Reference number  
DRS 397-3: 2018  
© RSB 2018  
  
 RWANDA 
STANDARD  DRS 
397-3 
 
1st edition  
2018 -mm-dd 
 
Use of Glass in Building s― 
Part 3: 
Energy and Light   DRS  397-3: 2018  
©RSB 2018  - All rights reserved                  ii 
 In order to match with technological development and to keep continuous progress in industries, 
standards are subject to periodic review. Users shall ascertain that they are in possession of the latest 
edition  
 
 
 
 
 
 
 
 
© RSB 2018  
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized 
in any form or by any means, electronic or mechanical, includin g photocopying and microfilm, without 
prior written permission from R SB. 
Requests for permission to reproduce this document should be addressed to : 
Rwanda Standards  Board  
P.O Box 7099 Kigali -Rwanda  
KK 15 Rd, 49  
Tel. +250 788303492  
Toll Free: 3250  
E-mail: info@rsb.gov.rw  
Website: www.rsb.gov.rw   
ePortal: www.portal.rsb.gov.rw   DRS 397-3: 2018  
iii                ©RSB 2018  - All rights reserved  
 Contents  Page 
Foreword  ................................ ................................ ................................ ................................ ..........................  ivv 
1 Scope  ................................ ................................ ................................ ................................ ......................  1 
2 Normative references  ................................ ................................ ................................ ............................  1 
3 Terms and definitions  ................................ ................................ ................................ ...........................  1 
4 Energy and Thermal Properties of Glass  ................................ ................................ ............................  1 
5 Light Reflectance  ................................ ................................ ................................ .............................  1715 
6 Total Solar Energy Transmittance (Solar Heat Gain Coefficient)  ................................ ...............  1917 
7 Total Solar Energy Transmission  ................................ ................................ ................................ .. 2725 
8 UV-Transmittance  ................................ ................................ ................................ ............................  2826 
Annex  A (normative)  Annex title  ................................ ................................ ................................ ................  2927 
A.1 General  ................................ ................................ ................................ ................................ .............  2927 
A.2 Clause  ................................ ................................ ................................ .....Error! Bookmark not defined. 27 
A.2.1  Subclause (level  1) ................................ ................................ ................ Error! Bookmark not defined. 27 
A.2.2  Subclause (level  1) ................................ ................................ ................ Error! Bookmark not defined. 27 
 DRS  397-3: 2018  
©RSB 2018  - All rights reserved                  iv 
 Foreword   
Rwanda Standard s are prepared  by Technical Committees and approved by R wanda Standards Board (RSB) 
Board  of Directors in  accordance with the procedures of RSB, in compliance with Annex 3 of the WTO/TBT 
agreement on the preparation, adoption and applicati on of standards.  
The main task of technical committees is to prepare national standards.  Final Draft Rwanda S tandards 
adopted by Technical committ ees are ratified  by me mbers of RSB Board of Director s for publication and 
gazettment as Rwanda Standards.  
DRS 397-3 was prepared by Technical Committee RSB/TC 009, Building Materials  and Civil Engineering . 
In the preparation of this standard, reference was made to the following standard s: 
1) AS 1288 – 1994: Glass in buildings – Selection and installation.  
2) BS 6262 - Code of practice for glazing for buildings  
The assistance de rived from the above source is hereby acknowledged with thanks.  
DRS 397 :  consists  of the following parts, under the general title Use of Glass in Buildings : 
 Part 1: Terminology  
 Part 2: General met hodology for selection  
 Part 3: Energy and Light  
Part 4: Fire and Loading  
Part 5: Safety related to Human Impact  
Committee membership  
The following organizations were represented on the Technical Committee on Building Materials and Civil 
Engineering (RSB/TC 009) in the preparation  of this Draft standard.  
City of Kigali  
Gasabo 3D  
GEE  DRS 397-3: 2018  
v                ©RSB 2018  - All rights reserved  
 INES - Ruhengeri  
NPD  
RHA  
RPPA  
RTDA  
RULIBA Clays Ltd  
University of Rwanda -CST 
Rwanda  Standards Board  (RSB) – Secretariat  DRS  397-3: 2018  
©RSB 2018  - All rights reserved                  vi 
 Introduction   
Glazing is an important item in buildin g construction and glass, the primary glazing element has to be selected 
to cater to several requirements. Fixing of glass a specialized operation, when properly done, will avoid the 
hazards of broken glass. Growing trend in resorting to glazed windows/doo rs in buildings and structures has 
considerably increased the importance of glazing and the need for proper workmanship.  
This standard ‘Code of Practice for Use of Glass in Buildings’ is envisaged to address the above and their 
engineering aspects, and is  brought out as various parts as listed in the foreword.   
This standard is intended to provide guidance on the energy and light aspects while choosing glazing in 
building construction. Standard specifications of various types of glass are not covered in th is standard, for 
which respective Rwandan Standards as mentioned in this draft may be referred to.  
Standard specifications of various types of glass are not covered in this standard, for which respective 
International Standards as mentioned/available may b e referred to.   
In the formulation of this standard, inputs were derived from:  
