AbstractOwing to the small thermal inertia of prefab houses, indoor thermal environment is poor and occupants are tormented especially under the high solar radiation. Retro-reflective materials can make it a reasonable choice to decrease radiation heat gain due to their high reflectivity for solar radiation. Comparative results show that the indoor air temperature of the prefab house by covering retro-reflectivity materials can reduce by more than 7°C, while the reduced value is close to 10°C for the peak radiant temperature and 7°C for indoor average radiant temperature. It shows retro-reflective materials have a significant effect on thermal performance improvement of prefab houses. Through the comparative study of different walls, it is found that the top, south and east walls are the better choices and that the north wall is the worst one to cover retro-reflectivity materials

Li, Pan

Liang, Weijie

Long, Enshen

Meng, Xi

Wang, Caixia

Wang, Suo

Elsevier - Publisher Connector 

 Procedia Engineering  121 ( 2015 )  1001 – 1007 
Available online at www.sciencedirect.com
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of ISHVAC-COBEE 2015
doi: 10.1016/j.proeng.2015.09.069 
ScienceDirect
9th International Symposium on Heating, Ventilation and Air Conditioning (ISHVAC) and the 3rd 
International Conference on Building Energy and Environment (COBEE) 
Thermal Performance Improvement of Prefab Houses by Covering 
Retro-reflective Materials 
Xi Menga, Caixia Wangb, Weijie Liangb, Suo Wanga, Pan Lia, Enshen Longa,b,* 
aCollege of Architecture and Environment, Sichuan University, Chengdu, China  
bInstitute for Disaster Management and Reconstruction, Sichuan University, Chengdu, China  
Abstract 
Owing to the s mall thermal inertia of prefab houses, indoor thermal environment is poor and occupants are 
tormented especially under the high solar radiation. Retro-reflective materials can make it a reasonable choice to 
decrease radiation heat gain due to their h igh reflectiv ity for solar radiation. Comparative results show that the 
indoor air temperature of the prefab house by covering retro-reflectiv ity materials can reduce by more than 7°C, 
while the reduced value is close to 10°C for the peak radiant temperature and 7°C for indoor average radiant 
temperature. It shows retro-reflective materials have a significant effect on thermal performance improvement of 
prefab houses. Through the comparative study of different walls, it is found that the top , south and east walls are the 
better choices and that the north wall is the worst one to cover retro-reflectivity materials. 
© 2015 The Authors. Published by Elsevier Ltd. 
Peer-review under responsibility of the organizing committee of ISHVACCOBEE 2015. 
Keywords: Retro-reflective materials; reflection; indoor temperature 
1. Introduction 
Prefab houses are widely used as temporary shelters, temporary houses after a disaster and temporary offices for 
construction projects, due to their easy transport, rapid construction, low cost and so on [1]. In 2008, 625 thousand 
prefab houses were built  as temporary shelters or houses after Wenchuan earthquake [2] and Fig. 1 shows one post-
disaster transitional settlement of prefab houses in Wenchuan earthquake. Before the completion of permanent 
buildings, victims had to live in these prefab houses for 2-3 years. However, due to the low thermal resistance and 
                                                                 
* Corresponding author. Tel.: +86-02808541015; fax: +86-02808541015. 
E-mail address: longes2@163.com; mengxihvac@163.com 
© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of ISHVAC-COBEE 2015
1002   Xi Meng et al. /  Procedia Engineering  121 ( 2015 )  1001 – 1007 
small thermal inert ia of prefab houses, indoor thermal environment was very poor, and thereby victims were 
tormented mentally and physically  [3-5]. In-situ measurement of p refab houses shows that indoor air temperature 
can reach 30°C, while inner surface temperature can be up to 55°C [6] and that solar radiation affects indoor thermal 
environment significantly and makes indoor air temperature far higher than outdoor air temperature. Decreasing 
solar radiation heat gain is necessary for refining indoor thermal environment in prefab houses. 
 
