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Contract No. DE-AC36-08GO28308

Long-Haul Truck Sleeper 

Heating Load Reduction 

Package for Rest Period Idling

Preprint

Jason Lustbader, Bidzina Kekelia, 

Jeff Tomerlin, and Cory Kreutzer

National Renewable Energy Laboratory

Steve Adelman and Skip Yeakel

Volvo Group North America

Zhiming Luo and John Zehme

Aearo Technologies LLC

To be presented at the SAE 2016 World Congress & Exhibition

Detroit, Michigan

April 12–14, 2016

Conference Paper

NREL/CP-5400-65442

March 2016
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2016-01-0258 

Long-Haul Truck Sleeper Heating Load Reduction Package for Rest Period Idling

Abstract

Annual fuel use for sleeper cab truck rest period idling is estimated at 

667 million gallons in the United States, or 6.8% of long-haul truck 

fuel use. Truck idling during a rest period represents zero freight 

efficiency and is largely done to supply accessory power for climate 

conditioning of the cab. The National Renewable Energy 

Laboratory’s CoolCab project aims to reduce heating, ventilating, and 

air conditioning (HVAC) loads and resulting fuel use from rest period 

idling by working closely with industry to design efficient long-haul 

truck thermal management systems while maintaining occupant 

comfort. Enhancing the thermal performance of cab/sleepers will 

enable smaller, lighter, and more cost-effective idle reduction 

solutions. In addition, if the fuel savings provide a one- to three-year 

payback period, fleet owners will be economically motivated to 

incorporate them. For candidate idle reduction technologies to be 

implemented by original equipment manufacturers and fleets, their 

effectiveness must be quantified. To address this need, several

promising candidate technologies were evaluated through 

experimentation and modeling to determine their effectiveness in 

reducing rest period HVAC loads. Load reduction strategies were 

grouped into the focus areas of solar envelope, occupant 

environment, conductive pathways, and efficient equipment. 

Technologies in each of these focus areas were investigated in 

collaboration with industry partners. The most promising of these 

technologies were then combined with the goal of exceeding a 30% 

reduction in HVAC loads. These technologies included “ultra-white”

paint, advanced insulation, and advanced curtain design. Previous 

testing showed more than a 35.7% reduction in air conditioning 

loads. This paper describes the overall heat transfer coefficient testing 

of this advanced load reduction technology package that showed 

more than a 43% reduction in heating load. Adding an additional 

layer of advanced insulation with a reflective barrier to the thermal 

load reduction package resulted in a 53.3% reduction in the overall 

heat transfer coefficient. 

Introduction

In the United States, long-haul trucks (trucks that travel more than 

500 miles per day) use approximately 667 million gallons of fuel 

annually for rest period idling [1]. Sleeper cab climate control is one 

of the primary reasons for idling the main engine during these driver 

rest periods. This rest period idling is approximately 6.8% of the total 

long-haul truck fuel use and represents a zero freight efficiency 

operating condition for the truck. Fuel is one of the largest trucking 

costs per mile, averaging 35% of the total from 2008 to 2013 [2]; 

therefore, the increasing cost and cost volatility of fuel provide a 

significant financial incentive to reduce fuel use. Recent federal,

state, and city anti-idling regulations [3] are providing further 

incentives to reduce truck idling. One example is the idle reduction 

technology credit in the Heavy-Duty Greenhouse Gas Emissions 

Standards, effective starting in 2014 [4].

By reducing thermal loads and enabling more effective idle-off 

climate control systems, there is an opportunity to reduce fuel use and 

associated emissions. Enhancing the thermal performance of longhaul truck sleepers will enable smaller, lighter, and more costeffective idle reduction solutions. In addition, if fuel savings from 

new technologies provide a one- to three-year payback period [5], 

fleet owners will be economically motivated to incorporate the new 

technologies. Therefore, financial incentive provides a pathway to 

rapid adoption of effective thermal load and idle reduction solutions.