1) ISO 9050:2003 Glass in building — Determination of light transmittance, solar direct  transmittance, 
total solar energy transmitta nce, ultraviolet transmittance and related glaz ing factors  
2) ISO 9845 (Part 1): 1992 Solar energy -- Reference solar spectral irradiance at the ground  at different 
receiving conditions Part 1: Direct normal and hemispherical solar irradiance for air mass 1,5  
3) IS 16231 - 2:  Glass in Buildings – Code of Practice -  part 2 - Energy and Light  
For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, 
observed or calculated, expressing the resul t of a test or analysis shall be rounded off in accordance wi th ISO 
17025; Guide to  the use of preferred numbers and of series of preferred numbers . The number of significant 
places retained in the rounded off value should be same as that of the specified value in this standard.DRS 397 -3: 2018  
1                ©RSB 2018  - All rights reserved  
 Use of Glass in Building s ― Part 3: Energy and Light   
1 Scope   
This Rwanda Draft standard covers guiding provision s for glazing in buildings with respect to their effect on 
energy, light  (Visual)  and solar environments in the building.   
This Standard does not cover the following:  
a) Glazing syste ms including Frameless glazing system  
b) Patent glazing  
2 Normative references  
The following documents are referred to in the text in such a way that some or all of  their content constitutes 
requirements of this document.  For dated references, only the editio n cited applies. For undated references, 
the latest edition of the referenced document (including any amendments) applies.  
ISO 9845 -1:1992: Solar energy – Reference solar spectral irradiance at the ground at different receiving 
conditions – Part 1: Direct normal and hemispherical solar irradiance for air mass 1,5  
ISO 10291:1994: Glass in building – Determination of steady -state U values (thermal transmittance) of 
multiple glazing – Guarded hot plate method  
ISO 10292:1994: Glass in building – Calculation of steady -state U values (thermal transmittance) of multiple 
glazing  
ISO 10293:1997: Glass in building – Determination of steady -state U values (thermal transmittance) of 
multiple glazing – Heat flow meter method  
3 Terms and definitions   
For the purposes of thi s standard, the terms and definitions given in  RS 397-1 shall apply . 
4 Energy and Thermal Properties of Glass  
4.1 General  
The energy and thermal properties of glass is defined by light transmission, internal and external reflection, 
solar heat gain coefficient and U value. The performance requirement for glazing shall be suitably selected.  
Minimum performance requirement is mentioned in the Annex B . Specific methods of calculating light and 
energy factors for glass in buildings are covered here and their charact eristic data can serve as a basis for  
©RSB 2018  - All rights reserved                  2 
 light, heating and ventilation calculations of rooms and can permit comparison between different types of 
glazing. It is applicable to all flat glass material.   
4.2 Energy  
4.2.1  General  
Glazed areas in buildings should be design ed so that account is taken of the overall energy balance in relation 
to the effects on the thermal comfort of occupants and the total annual energy implications of solar gain and 
energy loss. These should be examined separately, since factors affecting on e aspect of performance might 
have no effect on the other. Energy gain is due to transmission of solar radiant energy through the glass into 
the building. Solar energy is all at relatively short wavelengths and is controlled by using the glass to absorb or  
reflect the energy. Energy loss is due to the transfer of energy by conduction, convection and long wavelength 
radiation and can be influenced by a number of factors,  e.g. incorporating air cavities, including gases of low 
thermal conductivity and/or low  emissivity glass.  
4.2.2  Thermal comfort  
Energy transmission through glazing can significantly influence occupier comfort by raising or lowering room 
air temperatures. For example, hot sunny weather can give rise to excessive energy gains; and energy losses 
can occur through the glazing during cold weather or at night.  Thermal comfort can also be influenced by 
direct radiation through the glazing and/or by radiation exchange between the glazing and the occupants.  
4.2.3  Solar energy gain  
Factors that can influence the a mount of solar gain through the glazing include:  
a) orientation;  
b) glazed area;  
c) shading devices, both internal and external; and 
d) properties of the glazing.  
4.2.4  Total solar energy transmittance  
4.2.4.1  General  
Total solar energy transmittance is the proportion of solar r adiation at normal incidence transferred through 
the glazing. It is composed of the direct transmittance (short wave component) and the part of the solar 
absorptance dissipated inwards by long wave  radiation and convection (long wave component).  The total 
solar energy transmission properties of solar control glazing  shall be described by their shading coefficients. 
The shading coefficient is derived by comparing the properties of the solar control glazing with a clear float 
glass having a total solar energy  transmittance of 0.87(i.e., clear glass between 3 mm and 4 mm thick).  
NOTE  Total solar energy transmittance  is also known as g value.  DRS 397 -3: 2018  
3                ©RSB 2018  - All rights reserved  
 4.2.4.2  Solar control glass  
4.2.4.2.1  General  
Solar control glass can be manufactured in several forms. The function of solar control glas s is to reduce the 
total solar energy transmittance, which usually leads to a decrease in the transmission of the visible part of the 
solar spectrum. However some tints and coatings are able to attenuate preferentially non -visible solar 
radiation, leaving the transmission of the greater proportion of the visible radiation largely unchanged. The 
application of ceramic frit fired into the surface of the glass can also be used to modify the energy and light 
transmission of the glass. The effect can be varied a cross a pane depending on the screen print pattern. 
