Civil, Environmental, and Architectural EngineeringThe University of Kansas

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“Radiant Barrier Technology – A Must in Green Architecture”

Mario A. Medina, Ph.D., P.E.

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Introduction

““Preventing the sun's radiation from entering through the roof can make a significant contribution to comfort and reduction in cooling bills/needs.””

From: Sustainable Building Sourcebook Chapter: Energy  

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Definition

A radiant barrier consists of a layer of metallic foil, with low emittance, that significantly reduces the transfer of heat energy radiated from “hotter” surfaces to “colder” surfaces (e.g., the deck of an attic to the attic floor). Among the benefits of installing radiant barriers are energy savings, $ savings, and comfort.

(Source: Florida Solar Energy Center)

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Radiant Barriers

Installation Configurations

Pre-laminated Roof Sheathing

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How are they installed?

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Radiant Barriers

How are they installed?

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How they work:– Radiant barriers reduce radiated heat transfer rate by

the combination of the low emittance/high reflectance properties of the foil.

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Radiant Barriers

Modes of Heat Transfer

(Source: Btubusters)

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Radiant Barriers

Heat transfer schematic

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2,31,321

42

41

21

TTq

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21

42

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21

TTq

Radiant Barrier

Radiant Barrier

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Radiant Barriers

In the present study, the performance of radiant barriers was assessed via:

– Experiments• Side by side monitoring of pre- and post-retrofit data.

– Modeling• Mathematical representation of thermal sciences that describe

the processes that take place.

• Implemented using computer programming (e.g., FORTRAN).

– Model/Experiment Validation

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Experiments: Test Houses

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Experiments: Sensors

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Experiments: Monitoring Equipment

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Experimental Results: Calibration (No RB Case)Ceiling Heat Flux Indoor Air

Temperature

< 3 % < 0.3 oF

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Experimental Results: Calibration (RB Case)

Ceiling Heat Flux Indoor Air Temperature

< 3 % < 0.3 oF

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Experimental Results: Effect of Radiant Barriers (~28% Daily Heat Flow Reduction)

37.5%

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Experimental Results: Installation ComparisonsHorizontal Configuration vs. Truss Configuration?

Slight Advantage for the Horizontal Configuration

~ 5 %

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Experimental Results: Shingle Temperatures Horizontal Configuration Truss Configuration

vs. No RB Case vs. No RB Case

No difference in shingle temperature

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Experimental Results: Effects of Daily Solar Radiation

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Experimental Results: Effects of Attic Ventilation

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Experimental Results: Effects of Attic Insulation Level

42%

34%

25%

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Modeling: Based on Energy Balance Approach at Each Enclosing Surface

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ModelingEnergy Balance (General)

Energy Balance (Heat Transport Processes)

Outdoor Energy Balance

Indoor Energy Balance

Q Q Q Qconducted to from convected to from radiated net latent condensation evaporation( / ) ( / ) ( ) ( / ) 0

Y Tsi Tr X Tso Tr

CR q ho T Tso

hro T Tso q

i jj i

N S

i n j i jj i

N S

i n j

i o i n i amb i n

i sky surr i n sol i

,,

,

, ,,

,

,

( , ) ,

/ , ,

( ) ( )

" ( )

( ) "

0 1 0 1

1

0

Z Tsi Tr Y Tso Tr

CR q hi Tsi T

hri Tsi Tsi q

i jj i

N S

i n j i jj i

N S

i n j

i i i n i i n attic air n

i k i nk i

s s

k n latent i

,,

,

, ,,

,

,

( , ) , ,

, ,,

,

, ,

( ) ( )

" ( )

( ) "

0 1 0 1

1

1 10

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Modeling: Solar Modeling

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Verification of Model/Experiments (No RB Case)

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Verification of Model/Experiments

Horizontal Configuration Truss Configuration

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Verification of Model/Experiments (Winter)

No Radiant Barrier Configuration Horizontal Configuration

15 % Reduction in Heat Leaving Across the Attic

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Verification of Model/Experiments

No Radiant Barrier Configuration Horizontal Configuration

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Computer Simulations: Yearly Performance

Horizontal Configuration Truss Configuration

34 %Jun - Aug

32 %Jun - Aug

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Computer Simulations: Yearly Performance

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Computer Simulations: Attic Ventilation Pattern (Soffit/Soffit)

Jun - Aug

33.1% 31.6%

Horizontal

Truss

No RB

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Computer Simulations: Attic Ventilation Pattern (Roof/Soffit)