Fig. 1. One post -disaster transitional settlement of prefab houses in Wenchuan earthquake 
Retro-reflective materials make it  a  reasonable choice for decreasing radiation  heat gain and meeting the purpose 
of improving indoor thermal performance. Different from the specular reflect ion of conventional reflective materials 
[7], retro-reflective materials can reflect solar radiation in the opposite direction. And thereby retro -reflective 
materials can decrease radiation heat gain of prefab houses without increasing heat gain of other build ings. At the 
same time, retro-reflective materials, which have only 1mm th ickness, will not change the easy transport and rapid 
construction of prefab houses. 
At present, there is some research on conventional reflective materials. Synnefa et al. [8,9]researched reflective 
coatings on the effect of the urban environment and energy loads and thermal comfort in residential buildings . 
However, there is lack of the systemic research on retro-reflect ive materials. To the best knowledge of the authors, 
Sakai et al.[10]firstly researched these retro-reflective materials and measured their retro-reflective properties. 
In this paper, a contrast experiment was done on two models of prefab houses, one of which was covered with  
retro-reflective materials, and the contribution of ret ro-reflective materials was analysed on improving indoor 
thermal performance. 
2. Methods 
To reveal the thermal performance improvement of prefab  houses by covering retro-reflective materials, two  
prefab house models (M1 and M2) are built with  the dimensions of 800mm× 1000mm× 1300mm (L ×W ×H) as 
shown in Fig. 2, and M2 is covered by retro-reflective materials, while M1 is a original one as a comparison object. 
In addition, Fig. 2 also shows the locations of the temperature measurements. Tt , Tb, Te, Ts , Tw  and Tn measure 
the inner surface temperature of the top, bottom, west, south, west and north walls, while Tin  and Tout measure 
indoor and outdoor air temperature. At last, thermocouples, whose errors are lower than 0.3°C, were selected fo r 
experimental measurement. All measurement data are recorded by JTRG-II building thermal temperature automatic 
tester with the data-collection interval of 60 minutes.  
Fig. 3 shows the structure of M2 wall consisting of one 40mm polystyrene foam board  (XPS) layer, two 0.5mm 
stainless steel layers and one 1mm retro-reflective material layer. There is no the retro-reflective material layer in  
outer surface for M1. 
Fig. 4 d isplays the magnificat ion of retro-reflective materials with the magnify ing power of 180x and the 
reflectivity measurement value of retro-reflect ive materials by a spectrophotometer in the laboratory. It can be 
clearly seen the uniform distribution of glass beads on the membrane material surface. In addition, Figure 4(b) also 
gives the ground solar radiat ion energy distribution at the standard sea level with the secondary air quality. It can be 
1003 Xi Meng et al. /  Procedia Engineering  121 ( 2015 )  1001 – 1007 
found that retro-reflective materials have the high reflectivity for the visible light and near-infrared layer, which has 
the highest percentage of solar rad iation energy distribution, so retro-reflective materials can reflect the solar 
radiation by the reasonable choice of reflectivity wave lengths and refine the thermal performance of prefab houses. 
And the comprehensive reflectiv ity of retro-reflect ive materials can be gained as 0.543 by the weighted computation 
of the ground solar radiation energy distribution. 
 
Fig. 2. The arrangement diagram of thermocouples and two models of p refab houses 
 
Fig. 3.  A schematic diagram of wall structure for M2 
 
Fig. 4.  (a)The magnification of retro-reflective Materials with the magnifying power of 180x and (b) reflectivity of retro-reflective materials 
1004   Xi Meng et al. /  Procedia Engineering  121 ( 2015 )  1001 – 1007 
3. Results 
The measurement experiment was carried out from 4th to 9th in August 2014 in Sichuan University of Chengdu. 
Fig. 5 shows outdoor air temperature and horizontal total radiation measured by a meteorological station during the 
test period. It is easily found that there were overcast, cloudy and sunny days during the measurement period. The 
maximum temperature difference can be up to 10°C in a day and solar radiat ion can reach 850W/m2 at 14:00 during 
the cloudy and sunny days. 
 
Fig. 5.  Outdoor air temperature and horizontal total radiation during the test period 
Fig. 6 shows indoor air temperature in  M1 & M2 and outdoor air temperature during the test period. As shown in  
Fig. 6, compared to indoor air temperature in  M1, the reduced peak air temperature  is 7.1°C for the second day and 
7.4°C for the fourth day in M2 by covering retro-reflective materials and it shows retro-reflective materials have a 
significant effect on thermal performance improvement. In addition, it can be observed that indoor air temperature is 
almost the same with outdoor air temperature under the lower horizontal total radiation at  the first day and nights of 
the test period, while indoor air temperature in M1 & M2 is higher than outdoor air temperature under the higher  
horizontal total radiat ion. It shows solar radiation is a main factor causing the poor thermal environment. On the 
other hand, it reveals the effective cause of the retro-reflective materials on thermal performance improvement. 
Moreover, indoor air temperature in M1 & M2 fluctuates nearly in  sync with outdoor air temperature due to the low 
thermal resistance and the small thermal inertia of ultrathin envelope in the prefab house. 
 