Manufacturers should be consulted for specific details.  
The relationship between light transmission and total solar energy transmission is expressed as the 
light/energy ratio.   
NOTE  Some manufacturers  use this as a descriptive code, quoting a light transmission figure followed by a total 
solar energy transmission figure.  
There is no optimum light/energy ratio. The selection of an appropriate product depends on the requirements 
of the building. Solar gain may be relatively high - for making use of passive solar gains, or low - to reduce air 
conditioning loads. Light transmission will depend on the extent to which daylight is used to obviate artificial 
lighting.  
4.2.4.2.2  Body tinted glass (for increased absorption)  
Solar control properties and colour vary with the thickness of the glass. When used in insulating glass units, 
body tinted glass should be positioned as the outer pane because the energy due to the absorbed radiation is 
more easily dissipated to the outside . 
4.2.4.2.3  Reflective coated glass   
4.2.4.2.3.1  Reflective coated glass uses the principle of increasing the direct reflection to maximize solar 
energy attenuation. In comparison with clear glass, its absorption of solar energy is also increased.  
Reflectance  properties shall be selected as per cla use 5. 
NOTE  Compared with a float glass surface, these reflective coatings (owing to their composition) exhibit lower levels 
of emissivity which improves their U value.  
4.2.4.2.3.2  The coatings may be placed on to body -tinted glass to extend the range of performances.  
The advantages of such glass types shall be :  
a) greater performance range than body tinted glass;  
b) higher performances (greater solar energy attenuation);  
c) light/energy ratios nearer to the theoretical limit;   
©RSB 2018  - All rights reserved                  4 
 d) a range of colour appearances in transmission and reflection.  
NOTE  Performance  data in comparison to clear glass can be  had from manufacturers.  
4.2.4.2.4  Laminated glass  
Laminated glass  shall be  commonly constructed with clear glass and clear interlayers, but sol ar control 
properties can be incorporated into laminated glass by including either solar control glass or tinted interlayers 
or both. Laminated glass with clear interlayers and solar control glass exhibits similar properties to the solar 
control glass from  which it is made. Laminated glass with a tinted interlayer acts in a similar manner to body 
tinted glass, by absorbing the solar radiation, but with a different range of colours and performances.  
NOTE   Performances  of a typical range of laminated glass pr oducts with clear glass and tinted interlayers can be  had 
from manufacturers.  
4.2.4.2.5  Insulating glass units  
Insulating glass units that are used primarily for improved t hermal transmittance (U Value ) see 4.3, can also 
improve the total solar energy transmittance.  This improvement is a result of the incorporation of a second 
pane of glass together with the hermetically sealed air space. The second glass pane can be of any glass 
type. The main glasses used as the inner pane are clear glass, hard coat low emissivity glass or soft coat low 
emissivity glass.  
NOTE 1  Soft coat low emissivity glass can be  used as the hermetically sealed airspace protects the coating.  
NOTE 2  Performances  of a typical range of insulating glass units with clear float inner pane or with hard /soft coat low 
emissivity glass inner pane can be  had from manufacturers.  
4.2.4.3  Solar control plastics glazing sheet materials  
Various coloured plastics glazing sheet materials having ability to reduce the transmission of solar radiation 
can be used in consult ation with the manufacturers.  
4.2.4.4  Blind and louvers  
The use of blinds or louvres in windows affects the window shading coefficient. This depends upon the solar 
optical properties of the glazing and the material of the blind, on the coefficients of energy trans fer at the 
window surfaces, on the geometry and location of the blind, and the angular position of the sun. 
Manufacturers' advice may be consulted for use of these materials .   
4.3 Energy Loss  
4.3.1  General   
Energy loss is quantified by the thermal transmittance or U value. Glass and thin plastics glazing sheet 
materials readily conduct energy and so are poor insulators. To improve resistance to energy loss, insulating 
glass units or coupled glazing  should be used, since the air cavities provide extra thermal resista nce.  DRS 397 -3: 2018  
5                ©RSB 2018  - All rights reserved  
 Increasing the thickness of the glass or plastics glazing sheet material makes little difference to the U value of 
the glazing.  
4.3.2  Methods for improving thermal insulation  
4.3.2.1  Use of low emissivity coatings  
Low emissivity (low -E) coatings have surface emiss ivity of less than 0.2 ; the builder shall use low emissivity  
coating s on glass to improve  the thermal insulation. They are most efficient when used on the cavity surfaces 
of insulating glass units.  
4.3.2.2  Increasing the width of air space  
The width of the airspac e shall be increased to enhanced thermal insulation. However, there is a convection of 
the gas in the cavity.  
4.3.2.3  Using gases of lower thermal conductivity  
The air in the cavity with  should be replaced with gases of lower thermal conductivity such as , argon/kr ypton, 
so as to  improve the thermal insulation.  
4.3.2.4  Inhibiting convection within the air space  
Filling the cavity with cellular material reduces convection and makes the cavity a more efficient insulator. 
However, this usually results in loss of vision, since the materials are, at best, translucent.  
4.3.2.5  Evacuation of the air space  
In theory, a vacuum will eliminate energy transfer by conduction and convection. However, a vacuum puts 
high demands on the glass from the external air pressure.  
NOTE  Evacuation of the a ir space is currently considered to be technically impractical  
4.4 Typical U values of Glass Products  
Tables 1, 2 and 3 give typical U values  
Table 1: Thermal Transmittance ( U value) of glass products: single glazing  
 
