Reference Publication: Parker, D., Sherwin, J., "Monitored Summer Peak Attic Air Temperatures in Florida Residences," Presented at The 1998 ASHRAE Annual Meeting, Toronto, Canada, June 20-24, 1998.
Disclaimer: The views and opinions expressed in this article are solely those of the authors and are not intended to represent the views and opinions of the Florida Solar Energy Center.
Monitored Summer Peak Attic Air Temperatures in Florida Residences
Parker and John Sherwin
Florida Solar Energy Center (FSEC)
Florida Solar Energy Center (FSEC) has analyzed measured summer attic air temperature data taken for some 21 houses (three with two different roof configurations) over the last several years. The analysis is in support of the calculation within ASHRAE Special Project 152P which will be used to estimate duct system conductance gains which are exposed to the attic space. Knowledge of prevailing attic thermal conditions are critical to the duct heat transfer calculations for estimation of impacts on residential cooling system sizing.
The field data was from a variety of residential monitoring projects which were classified according to intrinsic differences in roofing configurations and characteristics. The sites were occupied homes spread around the state of Florida. There were a variety of different roofing construction types, roof colors and ventilation configurations. Data at each site were obtained from June 1st - September 30th according to the ASHRAE definition of summer. The attic air temperature and ambient air temperature were used for the data analysis. The attic air temperature was measured with a shielded type-T thermocouple at mid- attic height, halfway between the decking and insulation surface.
ambient air temperature was obtained at each site by thermocouple
located inside a shielded exterior enclosure at a 3-4m (10-12
foot) height. The summer 15-minute data from each site were sorted
by the average ambient air temperature into the top 2.5% of the
observations of the highest temperature. Within this limited group
of observations, the average outside air temperature, attic air
temperature and the coincident difference was reported.
Characterization of attic thermal performance in hot climates is vital to estimation of ceiling heat transfer to the interior space. However, equally important in air conditioned buildings is how prevailing attic thermal conditions impact duct heat transfer. This has been detailed in simulation analysis (Parker et al., 1991, 1993 and Gu et al., 1996), laboratory testing (Petrie and Christian, 1996) and field monitoring (Jump et al., 1996; Hageman and Modera, 1996).
FSEC has performed numerous experiments in test buildings over the last decade on the potential of a variety of methods to reduce attic air temperatures in Florida residences. This includes radiant barrier systems (Fairey et al., 1988), white reflective roofs (Parker et al., 1995), enhanced attic ventilation and roof tiles (Beal and Chandra, 1995) and sealed attics with insulation in the roof system (Rudd, et al., 1996). Attic air temperatures vary considerably depending on roofing type, color and ventilation. A good example of the importance of roof reflectance is shown in Figure 1 which plots the measured attic air temperature in a monitored residential site in Florida over the summer months in which the roof was made white on July 6th (Parker and Barkaszi, 1997).
Figure 1. Measured 15-minute attic air temperatures in a residence before and after roof was whitened in the summer of 1992. After roof was whitened attic air temperature was often lower than ambient due to heat transfer to the AC duct system.
The potential importance of attic ventilation has been investigated by an early series of NBS studies (Reppert ed., 1979).
field data presented are from a variety of projects associated
with these parameters and are classified according to intrinsic
differences. Each building is classified by a three letter/number
code; the sites are spread around the state of Florida.(1) There
are a variety of different roofing construction types, colors
and ventilation configurations represented. Truss-mounted radiant
barriers were present in two attic constructions.
data at each site was obtained from June 1st - September 30th
according to the ASHRAE definition of summer. The attic air temperature
and ambient air temperature were used for the data analysis. The
attic air temperature was taken at mid- attic, halfway between
the decking and insulation surface. The attic air temperature
was taken with a shielded type-T thermocouple. The ambient air
temperature was obtained at each site by a thermocouple located
inside a shielded exterior enclosure at a 3-4m (10-12 ft) height
adjacent to the building (See Figure 2).
Figure 2. Installation of a shielded thermocouple for measurement of site ambient air temperature. Houses in the background are those of a Habitat of Humanity development of which ten houses (see Table 1) have been metered for two years. House has light gray, blue and dark gray shingles.
The 15-minute summer data from each site comprised 11,712 observations. These were sorted by the average ambient air temperature into the top 2.5% of the observations (~293) of the highest temperature corresponding to the ASHRAE design condition. Within this limited group of observations, the average outside air temperature, attic air temperature and the coincident difference are reported. Figure 3 shows an example of the measured attic air temperature at one of the sites (HO2) over an entire summer.
Figure 3. Measured attic air temperature at site HO2 over the entire summer of 1996. House has a dark gray asphalt shingle roof and is located in Homestead, Florida.
