In the United States, most in-ground pools are uninsulated. In areas like Florida, this makes sense, since the ground temperature stays between the mid-60s and the low 80s—meaning that the pool temperature and surrounding soil temperature may be the same. However, the same pool in New Hampshire might lose 75,000 to 100,000 Btu per day to the earth. While this difference may seem drastic, the conductive heat loss in northern climates only contributes to about 5% of the pool’s heat loss.
In the grand scheme of pool conservation, insulating the walls of an in-ground pool picks the high-hanging fruit. The contribution of these losses is such a small portion of the total heat loss from the pool that any investment in insulation for the pool walls may be better spent somewhere else. Conductive losses to the earth are typically only a significant issue if the pool is built in an area with a high water table. A dynamic water table can more readily pull heat away from the pool’s perimeter than soil can. In this circumstance, insulation might be justified.
With an above-ground pool, there may be some advantage to adding rigid insulation to the pool walls, especially where there are large diurnal swings in air temperature during the swimming season or if the pool is located in a windy area. If the pool has a metal frame, rigid insulation may be able to be installed, although it will need to be covered to protect it from UV degradation. Above-ground pools with ribs may be more difficult to insulate.
Wind effects. Wind across a pool’s surface also steals some of the pool’s heat. For pools that are sheltered by buildings, vegetation, or fences, this heat loss is minimal. But in windy areas without protection, convection can contribute up to 20% to the pool’s total heat loss.
For pools located in unsheltered areas, windbreaks should be constructed. The windbreak might be a solid fence or shrubbery that protects the pool from the prevailing winds. Just keep it mind that you do not want your windbreak to shade the pool, since that could impact the passive solar heat delivered. The heat-robbing effects of wind can also be reduced by using a pool cover.
Even with a pool cover, above-ground pools in windy locations can experience significant heat loss through the pool walls. As a cooler wind blows against and past the pool walls, convection removes heat from the pool. An above-ground circular pool with an 18-foot diameter will lose roughly 120,000 Btu per day through its walls when there is a 15°F difference between the water temperature and the air temperature. This value climbs to 240,000 Btu per day with a 7 mph wind.
Surface evaporation & radiation. If a pool is left uncovered, the heat loss from the pool’s surface dwarfs the energy loss through the pool walls regardless of climate. Without a pool cover, the pool in New Hampshire could lose as much as 2,000,000 Btu per day through evaporation and radiation. That’s the energy equivalent of 25 gallons of propane used by an efficient pool heater.
Roughly 90% of the heat loss from a pool surface is due to evaporation. This is because each gallon of water that is evaporated from the pool removes more than 8,500 Btu of energy. One inch of evaporation from a pool’s surface can remove enough heat to cool the remaining water in the pool by 20°F.
This excessive heat loss from the pool surface is why covering the pool while vacant is so critical. If a pool blanket is used at night, the passive solar heat collected during the day is better retained through the night.
When used during times of inactivity, a pool cover by itself may be able to maintain adequate pool temperatures and avoid the need for additional heating in northern climates where pools are used seasonally. The U.S. Department of Energy estimates that an effective pool cover can reduce heat loss by 50% to 70%, and reduce the amount of water lost to evaporation by 30% to 50%. A bonus to reducing evaporation means that chemical loss is also reduced by 35% to 60%.