 
 Glass thickness  
mm U value  
W/m2 K 
4 5.8 
6 5.7 
10 5.6 
12 5.5  
©RSB 2018  - All rights reserved                  6 
  
Table 2 ― Thermal transmittance (U value ) of glass products : insulating glass units  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Table 3: Thermal Transmittance (U -Value) of Glass Products: Coupled Glazing  
Clear glass thickness and 
type  
mm Separation  
 
mm U-value  
 
W/m2K 
4+4 25 
75 2.8 
2.8 
4+4 Low E  25 
75 1.7 
1.9 Clear glass thickness and type  
mm Cavity 
width  U value  
W/m2K 
Air Argon  
4+4 6 
12 
16 
20 3.3 
2.9 
2.7 
2.8 3.1 
2.7 
2.6 
2.6 
4+4 Low E (€ d = 0.15)  6 
12 
16 
20 2.7 
1.9 
1.7 
1.7 2.3 
1.6 
1.5 
1.5 
4+4 Low E (€ d = 0.04)  6 
12 
16 
20 2.5 
1.6 
1.4 
1.4 2.0 
1.3 
1.2 
1.2 
4+4+4  6 
12 
16 
20 2.4 
1.9 
1.8 
1.8 2.1 
1.8 
1.7 
1.7 
4 Low E (€ d = 0.15) +4+4 Low E (€ d 
= 0.04)  6 
12 
16 
20 1.7 
1.7 
1.0 
0.9 1.4 
1.0 
0.8 
0.8 
4 Low E (€ d = 0.04) +4+4 Low E (€ d 
= 0.04)  6 
12 
16 
20 1.6 
1.0 
0.8 
0.7 1.2 
0.7 
0.6 
0.6 DRS 397 -3: 2018  
7                ©RSB 2018  - All rights reserved  
 Note 1  €d is the declared (normal) emissivity  
Table 4: Thermal Performance of Different Shading Devices  
4.5 Thermal Safety of Glass  
Thermal safety of glass should be assessed considering the amount of radiation incident o n the surface and 
the thermal capabilities of the glass. For example, the solar radiation intensity on the glass surface should be 
determined along with the air temperature range applicable to the location of the building. These 
measurements, together with  the energy transfer coefficients and the glass absorption allow determination of 
the appropriate basic temperature difference between the central area of the glass and its edge. The 
difference is related to the thermal stress and then modified for the typ e of glazing system, taking account of 
extraneous effects resulting from curtains, blinds, back -up walls, proximity to heaters, etc., to derive a stress 
of actual service conditions.  
The resultant service stress should then be compared with the design str ess for the glass. If on comparison, 
the service stress is less than or equal to the design stress, the glass and glazing system may be accepted as 
thermally safe provided, that the edges of the glass are of adequate quality.  
NOTE  1 High air temperatures,  low rates of air movement, and the insulation provided by curtains, blinds, back -up 
walls and multiple glazing tend to reduce the loss of energy and uphold the centre temperature. Low temperatures at the 
edges are maintained by conduction from the glass t hrough the frame to a cold building structure with a large thermal 
capacity.  S/No  
 
