night. Condensation on a closed screen and the humidity that follows are real operational limits on closure and are outside this calculation.
Greenhouse Thermal Screen Energy Saving Percentage — free, with the formula and a worked example, at Textile School.