The U.S. Department of Energy’s National Renewable Energy 

Laboratory’s (NREL’s) CoolCab project is researching efficient 

thermal management systems to maintain cab occupant comfort

without the need for engine idling. The CoolCab project uses a 

system-level approach that addresses three aspects: reducing the 

thermal loads, efficient delivery of climate control for occupant 

thermal comfort, and maximizing equipment efficiency. By reducing 

thermal loads, the occupant’s climate control needs are reduced, and 

reduced-capacity equipment can then provide the conditioning. To 

advance the goals of the CoolCab project and the broader goals of 

increased national energy security and sustainability, the CoolCab 

team works closely with industry partners to develop and apply 

commercially viable solutions to reduce national fuel use and 

industry costs. 

To achieve an effective solution, NREL first conducted baseline 

testing of vehicles to quantify their thermal behavior. This 

information was then used to build and validate a CoolCalc model. 

CoolCalc is NREL’s rapid HVAC load estimation tool [6]. CoolCalc 

thermal models were then used in conjunction with experimental 

screening tests to identify promising thermal load reduction 

technologies. The most promising technologies were combined into a 

Complete-Cab Thermal Load Reduction Package. Previous tests 

showed that this reduced air conditioning (A/C) electrical loads by 

more than 35.7%, exceeding the 30% goal [7]. This paper describes 

testing of the impact of this Complete-Cab Thermal Load Reduction 

Package on heat loads using an overall heat transfer coefficient (UA)

test procedure. This testing showed a 43% reduction in heating loads. 

The Complete-Cab Thermal Load Reduction Package was then 

updated with improved insulation (an additional thin layer of 

advanced insulation with a reflective barrier was added to the 

previously tested thermal insulation package), resulting in a UA 

heating load reduction of 53.3%. 

Approach

Complete-Cab Thermal Load Reduction Package

NREL has identified four key climate control load technology focus 

areas: conductive pathways, the solar envelope, occupant 

environment, and efficient equipment. The first three of these 

technologies impact the cab’s heat transfer and resulting thermal 

loads and are the focus of this report. The efficient equipment focus 

area translates the thermal loads into mechanical or electrical loads. 

The four technical focus areas are described below. 
2

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1. The conductive pathways focus area addresses heat transfer 

through walls and other surfaces of the cab/sleeper. 

Technologies in this area include insulation, advanced materials, 

and glass. 

2. The solar envelope describes the interaction of surfaces with 

radiant energy from the sun and the surrounding environment. It 

is predominantly driven by radiant heat transfer and is most 

relevant during daytime operation; however, nighttime radiation 

to the sky is also included. This focus area includes the study of 

opaque and transparent surface properties of paints and glass, 

respectively. It also includes devices to modify these properties, 

such as window shades. 

3. The occupant environment includes the volume of conditioned 

air, occupant heat exchange with the surroundings, and human 

factors such as thermal sensation/comfort. Designing the thermal 

environment to make every occupant comfortable rather than to 

meet the traditional temperature-based metric has a significant 

impact on design. Technologies in this area include sleeper 

curtains and control of the microenvironment. 

4. The use of efficient equipment impacts conversion of thermal 

loads to mechanical, electrical, or chemical loads. A range of 

technology options and design considerations falls into this 

category. These options include battery electric A/C, fuel-fired 

heaters, and auxiliary power units. For the purposes of this 

study, battery electric idle-off systems were used; however, the 

idle-off thermal load reduction technologies applied to the 

cab/sleeper are largely independent of the equipment used. 

Prior work used testing and modeling to down-select technologies 

from each of these focus areas to develop a Complete-Cab Thermal 

Load Reduction Package. The baseline experimental configuration 

for the test cab contained sections of insulation as part of the 

vehicle’s stock insulation configuration. The stock insulation was 

affixed to portions of select upholstery panels that composed the 

sleeper compartment (Figure 1A). For the advanced insulation 

package, prefabricated insulation panels were installed throughout the 

sleeper compartment to occupy the void space between the 

upholstery panels and exterior frame wherever possible (Figure 1B). 

