Are NVPH(Naturally Ventilated Polyhouses) cooler than outside air?

Polyhouses are of 2 types - naturally ventilated which do not need a fan-pad but leave a gap open at the top for air to escape, or the fan-pad climate controlled ones, which do not have any gaps and air entry exit is via pads/fans.

Question is - Do you think a naturally ventilated polyhouse can be cooler than outside?

Although many people believe so - the answer is a clear NO.

These structures control their inside temperature by natural convection - as soon as they start getting hotter than outside air, the hot air rises and escapes from top vents. This would NEED the polyhouse to get hotter than air outside for the inside air to rise and disperse.

Polyhouses with Open ventilation gaps at top

We have measured this internal temperature at many such polyhouses and it is always approximately 2 degrees hotter than outside, in areas like Nashik, Palghar and some more areas in Maharashtra.

Many growers we interacted with wrongly believed that their internal temperatures are colder than outside , and a simple handheld 300 Rs temperature probe is adequate to test and dispel this myth, although do read our blog article on temperature measurement pitfalls! This misunderstanding spreads easily as temperature thermometers are only placed inside, and not outside to compare minute by minute external to internal temp.

The advantage though of this naturally ventilated structure is that it gives you a weather protected low solar radiation structure which does not depend on electrical power to keep its temperature in check.

A fan-pad polyhouse has potential to reduce its temperature below outside, but since it has no vents at top - if power failure occurs, due to lack of natural convection - its internal temperature can go as hot as 15 degrees above outside. This implies power backup becomes mandatory for a fan-pad polyhouse - another investment on top of an already costly asset.