A traditional limewash reduced temperatures beneath insulated hive roofs by up to 10°C, cost only a few cents per hive, and consistently limited solar heating during the hottest hours of the day.
Measuring a centuries-old beekeeping practice with modern sensors.
Every summer, beekeepers look for ways to help their colonies cope with rising temperatures. Shade cloths, insulated roofs, ventilation systems and reflective paints are all common suggestions—but one of the oldest and least expensive solutions may simply be a bucket of lime and a paintbrush.
Across much of the Mediterranean, hive roofs have traditionally been whitewashed before the hottest months of the year. Walk through an apiary in Spain, southern France or Italy, and you will still find bright white roofs reflecting the summer sun.
The practice has been passed from one generation of beekeepers to the next for decades. But one question remained: how much difference does it actually make?
Rather than relying on tradition alone, we decided to measure it. Using BroodMinder-T sensors installed directly beneath several insulated hive roofs, we compared untreated metal roofs with reflective and limewashed roofs over a series of hot summer days. The results allowed us to quantify how much heat accumulated beneath each roof—and how much a simple limewash could reduce it.
The video shows the complete experiment, from preparing and applying the limewash to reviewing the sensor measurements.
Why Roof Temperature Matters
In our previous Summer Beekeeping study, we followed several colonies through an extreme heat wave during which a nearby weather station reached 47.4°C in full sun. One conclusion stood out: healthy colonies were remarkably capable of maintaining brood temperature close to 35–36°C, even during the hottest afternoons.
That naturally raised another question. If the bees were already working hard to remove excess heat, could we reduce the amount entering the hive in the first place?
The roof quickly became the obvious place to investigate. Throughout a sunny day, it receives the highest solar load of any part of the hive. Even when insulated, it continues absorbing sunlight and transferring heat downward. Therefore, reducing roof temperature should reduce the cooling workload imposed on the colony.
Read the first study: How do honey bee colonies cope when temperatures reach 47°C?
A Traditional Solution Based on Simple Physics
Limewashing is hardly a new idea. Long before reflective coatings became commercially available, beekeepers across southern Europe protected hive roofs with a simple mixture of hydrated lime and water.
It was the same principle that gave the traditional white villages of Spain, Greece and the Balearic Islands their distinctive appearance. Those bright white walls were not painted only for aesthetics: they reflected the intense summer sun and helped keep buildings cooler. Hive roofs benefit from exactly the same physics.
A bright white surface reflects a much larger proportion of incoming solar radiation than a dark metal roof. Less absorbed sunlight means a cooler roof and, potentially, less heat transferred into the hive.
The materials are inexpensive, natural and widely available. A 15 kg bag of hydrated lime cost approximately €12, enough to coat dozens of roofs. The cost per hive is therefore measured in only a few cents.
How We Measured the Difference
To quantify the effect, BroodMinder-T sensors were installed immediately beneath several insulated hive roofs. The comparison included an untreated matt metal roof, a reflective metal roof and two limewashed roofs.
Each roof contained approximately 4 cm of polystyrene insulation, so the comparison focused on the surface finish rather than insulation thickness. Measurements were then collected continuously over several sunny summer days.
White Roofs Stayed Much Cooler
The first chart compares temperatures measured directly beneath each roof. The untreated matt metal roof consistently became the hottest, reaching nearly 49°C during sunny afternoons, while the limewashed roofs generally peaked around 39–40°C.
The difference approached 10°C during the hottest part of the day. Just as importantly, the same pattern repeated from one day to the next. This was not an isolated measurement, but a consistent result throughout the experiment.
The Untreated Roof Became Hotter Than the Air
Adding the weather station measurement provides an even more interesting perspective. The untreated roof did not simply follow the surrounding temperature: it became significantly hotter.
Instead of merely receiving heat from the environment, the roof acted as a solar collector, absorbing energy and radiating heat toward the colony below. This happened despite the presence of 4 cm of insulation.
Insulation slows heat transfer, but it cannot prevent the roof surface from becoming extremely hot. Reflecting sunlight before it is absorbed is therefore a highly effective first line of defence.
The Measured Gain Reached 8–10°C
Looking only at the temperature difference makes the benefit immediately obvious. During every sunny afternoon, the limewashed roofs remained between 8 and 10°C cooler than the untreated roof.
Rather than appearing as a brief temperature spike, the reduction lasted for several hours each day—the period when colonies were exposed to the greatest solar heat load.
It is important to interpret this measurement correctly. A roof that is 10°C cooler does not mean the brood nest itself becomes 10°C cooler. Healthy colonies regulate brood temperature remarkably well. Instead, a cooler roof means less heat enters the hive, reducing the cooling effort required from the bees.
Simple, Natural and Inexpensive
One of the most impressive aspects of the experiment was not only the temperature reduction, but how little it cost. A few cents' worth of hydrated lime, a bucket of water and an ordinary brush were enough to coat an entire apiary.
Unlike many commercial coatings, limewash is mineral based, contains no synthetic resin and naturally weathers over time. That gradual weathering may even be an advantage: the reflective surface protects the hive during the hottest months, then slowly fades through autumn and winter before being renewed the following spring.
For many beekeepers, applying a fresh coat can simply become part of seasonal hive maintenance.
What We Learned
This experiment demonstrates that a centuries-old beekeeping practice still provides measurable benefits today.
Using BroodMinder sensors, we were able to quantify what generations of Mediterranean beekeepers had already observed in practice. The untreated roof became considerably hotter than the surrounding air, effectively turning it into an additional source of heat for the colony. By contrast, the limewashed roofs remained dramatically cooler throughout the hottest part of the day.
That difference does not mean the brood nest itself became 10°C cooler. Healthy colonies are remarkably good at regulating brood temperature. Instead, it means less heat entered the hive in the first place, reducing the cooling effort required from the bees.
Sometimes modern sensors do not replace traditional knowledge—they simply explain it.
Explore the Limewash Experiment Data
The video shows the experiment, while the BroodMinder dashboard contains the complete measurements. Use the interactive chart below to compare the different roof configurations over the entire monitoring period.
Open the interactive dashboard in a new tab
Key Takeaways
- Traditional limewashing reduced temperatures beneath insulated hive roofs by up to 10°C.
- Untreated metal roofs became hotter than the surrounding measured temperature, acting as a source of heat above the colony.
- The improvement remained consistent over several sunny days.
- The effect was measured despite approximately 4 cm of roof insulation.
- Hydrated lime cost only a few cents per hive.
- Modern sensors helped quantify the thermal benefit of a long-established beekeeping practice.
Measure the Temperature Inside Your Own Hives
The BroodMinder-T2 records internal hive temperature continuously, helping you compare roof treatments, follow brood development and evaluate management changes using real data.
BroodMinder Summer Beekeeping Series
This article is part of the BroodMinder Summer Beekeeping Series, where we use real-world sensor data to understand how honey bee colonies respond to hot summer conditions and evaluate practical, evidence-based ways to reduce heat stress.
- When Temperatures Reach 47°C: How Do Honey Bee Colonies Cope?
- Limewashing Beehive Roofs: How Much Cooler Do They Really Stay? (this article)
- Which Hive Material Stays Coolest in the Sun? (coming soon)
Every Hive Counts.
The BroodMinder Team