The advanced insulation package consisted of Thinsulate automotive 

acoustic insulation provided by Aearo Technologies. The areas of the 

sleeper cab insulation included the rear sleeper wall, sleeper ceiling, 

sleeper side walls, and portions of the cab ceiling. The installed 

insulation package contained a combination of one- and two-inchthick blanket insulation with a nominal thermal conductivity of 

between 0.03 and 0.05 W/m-K. Prior testing of this load reduction 

package showed it to be effective for electrical load reduction for an 

electric-powered A/C unit. To improve on this load reduction 

package, an additional 0.25-inch layer of Thinsulate insulation with a 

reflective radiation barrier was added between the interior trim and 

cab structure to reduce thermal shorts between the wall structure and 

the interior (Figure 1C). 

Figure 1. A (top), stock insulation; B (center), advanced insulation; C 

(bottom), additional layer with reflective barrier

Experimental Test Setup

An overall heat transfer coefficient (UA) test program was conducted 

at NREL’s Vehicle Testing and Integration Facility, shown in Figure 

2A, during the months of May through August. The facility is located 

in Golden, Colorado, at an elevation of 5,997 feet at latitude 39.7 N 

and longitude 105.1 W. The experimental setup included two current 

model Volvo cab test “bucks.” Both bucks were the cab section from 

a representative truck in current production provided by Volvo 

Trucks North America. One buck was utilized as the control buck, 

and the other was modified as the test buck. 

For the experimental setup, the test and control bucks were oriented 

facing solar south and separated by a distance of 25 feet to maximize 

solar loading and minimize shadowing effects. To keep the firewalls 

from receiving direct solar loads, a firewall shade cloth was 

implemented on both the control and test bucks. In each vehicle, the 

sleeper curtain and four shades were available for use, depending on 

the test being conducted. The shades available were the front privacy, 

cab skylight, and two bunk window curtains.

The UA test procedure used to measure the heating performance of 

the Complete-Cab Thermal Load Reduction Package was conducted 

at night to eliminate solar loading effects. The sleeper air temperature 

of both bucks was controlled to 32°C using a forced air heater

(Figure 2B). This temperature was selected to provide a sufficient 

temperature difference of at least 10°C from the environment over the 

extended test season. Unless noted otherwise, the sleeper curtain and 

all four shades were used on the vehicles. All curtains and shades 

were employed to match the expected standard configuration during a 

rest period operation. 

A

B

C
3

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Figure 2. A (left), NREL's Vehicle Testing and Integration Facility; B (right), 

UA test heater in sleeper

During initial shakedown tests the diffuser vent on the heater was 

oriented horizontally, directing exhaust hot air upwards and to the 

sides. This setup resulted in strong air stratification in the sleeper 

(Figure 3A). After re-orienting the diffuser in a vertical plane (as 

shown in Figure 2B) air stratification in the sleeper was essentially 

eliminated (Figure 3B). This heater design and diffuser vent exit 

orientation provided sleeper air temperature uniformity within ±2°C.

Figure 3. Test cab sleeper air temperatures normalized by temperature setpoint 

with horizontal (A) and vertical (B) orientation of heater diffuser vent

The UA value of the sleeper was calculated by measuring the heater 

power and the temperature difference between the interior air 

temperature and ambient temperature as described in Equation 1:

UUUU = QQ

TTSSSSSSSSSSSSSS−TTAAAAAAAA AAAA

(1)

where UA is the overall heat transfer coefficient [W/K], Q is the 

heater power [W], TSleeper [°C] is the average sleeper cab air 

temperature, and TAmbient [°C] is the local air temperature taken inside 

a naturally aspirated radiation shield at the weather station collocated 

at the test site. 

If the total area of heat transfer in the sleeper is known, this test can 

be used to determine a truck’s thermal resistance, or R-value, as 

described in Equation 2: 

RR = AA

UU (2)

where R [m2·K/W] is the total sleeper cab resistance, and A is the 

total area of the sleeper boundaries [m2

]. Due to the uncertainty in 

defining the total area of heat transfer in the sleeper, results are 

reported in terms of UA. It is worth noting that UA is inversely 

proportional to the overall resistance.

