Thermal Performance of Radiant

Barrier in a roof assembly by using

CFD methodBy

Ar. Dr. Lim Chin Haw

&

Zahin Ashhar

Previous research conducted by SERI UKM

Field MeasurementUsing Test Cells(In collaboration between UKM & San Miguel Yamamura Woven Products)

Lab MeasurementUsing Heat Flow Meter[In collaboration with Greenbuildingindex SdnBhd (GBI)]

What is CFD?

1. CFD is the analysis of systems by mathematical prediction involving fluid

flow and heat transfer by means of computer simulation.

2. Several areas of CFD application:

• Environmental Sustainability Design consulting

• Aerodynamics and Wind engineering

• Optimization of Industrial Ventilation (HVAC)

• Indoor Air Pollution assessment

• Naval architecture and Marine engineering

• Offshore, Oil and gas engineering

• Electronics Cooling and Thermal management

• Aerospace and aeronautics engineering

• Automotive engineering

FloEFD by Mentor Graphics

1. FloEFD is a Concurrent CFD tool where it combines all the simulation steps

from 3D model design, to mesh generation, solving and results visualization.

2. FloEFD is a powerful simulation tool to:

• Improve product performance, functionality and reliability

• Reduce physical prototyping and production costs

• Minimize risk of making design mistakes

Background

– Literature Review

1. According to S. Lee et al. (2016), the most common method used in

studying the performance of radiant barrier is by laboratory measurement

and empirical experiment.

2. The use of CFD in assessing radiant barrier performance for entire roof

assembly is still very limited.

3. Out of all the studies conducted in studying performance of radiant

barrier, only 19% uses computer simulation model.

4. The usage of CFD in heat transfer analysis as the 3D simulation should

be explored as it would provide accurate predictions on the heat transfer

mechanisms and gives us visual representation on the temperature

gradient, heat flux penetration and air flow.

Objective

The objective of this research is to evaluate the thermal performance of radiant

barrier for an entire roof assembly of various reflective air space thicknesses

and roof pitches, by using Computational Fluid Dynamics (CFD) simulation.

This is done by evaluating few parameters such as:

1. The temperature of various roof surfaces

2. Reduction of heat flux through the reflective air space and the ceiling

3. The RSI value of the roof assembly

Research Methodology

1. The entire roof assembly was modelled in a 3D CAD software from the

roof tiles to the ceiling.

2. Material properties of each components were assigned to model an actual

roof assembly.

3. Certain boundary conditions were fixed to perfectly simulate an actual

external environment conditions.

4. Several key parameters were collected from the simulation and the RSI

values were derived.

5. The results obtained through simulation was validated against empirical

study from the test cells measurement conducted previously.

Research Methodology

Ceiling

Reflective air space is

the air space between

the roof tiles and the

aluminum foil

Attic is the air space

between aluminum

foil and the ceiling

T1T2

T3

QA

QB

• The average temperature of the

roof, T1.

• The average temperature of the

insulation material, T2.

• The average temperature of the

top of the ceiling, T3.

• The average heat flux across the

reflective air space, QA.

• The average heat flux across the

ceiling, QB.

Research Methodology

Equations used to calculate RSI values:

𝑅𝑆𝐼𝑎𝑖𝑟 𝑠𝑝𝑎𝑐𝑒 =𝑇1−𝑇2

𝑄𝐴, 𝑅𝑆𝐼𝑐𝑒𝑖𝑙𝑖𝑛𝑔 =

𝑇2−𝑇3

𝑄𝐵

𝑅𝑆𝐼𝑜𝑣𝑒𝑟𝑎𝑙𝑙 = 𝑅𝑆𝐼𝑎𝑖𝑟 𝑠𝑝𝑎𝑐𝑒 + 𝑅𝑆𝐼𝑎𝑡𝑡𝑖𝑐

Key parameters obtained from the CFD simulation:

1. The average temperature of the roof, T1.

2. The average temperature of the insulation material, T2.

3. The average temperature of the top of the ceiling, T3.

4. The average heat flux across the reflective air space, QA.

5. The average heat flux across the ceiling, QB.

Positions & Number of Heat Flux sensors

Total number of

HF sensors:

27 pieces.(18 pieces on roof

9 pieces on ceiling)

Heat Flux

sensors

Heat Flux sensors

Research Methodology

Boundary Conditions

Solar radiation 600 W/m2 to 1000 W/m2

Reflective air space thickness 25mm, 50mm, 75mm, and 100mm

Roof pitch 30° and 45°

Ambient temperature Ambient temperature of 30°C

Wind speed 0.5 m/s

Humidity 60%

Main Material PropertiesProperties Material Value

Emissivity, ε

Clay tiles 0.9

Concrete tiles 0.9

Gypsum board 0.85

Wood 0.90

Aluminum foil 0.03

Thermal conductivity (W/m·K)

Clay tiles 0.84

Concrete tiles 1.10

Gypsum board 0.16

Wood 0.14

Aluminum foil 0.896

Specific heat capacity (J/K·kg)

Clay tiles 800

Concrete tiles 837

Gypsum board 840

Wood 1200

Aluminum foil 167

Validation of CFD against Empircal

Validation against empirical measurements using Mean absolute percentage

error calculation (MAPE)

Roof type and

Configuration

Empirical

RSI value

CFD simulation

results

Percentage error

(%)

25 mm air space

with clay tiles

(phase 5)

2.4 2.37 1.25

25 mm air space

with concrete

tiles

(phase 3)

2.15 2.33 8.37

25 mm air space

with concrete

tiles

(phase 5)

2.26 2.33 3.10

25 mm air space

with concrete

tiles

(phase 1)

2.37 2.33 1.69

Mean absolute percentage error 3.60

𝑀𝐴𝑃𝐸 =1

𝑁 𝑘=1

𝑁|𝐴𝑡−𝐹𝑡|

𝐴𝑡

where,

At is the actual value.

Ft is the forecast value.

N is the number of fitted points.

CFD GRAPHIC RESULTS

Results and DiscussionsComparison between with and without radiant barrier @ pitch 30° on Clay Roof Tiles

Without RB

With RB

t = 25 mm

Average Ceiling

temperature: 41.87°C

Average Heat flux @

ceiling: 31.3 𝑊/𝑚2

RSI: 0.8 𝑚2K/W

Average Ceiling

temperature: 34.64°C

Average Heat flux @

ceiling: 8.45 𝑊/𝑚2

Temperature reduction:

17.3%

Heat flux reduction: 73%

RSI: 2.37 𝑚2K/W

Results and DiscussionsComparison between with and without radiant barrier @ pitch 45°& 25mm air space

Without RB

With RB

Average Ceiling

temperature: 41.37°C

Average Heat flux

through the ceiling:

29.6 𝑊/𝑚2

Average Ceiling

temperature: 35.38°C

Average Heat flux

through the ceiling:

6.37𝑊/𝑚2

Temperature reduction:

14.5%

Heat flux reduction: 78%

Ceiling temp

Ceiling temp

Results and DiscussionsReflective air space thickness: variable 25mm & 50mm (fixed @ pitch 30°)

25 mm

50 mm

Average Ceiling

temperature: 34.59°C

Average Heat flux

through the ceiling:

7.43 𝑊/𝑚2

Temperature reduction:

1.15%

Heat flux reduction:

12%

Average Ceiling

temperature: 34.64°C

Average Heat flux

through the ceiling:

8.45𝑊/𝑚2

Results and DiscussionsReflective air space thickness variable: 75mm & 100mm (pitch 30°)

75 mm

100 mm

Average Ceiling

temperature: 33.24°C

Average Heat flux

through the ceiling:

6.48 𝑊/𝑚2

Temperature reduction:

3%

Heat flux reduction:

7.4%

Average Ceiling

temperature: 34.47°C

Heat flux through the

ceiling:

7.00 𝑊/𝑚2

Temperature reduction:

Heat flux reduction:

Results and DiscussionsReflective air space thickness variable 25mm & 50mm (pitch 45°)

25 mm

50 mm

Average Ceiling

temperature: 34.38°C

Average Heat flux

through the ceiling:

4.27 𝑊/𝑚2

Temperature reduction:

0.2%

Heat flux reduction:

10%

Average Ceiling

temperature: 34.43°C

Average Heat flux

through the ceiling:

4.75 𝑊/𝑚2

Results and DiscussionsReflective air space thickness (pitch 45°)

75 mm

100 mm

Average Ceiling

temperature: 33.42°C

Average Heat flux

through the ceiling: 3.76

𝑊/𝑚2

Temperature reduction:

2.68%

Heat flux reduction:

1.75%

Average Ceiling

temperature: 34.34°C

Average Heat flux

through the ceiling:

3.82 𝑊/𝑚2

Results and DiscussionsComparison between pitch 30° and pitch 45° (both without radiant barrier)

Pitch 30°

Pitch 45°

Average Ceiling

temperature: 41.87°C

Average Heat flux

@ ceiling: 31.3 𝑊/𝑚2

Average Ceiling

temperature: 41.37°C

Average Heat flux

@ ceiling: 29.6 𝑊/𝑚2

Temperature reduction:

1.19%

Heat flux reduction:

5.4%

CFD ANIMATION RESULTS

Without RB, 30deg pitch

With RB, 30deg pitch,

100mm air space

CFD ANIMATION

• Overall slower air

velocities in the attic

Without Radiant Barrier

@45deg pitch

With Radiant Barrier

@45deg pitch with

100mm air space

CFD ANIMATION

• Vortices decreases in the attic

• Air velocities

decreases/slower

CFD ANIMATION

Without Radiant Barrier

@45deg pitch

With Radiant Barrier

@45deg pitch with

100mm air space

• Cooler temperature at

the attic space

• Convective heat is

lesser (vortices lesser)

• Air velocities decreases

in the attic

Without Radiant Barrier

@45deg pitch

With Radiant Barrier,

100mm air space

@45deg pitch

TEMPERATURE

GRADIENT VIEW

CFD ANIMATION

• Temperature

decreases at the

ceiling level

30deg pitch roof

With 25mm air space

30deg pitch roof

With 100mm air space

CFD ANIMATION

• Air velocities increase

• Vortices increase due to

bigger air space

CFD SIMULATION RESULTS

Results and Discussions

RSI value of reflective air space only

Results and Discussions

RSI value of attic in relation to thickness of reflective air space

Results and Discussions

Pitch 45°

Pitch 30°

RSI value of Entire Roof Assembly

Results and Discussions

The reflective air space plays more effective role is reducing the heat flux compares to attic

FOR 30 degree pitch roof

Results and DiscussionsFOR 45 degree pitch roof

The reflective air space plays more effective role is reducing the heat flux compares to attic

RSI values summary

Roof TypeRoof

Pitch

Reflective air space thickness

25 mm 50 mm 75 mm 100 mm

R-Air

space

R-

Attic

R-Air

space

R-

Attic

R-Air

space

R-

Attic

R-Air

space

R-

Attic

Clay Tiles30° 0.9 1.47 1.24 1.308 1.55 1.29 1.76 1.26

45° 0.78 2.31 1.19 2.12 1.46 2.12 1.65 2.16

Concrete Tiles30° 0.83 1.5 1.2 1.32 1.5 1.26 1.69 1.24

45° 0.72 2.32 1.17 2.11 1.43 2.14 1.63 2.11

Roof Type Roof PitchTotal RSI value of roof assembly

25 mm 50 mm 75 mm 100 mm

Clay Tiles30° 2.37 2.55 2.84 3.02

45° 3.09 3.30 3.58 3.81

Concrete

Tiles

30° 2.33 2.52 2.76 2.93

45° 3.04 3.28 3.57 3.74

Thank you