Jun - Aug

31.4% 26.2%

Horizontal

Truss

No RB

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Computer Simulations: Attic Ventilation Pattern (Soffit/Ridge)

Jun - Aug

32.3% 28.2%

Horizontal

Truss

No RB

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Computer Simulations: Impact of Radiant Barrier on Cooling Demand as a Function of Insulation Degradation

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Computer Simulations: Climate Influence

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Computer Simulations: Climate Influence

Climate

SummerMonthly

Dry Bulb Air Temperature

oC(oF)

SummerMonthly RelativeHumidity

(%)

Summer Monthly Wind

Speedkm/h(mi/h)

Area Covered

km2

(mi2)

Percent Area

Covered(%)

Humid Subtropical29

(84)68

13.7(8.5)

1,939,636(750,430)

24.03

Humid Continental Warm Summer

25(77)

7014.1(8.8)

1,655,112(640,350)

20.50

Desert28

(83)47

13.0(8.1)

1,223,467(473,350)

15.16

Humid ContinentalCool Summer

21(70)

6714.0(8.7)

905,291(350,250)

11.21

Steppe17

(62)43

12.7(7.9)

739,043(285,930)

9.15

Marine West Coast15

(59)80

13.3(8.3)

560,259(216,760)

6.94

Mediterranean17

(63)74

16.1(10.0)

508,837(196,865)

6.30

Western High Areas20

(68)50

13.7(8.5)

481,581(186,320)

5.97

Tropical Savanna28

(83)77

12.9(8.0)

59,484(23,014)

0.74

TOTAL8,072,711

(3,123,269)100.00

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Computer Simulations: Climate Influence

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Computer Simulations: Climate Influence

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Computer Simulations: Climate Influence

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Computer Simulations: Climate Influence

0

50

100

SIPR PHPR

0

50

100

SIPR PHPR 0

50

100

SIPR PHPR

0

50

100

SIPR PHPR

0

50

100

SIPR PHPR

0

50

100

SIPR PHPR 0

50

100

SIPR PHPR

0

50

100

SIPR PHPR

0

50

100

SIPR PHPR

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Computer Simulations: Climate Influence

Climate Sample Station

Sample Summer

Integrated Percent

Reduction(SIPR)

(%)

Average

Peak-Hour Percent

Reduction (PHPR)

(%)

Humid SubtropicalSan Antonio, TXNew York- NY

Atlanta, GA

34.332.538.5

35.1 31

Humid ContinentalWarm Summer

Topeka, KSIndianapolis, IN

30.030.1

30.5 46

DesertLas Vegas, NV

Tucson, AZ19.223.0

21.1 23

Humid Continental CoolSummer

Minneapolis, MNDetroit, Michigan

25.724.3

25.0 54

SteppePocatello, IDHelena, MT

16.013.7

14.9 36

Marine West Coast Astoria, OR 9.6 9.6 ~100

Mediterranean San Francisco, CA 2.3 2.3 97

Western High Areas Boulder, CO 19.7 19.7 44

Tropical Savanna Miami, FL 36.8 36.8 42

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Parametric Analyses: Outdoor Air Temperature

0

5

10

15

20

25

30

35

40

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0 10 20 30 40 50 60 70 80 90

Average Hourly Ambient Temperature for Period (deg F)

Per

cen

tag

e R

edu

ctio

n i

n C

elin

g

Hea

t F

lux

for

Per

iod

(%)

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Parametric Analyses: Mean Hourly Relative Humidity

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Parametric Analyses: Mean Hourly Global (H) Radiation

0

5

10

15

20

25

30

35

40

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0 50 100 150 200

Mean Hourly Global Horizontal Solar Radiation for period(Btu/h-sf)

Per

cen

tag

e R

edu

ctio

n i

n C

eili

ng

Hea

t F

lux

for

Per

iod

(%)

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Parametric Analyses: Latitude

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50

Latitude of Location(deg N)

Per

cen

tag

e R

edu

ctio

n i

n C

eili

ng

Hea

t F

lux

for

Per

iod

(%)

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Parametric Analyses: Altitude

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Parametric Analyses: Roof Solar Absorptivity

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Parametric Analyses: Radiant Barrier Emissivity

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Parametric Analyses: Attic Airflow Rate

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Parametric Analyses: Roof Slope

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In Conclusion….

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THANK YOUTHANK YOU