To account for normal day-to-day variations in weather conditions, 

the control buck was used as a reference. All vehicles will have some 

thermal performance variation due to differences in climate control 

systems, manufacturing, leakage, and other factors. To account for 

this, the control cab was calibrated to the test cab while both bucks 

were in a baseline configuration. The control cab can then be used as 

an accurate reference for the behavior of the test cab in a baseline 

configuration.

To further minimize the impact of weather variation on the test 

results, environmental screening criteria were established for a valid 

heating test day. Net downwelling infrared radiation (IR) was 

measured using Kipp & Zonen CG4 pyrgeometer located at NREL's 

Solar Radiation Research Laboratory's weather station to characterize 

the cloud cover at night. Net downwelling IR is the difference 

between upward radiation from a surface to the sky and the 

downwelling, incoming radiation from the atmosphere. Cloud cover 

will increase the downwelling IR from the atmosphere to the surface 

and thus decrease the net loss of heat from the surface. A net 

downwelling IR of more than 85 W/m2 lost from the surface was 

determined to indicate a clear night sky condition. Hourly average 

wind speeds were filtered to below 3.58 m/s to limit the impact of 

wind variability. Wind speed below this limit was found to have little 

influence on UA.

To minimize the impact of transient effects, one-hour segments were 

selected that met stability criteria for ambient temperature and heater 

power. The maximum allowable ambient temperature change in the 

hour interval was 3°C, and the maximum allowable heater power 

change was 15%. The one-hour segments also had to be after 

midnight and before daybreak to avoid any influence of sunset or 

sunrise.

The average interior sleeper air temperature was calculated by 

averaging eight thermocouples, with six located in accordance with 

the American Trucking Association Technology Maintenance 

Council’s recommended practice RP422A [8], as shown in Figure 

4B. Similarly, the average cab air temperature was calculated by 

averaging six thermocouples with four located in accordance with 

RP422A, illustrated in Figure 4A. The addition of two thermocouples 

located in both the cab and sleeper air spaces improved the average 

air temperature measurement by more accurately capturing the air 

temperature distribution.
4

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Figure 4. (A) Cab and (B) Sleeper thermocouple locations. Dimensions: A = 

12”, B = 6”, C = 18”. Blue: TMC standard [8]; red: NREL added.

A National Instruments SCXI data acquisition system was used to 

record measurements at a sampling frequency of 1.0 Hz, which was 

averaged over 1-minute intervals. Each cab had over 40 calibrated 

type K thermocouples for a variety of surface and air temperature

measurements. An isothermal bath and a reference probe were used 

for thermocouple calibration, achieving a U95 uncertainty of ±0.32°C 

in accordance with American Society of Mechanical Engineers 

standards [9]. Air temperature sensors were equipped with a double 

concentric cylindrical radiation shield to prevent errors due to direct 

solar radiation. The heater electrical power consumption was 

measured using Yokogawa watt meters with the accuracy of ±0.2% 

reading. Weather data were collected from both NREL’s Solar 

Radiation Research Laboratory and NREL’s Vehicle Testing and 

Integration Facility [10] weather stations, which together feature 

more than 160 instruments dedicated to high-quality measurements of 

solar radiation and other meteorological parameters.

Results

Baseline Testing

For the Complete-Cab Thermal Load Reduction Package

experimental evaluation, the heater system was calibrated for the test 

and control bucks using the baseline configuration in both bucks. 

Two configurations were calibrated: a standard original equipment 

manufacturer insulation package with standard sleeper curtain and 

privacy shades, and a privacy-shade-only case with no sleeper 

curtain. The bucks were painted different colors, but because the 

testing was conducted at night and the paint emissivity was very 

similar, any effects due to this were negligible. The calibration data 

for the Complete-Cab Thermal Load Reduction Package UA baseline 

is shown in Figure 5. The figures contain only data that meet the 

weather and stability screening criteria described in the approach 

section. Both data sets show a strong linear correlation with a 

coefficient of determination (R2

) of 0.934 and 0.962 for the standard 

and no-curtain configurations, respectively. 