The tabular results from the above analysis are given below in Tables 1 -5 and in Figure 4 as classified by roof type, color and ventilation.
Figure 4. Coincident attic to ambient air temperature difference at ASHRAE summer design condition.
It is worth noting that light gray or "white" asphalt shingles have a measured solar absorptance of approximately 75% as opposed to true reflective roofing systems which have absorptances less than 30% (Parker et al., 1993B). Dark gray shingles have a solar absorptance of about 90%.
Shingle Roofs with Soffit and Ridge Vents
Lt. Gray shingle
Lt. Gray shingle
Lt. Gray shingle
Dark Gray shingle
Med. Blue shingle
Lt. Gray shingle
Dark Gray shingle
Med. Blue shingle
Dark Gray shingle
Lt. Brown shingle
Dk. Brown shingle, no ducts
Soffit & Ridge vent
Dk. Brown, Soffit vent only
|RC5||96.6o||107.2o||10.5o||Merritt Island||1993||Gray tile|
|DSP||94.9o||88.6o||-6.1o||Cocoa Beach||1993||White shingle ridge/soffit vents|
|RC5||94.4o||91.6o||-2.9o||Merritt Island||1995||White tile (2nd yr.)|
|RC3||91.7o||93.6o||1.9o||Nobleton||1994||White (2nd yr.)|
|RC9||93.1o||84.0o||-9.0o||Cocoa||1995||White gravel (2nd yr.)|
|PT2||91.7o||95.2o||3.4o||Palm Bay||1995||White shingle (2nd yr.)|
|RC4||93.4o||87.8o||-6.6o||Miami||1994||White gravel (2nd yr.)|
|LS1||95.1o||103.4o||8.3o||Merritt Island||1995||White shingle (2nd yr.)|
data set highlights prominent influences on attic thermal performance.
The first set of data are from ten 1,100 square foot (100m2)
homes of virtually identical construction in Homestead, Florida.
Each has both soffit and ridge venting. Only the shingle color
varies, an important influence as suggested by the temperature
data shown both in tabular form and illustrated in Figure 5.
Figure 5. Influence of asphalt shingle color on measured average attic air temperature at two adjacent Homestead, Florida sites (HO2 and HO5) during the summer of 1996.
Data from the second group of four homes suggests that soffit venting only can result in substantially higher temperatures than with soffit and ridge venting. This is likely a result of increased ventilation. Note that there are two data sets for location CR1. The first is in its initial condition with a black roof and very little ventilation; the second year is with an attic radiant barrier and soffit and ridge venting. The attic to ambient dT is reduced by 16oF (8.9oC). There are also two years with site RC5, one with gray tile and the other with white tile - both configurations showing better performance than shingle roofs.
The data from the homes with white roofs show some variance, but generally indicate much lower coincident attic air temperatures during summer peak conditions. Site RC3 has data from a period with a dark shingle roof and a white roof with dramatic differences. Often with a white roof the attic to ambient temperature difference is negative - seemingly counterintuitive. There are several reasons:
Figure 6. Influence of duct system heat gain on measured attic air temperature site WD1, August 6, 1997. Short term fluctuations of attic air temperature appear correlated with wind conditions.
Although the 21-house data set is not large enough to comprise a statistical sample, it does suggest some important influences on summer attic design temperatures in Florida. Light roof colors, deck-mounted radiant barriers, added attic ventilation and tile roof construction were all shown to reduce peak attic air temperatures. A simple characterization of the collected data would be as follows:
Comparison of Coincident Attic
to Ambient Design Temperature Difference
|Case||Coincident 2.5% Attic Design Temperature|
|Shingle roof, soffit vent only||Ambient
+ 35oF (19.4oC)
soffit and ridge venting
+ 22oF (12.2oC)
roof, Radiant Barrier
soffit vent only
+ 25oF (13.9oC)
Note that this summary does not represent a statistical sample and may only be representative for Florida conditions. Even so, it is likely that the general ranking of identified influences will be observed in measurements elsewhere.
