Secondary Author(s)

Sherwin, John

Report Number




Buildings; Roofs


An experimental evaluation has been conducted on a night sky cooling system designed to substantially reduce space cooling needs in homes in North American climates. The system uses a sealed attic covered by a highly conductive metal roof (a roof integrated radiator) which is selectively linked by air flow to the main zone with the attic zone to provide cooling--largely during nighttime hours. Available house mass is used to store sensible cooling. Additional dehumidification is done during the evening hours as warranted by interior conditions.

An initial report describes a detailed simulation model of the relevant night cooling phenomenon, examining potential performance (Parker, 2005). A second report summarized an experimental evaluation of concept performance using two highly instrumented test sheds using short term data in the autumn of 2006 (Parker, 2007). These evaluated passive performance of the building thermal performance under static conditions (NightCool not operating), and also in a circumstance where NightCool is operating via natural convection alone between the interior of the test building and the sealed attic.

Here, similar data is presented on the long-term comparative with all of NightCool system fully operational, with circulating fans when attic conditions are favorable for nocturnal cooling and with conventional air conditioning at other times. Data is included for a full year of the cooling season in Central Florida, which stretches from April to November of 2007.

Average long-term performance was somewhat lower than the previous simulation analysis. The delivered cooling rate averaged about 1.5 - 3.0 Btu/hr/ft2 (5 - 10 W/m2) of roof surface on the average evening, implying that NightCool in a full scale 2,000 square foot home would cool at a rate of 4,000 - 8,000 Btu/hr depending on the season. Daily runtime fractions during which the NightCool fan operated varied from 12% (3 hours) in August - September to 36% to 8 hours in May. Over a typical 6 hour operating period, this would produce about 0.2 ton-hours of sensible cooling of 2 ton-hours in a full scale home. Average long-term monthly energy efficiency ratios (EERs) ranged from 16 - 32 Btu/Wh with a mean of 25 Btu/Wh over the cooling season. As expected, performance was best during the spring and fall months.

Date Published


Local Subjects

Buildings - Roofs


FSEC Energy Research Center® Collection



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