Figure 5. UA calibration results: A. Sleeper with standard sleeper curtain and 

shades, B. Sleeper with window shades only and no sleeper curtain

Thermal Load Reduction Package Testing

The test buck thermal load reduction package was tested for multiple 

configurations as summarized in Tables 1, 2 and 3. As previously 

discussed, one-hour segments that met the stability criteria were 

selected for multiple test days. These stable segment averages were 

then averaged to calculate the UA reduction and corresponding 

standard deviations (σ) for each configuration. Applying only the 

advanced curtains and shades to the standard insulation resulted in a 

20.6% reduction in UA. The advanced insulation alone was also 

effective at reducing the heating load, yielding a 20.7% reduction in 

UA. Applying the full thermal load reduction package with both 

advanced curtains and shades resulted in a 43% reduction in UA.

Table 1. Thermal load reduction with advanced insulation

To build on these results, the insulation was improved further by 

adding an additional insulation layer with a reflective barrier to the 

walls and ceiling as previously described. This is the “advanced plus 

thermal load reduction” configuration. This layer fit between the 

structural members and the inner panels. The “advanced plus 

configuration” results are shown in Table 2. This insulation 

improvement resulted in an increase in the UA reduction for the 

insulation-only case with a 33.6% reduction in UA. Adding this 

insulation improved the full thermal load reduction package, reducing 

UA by 53.3%. The third case in Table 2 compares the advanced 

insulation and privacy shades without a sleeper curtain to the baseline 

with a standard curtain. In some cases, drivers may want a more 

spacious feeling and not use a sleeper curtain during rest periods. 

Configuration Insulation Privacy shades 

on windshield

Sleeper 

curtains

Reduction 

in UA

Standard 

Deviation 

(σ)

Baseline with sleeper curtains closed Standard Standard Standard - -

Advanced curtains Standard Advanced Advanced 20.6% 0.9%

Advanced insulation Advanced Standard Standard 20.7% 0.4%

Complete cab solution Advanced Advanced Advanced 43.0% 1.6%
5

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This would provide more space to the occupant, but does reduce the 

overall improvement substantially. By opening the curtain, a larger 

volume of air will be conditioned and walls without improved 

insulation will be exposed to higher interior temperatures as 

compared to the baseline case with the standard curtains closed, 

resulting in higher thermal losses to the colder surroundings.

Table 2. Thermal load reduction with advanced plus insulation

The second calibration process was used to investigate the impact of 

the insulation and privacy shades on a vehicle not using a sleeper 

curtain, as shown in Figure 5B. The impact of the insulation 

technologies was then compared against the baseline with no sleeper 

curtain as shown in Table 3. These results also show strong 

improvements over the no-sleeper curtain baseline.

Table 3. Thermal load reduction with baseline sleeper curtains open

Summary/Conclusions

NREL’s CoolCab project aims to reduce HVAC loads and resulting 

idling fuel consumption while maintaining driver comfort when longhaul trucks are parked during a rest period. To achieve this, NREL 

researchers have collaborated with partners Volvo Group North 

America, Aearo Technologies, and PPG Industries to develop and 

test a thermal load reduction package that uses ultra-white paint, 

advanced insulation, and advanced curtains and window shades. 

Overall heat transfer coefficient (UA) testing of this package 

demonstrated a 43% reduction in heating loads, complementing the 

35.7% reduction in cooling loads shown previously and exceeding 

the 30% reduction goal. Adding an additional layer of advanced 

insulation with a reflective barrier improved this to a 53.3% reduction 

in heating loads. Future work will use NREL's rapid HVAC load 

analysis tool, CoolCalc, to model the baseline insulation and 

advanced insulation packages. Simulations over a typical 

meteorological year will be performed for 200 weather stations 

identified as most representative of long-haul truck vehicle miles 

traveled. This will be combined with air conditioning and vehicle 

models to quantify fuel use and payback period impacts of these 

thermal load reduction technologies.

References

1. Gaines, L., Vyas, A., and Anderson, J., “Estimation of Fuel Use 

by Idling Commercial Trucks.” Presented at 85th Annual 

Meeting of the Transportation Research Board, Washington, 

D.C., Jan. 22–26, 2006; Paper No. 06-2567.

2. Torrey, F., and Murray, D., “An Analysis of the Operational 

Costs of Trucking: A 2014 Update.” American Transportation 

Research Institute. Atlanta, GA. 2014.

