Thermal Performance of a "Cool Roof" Attic Design José B. Dávila Christian Brothers University Abstract This research consisted of analysis of a large volume of temperature and heat flux data pertaining to the thermal performance of an improved attic design. The objective was to reduce the heating and cooling load of a conditioned zone located underneath the attic. Data obtained for the improved attic design were compared with those obtained for a standard-construction roof. The results show a significant reduction of both
Transcript
Thermal Performance of a "Cool Roof" Attic Design José B. DávilaChristian Brothers University
Abstract
This research consisted of analysis of a large volume of temperature and heat flux data pertaining to the thermal performance of an improved attic design. The objective was to reduce the heating and cooling load of a conditioned zone located underneath the attic. Data obtained for the improved attic design were compared with those obtained for a standard-construction roof. The results show a significant reduction of both cooling and heating load.
A Tale of Two AtticsTemperature and heat flux data were obtained, using thermocouples and heat flux transducers, at an outdoor multiple-lane roof facility at Oak Ridge National Laboratory, for two very different attic structures: the “cool roof” and the standard-construction (reference).
The conditioned zone below the attic was maintained at a constant temperature.
Start date: March 14, 2008 (the start of week 1)End date: March 19, 2009 (the end of week 53)
“Cool Roof” Attican improved attic designed by the Building Envelopes Group of the
Building Technologies and Research Center at ORNL
• The uppermost surface of the roof is made of metal sheets with a reflective surface.
• A roof board is installed between the above-mentioned metal sheets and the main attic plenum thus creating a circulation channel, vented at the top and bottom of the roof incline.
• A metal sheet is installed below the above-mentioned board to serve as a radiation shield.
• Twelve-inch thick layer of fiberglass insulation is laid on the floor board.
• The upper four inches of the insulation layer are laced with encapsulated phase-change material (PCM).
“Cool Roof” SchematicJ. Kosny et al: “Theoretical and Experimental Thermal Performance Analysis of Building Shell Components Containing Blown Fiberglass Insulation Enhanced with Phase-Change Material (PCM),” ASHRAE Buildings IX, 2009.
Standard-construction Attic
• Standard shingle on roof board• No vented circulation space• No radiation shield• No insulation• No PCM
Measurements:• Outdoor, plenum, base board, roof board, attic plenum and insulation temperatures.
• Base board and roof board heat flux.
Interval: 15 minutesHeat flux integration over one-week periods.
Quantification of cooling load, according to ambient temperature and heat flux data:
If ambient temperature is higher than 60 ºF, then heat flux is classified as “cooling load.”
• If flux is into the zone, then it is positive. That is, the zone gains heat (because of the hot weather), and therefore it requires cooling.
• If flux is out of the zone, then it is negative.That is, the zone loses heat (despite the hot weather, due to thermal mass effects and/or solar radiation), and therefore some of the cooling requirement is relieved.
Quantification of heating load, according to ambient temperature and heat flux data:
If ambient temperature is lower than 60 ºF, then heat flux is classified as “heating load.”
• If flux is out of the zone, then it is positive. That is, the zone loses heat (because of the cold weather), and therefore it requires heating.
• If flux is into the zone, then it is negative.That is, the zone gains heat (despite the cold weather, due to thermal mass effects and/or solar radiation), and therefore some of the heating requirement is relieved.
Positive cooling load [Btu/ft2] vs week number
0 10 20 30 40 50 60-1000
100200300400500600700800900
100011001200
Cool RoofReference Roof
Negative cooling load [Btu/ft2] vs week number
0 10 20 30 40 50 60-1000
100200300400500600700800900
100011001200
Cool RoofReference Roof
Positive heating load [Btu/ft2] vs week number
Negative heating load [Btu/ft2] vs week number
0 10 20 30 40 50 60-1000
100200300400500600700800900
100011001200
Cool RoofReference Roof
0 10 20 30 40 50 60-1000
100200300400500600700800900
100011001200
Cool RoofReference Roof
Summary
Energy [Btu] defective year (46 weeks)
Cool Roof Reference Roof
Positive Cooling Load [Btu/ft2]
238 3627
Negative Cooling Load [Btu/ft2]
385 459
Net Cooling Load [Btu/ft2]
-146 3167
Positive Heating Load [Btu/ft2]
3030 18543
Negative Heating Load [Btu/ft2]
2 12
Net Heating Load [Btu/ft2]
3027 18531
Observations and concluding Remarks
The Cool Roof has reduced the cooling load, not merely by reducing the heat gain, but by reversing the net heat transmission through the roof. Compared to the net cooling load of the Reference Roof, the cooling load savings are 3313 Btu/ft2. The reversal does not occur in the heating load. But the heating load savings are much larger: 15504 Btu/ ft2, representing a reduction of 83.7 percent.
The load reduction advantage of the Cool Roof over the Reference Roof was significant for both heating and cooling. The results suggest that Cool Roof can effectively eliminate the roof as a cooling load source, and substantially reduce the heating load.