 Name of the shading device  
 
 Transmittanc e 
U-Value  Shade 
Factor  
i) 3mm  
(pane area up to 0.1 m2) 2 - 
ii) For pane area >  0.1m2 2 - 
iii) 3 - 2 
iv) 4 2 - 
v) 4 - 2 
vi) 5 2 2 
vii) 6 2 2 
viii) 8 - 3 
ix) 10 - 3 
x) 12 - 3 
xi) 15 - 5 
xii) 19 - 5 
xiii) 25 - 5  
©RSB 2018  - All rights reserved                  8 
 NOTE 2  For new innovations and new products outside the scope of this standard, it is important to seek advice  from 
the manufacturer of glass as to the methods for assessing the t hermal safety of  the glass.  
NOTE 3  where the application of a solar control film is being considered to existing glazing, advice should be sought 
from the manufacturer on the effect of any additional thermal stress likely to be induced in the glass.  
Normalized relative spectral distribution of solar radiation (direct+diffuse) Sλ for air mass = 1.5 calculated from 
the values given in ISO 9845 -1, multiplied by the wavelength interval Δλ. The values in  this table are 
calculated according to the trapezoidal rule.   
TABLE 5: Normalized Relative Spectral Distribution DΛV(Λ) ΔΛ  
  
λ 
nm DλV(λ)Δλ x 102 λ 
nm DλV(λ)Δλ x 102 
380  0  600  5.3542  
390  0.0005  610  4.2491  
400  0.003  620  3.1502  
410  0.0103  630  2.0812  
420  0.0352  640  1.381  
430  0.0948  650  0.807  
440  0.2274  660  0.4612  
450  0.4192  670  0.2485  
460  0.6663  680  0.1255  
470  0.985  690  0.0536  
480  1.5189  700  0.0276  
490  2.1336  710  0.0146  
500  3.3491  720  0.0057  
510  5.1393  730  0.0035  
520  7.0523  740  0.0021  
530  8.799  750  0.0008  
540  9.4427  760  0.001  
550  9.8077  770  0  
560  9.4306  780  0  
570  8.6891  
580  7.8994  
590  6.3306  
Normalized relative spectral distrib ution Dλ of illuminant D65 multiplied by the spectral luminous efficiency V(λ) and by the 
wavelength interval Δλ. The values in this table are calculated  DRS 397 -3: 2018  
9                ©RSB 2018  - All rights reserved  
 The normal mode of thermal b reakage of glass is by the action of tensile stress located in and parallel to an 
edge, and so the breaking stress of the glass is mainly dependent on the extent and position of flaws in the 
edges. The condition of the glass edge is therefore extremely imp ortant.   
©RSB 2018  - All rights reserved                  10 
 TABLE 6 – Normalized  relative spectral distribution of global solar radiatio n 
λ  
nm  SλΔλ  λ  
nm  SλΔλ  
300  0  680  0.012838  
305  0.000057  690  0.011788  
310  0.000236  700  0.012453  
315  0.000554  710  0.012798  
320  0.000916  720  0.010589  
325  0.001309  730  0.011233  
330  0.001914  740  0.012175  
335  0.002018  750  0.012181  
340  0.002189  760  0.009515  
345  0.00026  770  0.010479  
350  0.002445  780  0.011381  
355  0.002555  790  0.011262  
360  0.002683  800  0.026718  
365  0.00302  850  0.04824  
370  0.003359  900  0.040297  
375  0.003509  950  0.021384  
380  0.0036  1000  0.036097  
385  0.003529  1050  0.03411  
390  0.003551  1100  0.018861  
395  0.004294  1150  0.013228  
400  0.007812  1200  0.022551  
410  0.011638  1250  0.023376  
420  0.011877  1300  0.017756  
430  0.011347  1350  0.003743  
440  0.013245  1400  0.000741  
450  0.015343  1450  0.003792  
460  0.016166  1500  0.009693  
470  0.016178  1550  0.013693  DRS 397 -3: 2018  
11                ©RSB 2018  - All rights reserved  
   