3. “Idling Regulations Compendium.” American Transportation 

Research Institute, accessed September 16, 2013: http://atrionline.org/2013/02/20/idling-regulations-compendium/.

4. “Greenhouse Gas Emissions Standards and Fuel Efficiency 

Standards for Medium- and Heavy-Duty Engines and Vehicles, 

Final Rule.” Federal Register 76 (15 September, 2011): 57106-

57513.

5. Roeth, M., Kircher, D., Smith, J., and Swim, R. Barriers to the 

Increased Adoption of Fuel Efficiency Technologies in the North 

American On-Road Freight Sector. Report for the International 

Council for Clean Transportation. NACFE. July 2013.

6. Lustbader, J., Kreutzer, C., Adelman, S., Yeakel, S., et al. 

“Impact of Paint Color on Rest Period Climate Control Loads in 

Long-Haul Trucks.” Presented at SAE World Congress, April 8, 

2014. http://www.nrel.gov/docs/fy14osti/61084.pdf 

7. Lustbader, J., Kreutzer, C., Adelman, S., Yeakel, S., et al., 

"Sleeper Cab Climate Control Load Reduction for Long-Haul 

Truck Rest Period Idling," SAE Technical Paper 2015-01-0351, 

2015, doi:10.4271/2015-01-0351.

8. “Cab Insulation Testing Methodology.” RP422A-1-9. 

Technology and Maintenance Council’s Recommended 

Maintenance Practices Manual, 2010–2011 Edition. Arlington, 

VA: American Trucking Association, p. RP422A 1.

9. Dieck, R.H., Steele, W.G., and Osolsobe, G., Test Uncertainty. 

ASME PTC 19.1-2005. New York, NY: American Society of 

Mechanical Engineers. 2005.

10. “Vehicle Testing and Integration Facility.” National Renewable 

Energy Laboratory, http://www.nrel.gov/midc/vtif_rsr/ 

Contact Information

Jason A. Lustbader

Heavy Vehicle Thermal Management Team Lead

National Renewable Energy Laboratory

Jason.Lustbader@nrel.gov

303-275-4443

Acknowledgments

1. Special thanks to our additional industry partner PPG Industries.

2. Additional thanks to John Rugh and Lisa Fedorka (NREL).

This work was supported by the U.S. Department of Energy under 

Contract No. DE-AC36-08GO28308 with the National Renewable 

Energy Laboratory. Funding was provided by U.S. DOE Office of 

Energy Efficiency and Renewable Energy Vehicle Technologies 

Office.

The U.S. Government retains and the publisher, by accepting the 

article for publication, acknowledges that the U.S. Government 

retains a nonexclusive, paid-up, irrevocable, worldwide license to 

publish or reproduce the published form of this work, or allow others 

to do so, for U.S. Government purposes.

Definitions/Abbreviations

A area

A/C air conditioning

HVAC heating, ventilating, and air 

conditioning

IR infrared radiation

Configuration Insulation Privacy shades 

on windshield

Sleeper 

curtains

Reduction 

in UA

Standard 

Deviation 

(σ)

Baseline with sleeper curtains closed Standard Standard Standard - -

Advanced+ insulation Advanced+ Standard Standard 33.6% 0.7%

Complete cab solution+ Advanced+ Advanced Advanced 53.3% 0.7%

Adv+ insulation & adv shades Advanced+ Advanced - Open - 21.6% 0.5%

Configuration Insulation Privacy shades 

on windshield

Sleeper 

curtains

Reduction 

in UA

Standard 

Deviation 

(σ)

Baseline with sleeper curtains open Standard Standard - Open - - -

Adv insulation & adv shades Advanced Advanced - Open - 27.4%* 0.7%

Adv+ insulation & adv shades Advanced+ Advanced - Open - 33.9%* 0.4%

*Calibrated with open sleeper curtains in both test and control cabs
6

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NREL National Renewable Energy 

Laboratory

Q heat power

R thermal resistance

R2 coefficient of determination

T temperature

UA overall heat transfer 

coefficient