Fig. 6.  Indoor air temperature in M1 & M2 and outdoor air temperature during the test period 
It is generally known that radiant temperature of inner surfaces is another important parameter affecting thermal 
performance besides inner air temperature. And Fig. 7 shows top inner surface temperature in M1 and M2 during the 
test period. As shown in figure, compared to inner surface temperature of top wall in M1, the reduced peak air 
temperature is 9.6°C for the second day and 9.9°C for the fourth day in M2 by covering retro-reflective materials. 
1005 Xi Meng et al. /  Procedia Engineering  121 ( 2015 )  1001 – 1007 
And retro-reflective materials have the efficient improvement on thermal performance under the higher horizontal 
total radiation, which is consistent with conclusion in the Fig. 6.  
 
Fig. 7.  Top inner surface temperature in M1 and M2 during the test period 
To farther research the influence of retro-reflective materials on the radiant temperature, Fig. 8 shows inner 
surface temperature of the east, south, west, north walls. It can be seen clearly that when retro-reflective materials 
are covered on the south wall, the best effect of thermal performance has been obtained; Next is the east wall; The 
poorest is the north wall and no obvious refinement can be observed. It shows the walls with the different directions, 
on which retro-reflect ive materials are covered, have a different refinement effect due to that the walls with the 
different orientation can obtain the different solar radiation strength.  
 
Fig. 8.  Inner surface temperature in M1 and M2 during the test period: (a) East wall; (b)  South wall; (c) West wall and (d) North wall 
Above research on inner surface temperature of walls with  the different d irections shows that the top wall, the 
south wall and the east wall are the better choices and the north wall is the worst choice to cover ret ro-reflective 
materials.To synthesize the radiant temperature of each inner surface, indoor average radiant temperature is 
introduced and defined as following: 
1006   Xi Meng et al. /  Procedia Engineering  121 ( 2015 )  1001 – 1007 
average i i i
1 1
N N
i i
T T S S
  
 ¦ ¦    (1) 
Where, Ti is inner surface temperature of the wall, °C; Si is inner surface area of the wall, m2. 
Fig. 9 shows indoor average radiant temperature in M1 and M2 during the test period. It can be seen clearly that 
indoor average radiant temperature is 6.3°C for the second day and 6.9°C for the fourth day lower than that in M2. It  
shows retro-reflect ive materials improve thermal performance of prefab houses obviously in summer by decreasing 
indoor air temperature and average ambient temperature owing to their high reflectivity. 
 
Fig. 9.  Indoor average radiant temperature in M1 and M2 during the test period 
4. Discussion  
In our study, the thermal performance improvement of prefab house is experimentally researched by covering 
retro-reflective materials . Different  from the conventional constructions, indoor air temperature fluctuates in  sync 
with outdoor ambient temperature due to the low thermal resistance and the small thermal inert ia of ultrathin  
envelope in the prefab houses. Therefore, the high solar radiation will further worse the poor thermal environment. 
And retro-reflective materials will increase the reflectivity of outer wall surface and decrease its absorptive 
correspondingly. And then the purpose of the thermal performance improvement is obtained. Experimental results 
show the indoor air temperature of p refab houses can be reduced by more than 7°C, while the reduced value is close 
to 10°C for the peak radiant temperature and 7°C for indoor average radiant temperature. It shows retro-reflective 
materials have a significant effect on thermal performance improvement of prefab houses. In addition, through the 
comparative study of walls  with  different d irections, on which retro-reflective materials are covered, it is found that 
the top wall, the south wall and the east wall are the better choices and that the north wall is the worst on, which can  
guide the engineering application of retro-reflective materials. 
5. Conclusions  
Retro-reflective materials can improve the thermal performance of prefab houses significantly in the summer with 
the high solar intensity. And the top wall, the south wall and the east wall are the best choices to cover retro -
reflective materials on in Chengdu, China. 
Acknowledgements 
The authors gratefully acknowledge the financial support from the National Nature Science Foundation of China 
under Grant No.51478280 and No.51178282. 
1007 Xi Meng et al. /  Procedia Engineering  121 ( 2015 )  1001 – 1007 
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Thermal Performance Improvement of Prefab Houses by Covering Retro-reflective Materials 

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Thermal Performance Improvement of Prefab Houses by Covering Retro-reflective Materials

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