Introduction
This research paper explains the effects of reflective roof coatings in reducing the heat load on rooftop airconditioning units.
A black EPDM roof was selected for this study. EPDM is an inexpensive black roofing material that resembles tire tubes. The typical EPDM roof lasts 15 years.
Dismantling and disposal of EPDM is expensive so an elastomeric roof coating system was specified. Elastomeric roof coatings are an excellent, environmentally friendly alternative to roof replacement. It solves roof leaks, extends the life of the roof and reflects the heat from the sun.
This EPDM roof is in good condition but with some ponding water.
Objective
- Determine if a reduction in roof surface temperature reduces intake air temperature going through the roof-top units.
- Determine if a cooler roof reduces plenum air temperatures above the ceiling.
- Determine other factors that degrade the efficiency of the A/C system that translates to higher cooling costs.
Roof Restoration Process
Crews spent an inordinate amount of time on washing and preparing the EPDM roof. The entire roof surface was power washed, broomed and allowed to dry.
Seams were prepared by applying a acrylic bonding primer reinforced with polyester roofing fabric and allowed to dry.
An acrylic primer was applied to the entire roof surface to provide positive adhesion for the elastomeric roof coating.
The elastomeric roof coating was applied with a hydraulic airless sprayer stationed on the ground. This scenario provided the perfect opportunity for us to conduct thermal efficiency studies on both the coated and uncoated sections of the roof.
Temperature Studies – Rooftop
Rooftop Data before Coating Application
| Time | Ambient (Ground) | Ambient (Roof) | Roof Surface | Rooftop A/C |
|---|---|---|---|---|
| 11:00AM | 89.2°F | 99.7°F | 149.0°F | 94.6°F |
| 12:00NN | 94.3°F | 102.9°F | 170.0°F | 97.9°F |
| 1:00PM | 94.8°F | 106.2°F | 172.0°F | 103.1°F |
| 2:00PM | 97.5°F | 106.5°F | 176.5°F | 104.9°F |
| 3:00PM | 95.9°F | 113.5°F | 165.0°F | 103.9°F |
| 4:00PM | 96.6°F | 110.3°F | 168.0°F | 101.7°F |
| 5:00PM | 95.5°F | 103.5°F | 151.0°F | 99.0°F |
*Measurements taken July 26, 2005
These initial readings were taken during summer in Southern Georgia. The heat index was 105°F and at 2:00PM roof surface temperature reached a scorching 181°F on certain sections of the black EPDM roof. For this study, we use the lowest reading from 6 test points all over the roof.
On average, a black, non-reflective roof raises the ambient air temperature on the roof about 10°F (10%). I was getting lower readings from thermocouples placed near the A/C units. Airflow had a cooling effect on the thermocouples (even with cones), thus lowering the measured temperature. Direct sunlight also skewed the readings so we had to keep the thermocouples in the shade for the duration of the test.
Rooftop Data after Coating Application
| Time | Ground Ambient |
Roof Ambient |
Uncoated Surface |
Coated Surface |
A/C Area Uncoated |
A/C Area Coated |
|---|---|---|---|---|---|---|
| 11:00AM | 88.9°F | 96.8°F | 149.0°F | 94.6°F | 102.9°F | 97.9°F |
| 12:00NN | 92.5°F | 98.5°F | 159.0°F | 97.9°F | 103.2°F | 98.3°F |
| 1:00PM | 92.9°F | 99.3°F | 163.0°F | 103.1°F | 103.9°F | 98.6°F |
| 2:00PM | 93.2°F | 99.8°F | 168.2°F | 104.9°F | 104.2°F | 98.5°F |
| 3:00PM | 93.0°F | 99.9°F | 165.0°F | 103.9°F | 104.1°F | 98.5°F |
*Measurements taken August 4, 2005
These readings were taken a few days after the roof was coated with reflective roof coatings. There was a significant reduction in roof surface temperatures! The uncoated area measured 168.2°F; the coated area was 94.5°F. It is a heat reduction of 73.7°F or 98%. Even air around the coated and uncoated A/C units had a temperature differential of almost 5°F.
Rooftop A/C
Left: Infrared Thermography taken on July 26, 2005 at 2:00PM clearly shows higher temperatures near the roof due to superheated air on the roof surface.
Right: Rooftop units are running cooler and more efficiently since the surrounding air temperature has been lowered by reflective roof coatings.
The Plenum
The plenum is the space between a false ceiling and the actual ceiling that serves as a distribution area for heating or cooling systems. For unducted air returns, this is also called the return plenum and a significant amount of cooling energy is lost here.
So much attention has been placed on achieving optimum R-values and SEER ratings that almost no emphasis was given to efficiency lost during distribution. The heat load on the roof raises plenum temperatures significantly and this directly affects the cooling efficiency of the system since cool air is being reheated as it enter the plenum and then re-cooled.
Ceiling Grid
Heat transfer from the plenum to the room was clearly evident in this IR thermograph. The ceiling grids become “heat sinks” and represent 8.3% of the surface area of the ceiling if taken collectively.
Conclusion
The performance of A/C system can be severely degraded by heat gain coming from the roof. An A/C system works by removing heat from the room and dissipating it outdoors. Any heat gain around the supply ducts, return plenum, ceiling and roof-top units contribute to this degradation. In this case study, the heat gain on the A/C system probably reduced the cooling efficiency by as much as 40%.
The owners of this building not only saved re-roofing dollars by extending the service life of this roof, they also will save money in the long run by enhancing the performance of their A/C system.
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