A simple calculation illustrates the importance of controlling the peak attic air temperatures measured in this study. As example, consider a residence on a a peak summer day at 95oF served by a three ton cooling system with a sensible capacity of 27,000 Btu/hr and an EER of 9.0 Btu/W. The assumed residence has a 1,800 square foot ceiling with R-30 attic insulation. Supply ducts typically comprise a combined area of ~25% of the gross floor area (see Gu et al. 1997, Appendix G, and Jump and Modera, 1994), but are only insulated to R-4. With the peak attic temperatures for a shingle roof with poor ventilation estimated at 130oF, and 75 maintained inside, a UA dT calculation shows a ceiling heat gain of 3,300 Btu/hr. With R-4 ducts in the attic and a 57 air conditioner supply temperature, the heat gain to the duct system is 8,212 Btu/hr if the cooling system ran the full hour under design conditions-- more than twice the ceiling flux. However, the magnitude of both ceiling and duct heat gain is 43% of the air conditioner's design sensible cooling load. Thus, attic heat to ceiling and attic to duct heat gains are a major portion of the design cooling load for residences.
the example the attic related gains are also responsible for a
1.28 kW increase in peak air conditioning electric demand. As
a contrast, with a white roof system, the estimated attic air
temperature would be 93.5oF, with a total ceiling and
duct heat transfer rate of 5220 Btu-- a reduction of 6,300 Btu/hr
and a drop in electrical demand of 700 W if the system was at
design capacity with the dark roof.
work was supported by funding from the Florida Energy Office,
whose sponsorship is gratefully acknowledged. Special thanks also,
to Iain Walker, at Lawrence Berkeley National Laboratory for helpful
comments throughout the analytical process.
D. Beal and S. Chandra, 1995. "The Measured Performance of Tile Roof Systems and Attic Ventilation Strategies in Hot and Humid Climates," Thermal Performance of the Exterior Envelopes of Buildings VI, ASHRAE/DOE/BTECC, p. 753, December, 1995.
P. Fairey, M. Swami and D. Beal, 1988. Radiant Barrier Systems Technology: Task 3 Report, FSEC-CR-211-88, prepared for the U.S. Department of Energy, Florida Solar Energy Center, Cocoa, FL. L. Gu, J.E. Cummings, M.V. Swami, P.W. Fairey and S. Awwad, 1996. Comparison of Duct System Computer Models That Could Provide Input to the Thermal Distribution System Standard Method of Test (SPC-152P), FSEC-CR-929-96, ASHRAE Project 852-RP, Florida Solar Energy Center, Cocoa, FL.
R. Hageman and M.P. Modera, 1996. "Energy Savings and HVAC Capacity Implications of a Low-Emissivity Interior Surface for Roof Sheeting," Proceedings of the ACEEE 1996 Summer Study on Energy Efficiency in Buildings, Vol. 1, p. 117, American Council for an Energy Efficient Economy, Washington D.C.
D.A. Jump, I.S. Walker and M.P. Modera, 1996. "Measurements of Efficiency and Duct Retrofit Effectiveness in Residential Forced Air Distribution Systems," Proceedings of the ACEEE 1996 Summer Study on Energy Efficiency in Buildings, Vol. 1, p. 147, American Council for an Energy Efficient Economy, Washington D.C.
D.S. Parker, P.W. Fairey and L. Gu, 1991. "A Stratified Air Model for Simulation of Attic Thermal Performance," Insulation Materials: Testing and Applications, ASTM 1116, American Society of Testing and Materials, Philadelphia, PA.
D. Parker, P. Fairey and L. Gu, 1993. "Simulation of the Effects of Duct Leakage and Heat Transfer on Residential Space Cooling Energy Use," Energy and Buildings 20, p. 97-113, Elsevier Sequoia, Netherlands.
D.S. Parker, J.E.R. McIlvaine, S. Barkaszi and D.J. Beal, 1993B. Laboratory Testing of Reflectance Properties of Roofing Materials, FSEC-CR-670-93, Florida Solar Energy Center, Cocoa, FL.
D.S. Parker and S.F. Barkaszi, Jr., 1997. "Roof Solar Reflectance and Cooling Energy Use: Field Research Results from Florida," Energy and Buildings 14, Elsevier Sequoia, Netherlands.
M.H. Reppert ed., 1979. Summer Attic and Whole House Ventilation. NBS Special Publication 548, National Bureau of Standards, Washington, DC.
T.W. Petrie and J.E. Christian, "Details on Duct Tests ongoing in the Large Scale Climate Simulator," memorandum to Esher Kweller dated September 9, 1996, Oak Ridge National Laboratory, Oak Ridge, TN.
A.F. Rudd, J.W. Lstiburck and N.A. Moyer, 1996. "Measured of Attic temperatures in Vented and Sealed Attics in Las Vegas, Nevada," Proceedings of the 14th Annual Excellence in Building Conference, Energy Efficient Building Association, Minneapolis, MN.
1. Homestead is some 50 km south of Miami, Nobleton is in the West Central part of the state; Hollywood is just north of Miami, and Merritt Island, Palm Bay and Cocoa are on the Atlantic coastal zone in central Florida. Belle Glade is in the South Central part of the state
The 1998 ASHRAE Annual Meeting
June 20-24, 1998