In a 2011 Las Vegas test, a 15.24-cm wet roof pond lowered indoor peak operative temperature by about 8.5°C versus the outdoor maximum

Air conditioning is the go-to method for dealing with hot summers in many parts of the American Southwest and Las Vegas is one of the hottest spots in America where other options can be tried. Daytime temperatures stay above the comfort level for months and the dry climate ensures that air conditioning becomes an expensive venture for homeowners. An experimental building in Las Vegas demonstrated in summer 2011 that water retained in an open roof pond could take care of much of the cooling requirement without air conditioning.

Roof ponds are an old concept: they have been investigated since the 1960s as a passive technique that makes use of the thermal mass characteristics of water, absorbing heat during the day and releasing it at night, a history later synthesized in a 2015 systematic review published in the journal Applied Energy. Separately, the 2011 Las Vegas experiment made a direct comparison between dry and wet roof ponds and indicated the level of shortfalls of each in terms of comfort.

A 15-centimeter wet pond cut the peak operative temperature by 8.5°C.


Characterization of Thermal Comfort in a Passively Cooled Building Located in a Hot-Arid Climate (Fernandez-Gonzalez, 2014), presented at Windsor Conference 2014, examined a test building of standard residential construction fitted with a 15.24 cm depth roof pond during the summer of 2011 in Las Vegas. The study compared two modes of operation, dry and wet, in the same roof pond building.

In dry mode, the roof pond lowered the indoor peak operative temperature by roughly 5.1°C compared with the outdoor maximum temperature. In the case of introducing water into the dry roof pond, the difference widened due to an additional 3.4°C lowering of the indoor peak temperature, thereby resulting in a difference of 8.5°C less than the outdoor maximum temperature. The cooling in the wet mode came from evaporation from the roof pond.

Nighttime temperatures showed the reverse pattern

Similarly, the performance also reversed during nighttime. For dry operation, the lowest indoor temperature was 1.8°C higher compared to the lowest outdoor temperature, indicating that the building still had some heat from its previous exposure during the day. On the other hand, for wet operation, the lowest indoor temperature was 2.2°C lower than the lowest outdoor temperature. Neither design provided continuous comfort throughout the day during the most extreme days of a Las Vegas summer. Combining the wet roof pond design with better insulation and ceiling fans might reduce discomfort to only a few hours during the hottest part of the afternoon.

Other roof pond studies land in a similar range

Roof ponds as passive heating and cooling systems: A systematic review, a 2015 systematic review in Applied Energy, provided a synthesis of 19 different roof pond cooling systems based on the results of prior research. Despite differences in climate and system designs, the cooling systems presented in the paper maintained indoor temperatures a few degrees below outdoor temperatures. In some tropical climates, indoor temperatures were lowered by only 2°C to 4°C, a smaller cooling effect than the roughly 8.5°C achieved by the wet roof pond in the hot-arid Las Vegas study.

This paper also indicated the factors affecting the efficiency of roof pond systems. These factors included water depth, roof deck type, insulation thickness and weather conditions. Wet gunny-bag ponds, shaded ponds, ventilated ponds and movable insulation ponds were cited as some of the most efficient among the 19 different roof pond designs.

How roof ponds fit into passive home cooling

Roof pond systems are only one of many passive-cooling options. The U.S. Department of Energy recommends fans, evaporative coolers, heat pumps, insulation, shading and ventilation to help keep homes comfortable while reducing energy use. None of those technologies will provide an alternative to air conditioning in extremely tough conditions, and the case in Las Vegas illustrates the limitations of passive cooling instead of being a successful example.