 
 
 
 
 
 
 
 
 
 
 
 
 
Solar control glasses should not be nipped to size and any panes with shelled or vented edges should not be 
accepted for glazing in orientations subject to direct sunlight.  
Although a wheel -cut edge is the most satisfactory, laminated glasses with worked edges m ay be used.  
Where clean -cut edges are not permitted, arises  should be created by a wet process, working parallel to the 
edge and not across the thickness, and the design implications of such an action should be examined.  
Where solar control glasses are t o be used in sliding doors and windows there is always the possibility that, 
when opened during sunny periods, the overlapping will function as double glazing with little ventilation in the 
air space, and it is this condition that should be assumed in asse ssing the thermal safety of glass.  
Thermal safety assessment shall be  based on the behavior of glass in good condition and properly glazed. 
Even if the glass is s hown to be thermally safe, this depend s on close adherence to the recommend  glazing 
procedure s. All necessary precautions should be taken to see that only glass with edges of an acceptable 
condition is used. The glass should be stored and handled so that no contact with hard bodies can damage 
the edges and each pane or insulating glass unit should  be carefully examine immediately before glazing.  
Some solar control glasses can be toughened or heat strengthened and gives a means of raising the design 
stress and ensuring safety from thermal fracture.  480  0.016402  1600  0.012203  
490  0.015794  1650  0.010615  
500  0.015801  1700  0.007255  
510  0.015973  1750  0.007183  
520  0.015357  1800  0.002157  
530  0.015857  1850  0.000395  
540  0.015827  1900  0.000082  
550  0.015844  1950  0.001087  
560  0.01559  2000  0.003024  
570  0.015255  2050  0.003988  
580  0.014745  2100  0.004229  
590  0.01433  2150  0.004142  
600  0.014663  2200  0.00369  
610  0.01503  2250  0.003592  
620  0.014859  2300  0.003436  
630  0.014622  2350  0.003163  
640  0.014526  2400  0.002233  
650  0.014445  2450  0.001202  
660  0.014313  2500  0.000475  
670  0.014023     
©RSB 2018  - All rights reserved                  12 
 4.6 Energy Conservation  
4.6.1  The design of glazing op enings or curtain walls can, by reducing reliance on artificial lighting, can be 
one of the largest signal means of saving energy and should be carefully considered.  
4.6.2  Account should be taken of shape of the room, window orientation, occupancy pattern s and task, 
together with the relationship of windows to surrounding buildings and other obstruction. As with task lighting, 
possible solar overheating might result and similar precautions should be taken.  
4.7 Light Transmission Properties of  openings  
4.7.1  Light transmission is defined as the fraction of visible light at normal incidence transmitted through 
the glazing. For properties of plastic glazing sheet materials, manufacturers may be consulted.  
4.7.2  Dirt on glazing reduces the light transmission, oft en by an appreciable extent before becoming 
noticeable. To ensure day -lighting levels are adequate, an allowance for the reduced light transmission should 
be made in day -lighting calculation by introducing a 'dirt factor' between 0.7 and 1.0. Regular clean ing of glass 
and plastic glazing sheet materials therefore call their importance.  
4.8 Glare  
4.8.1  General  
Glare results from excessive contrast of illumination, or from an excess of illumination in the field of view. 
Reaction to it is subjective. When correctly desi gned, natural lighting should not be a glare problem.  
NOTE  Contrasts  in excess of 10:1 in illumination in different parts of the field of view might give rise to glare in some 
from.  
4.8.2  Disability glare  
4.8.2.1  Reducing the light transmission of glazing from 87% to 60% produces a just perceptible reduction in 
disability glare caused by direct sunlight. Even when the light transmission of the glazing is as low as 10% 
some 10,000 lux can still be experienced and glare will almost certainly occur.  
4.8.2.2  Glare can be reduced by some form of mechanical shading, e.g., a canopy, an overhanging floor, a 
balcony or a louvre system. Alternatively, internal screening can be provided by louvres or blinds.  
It might also be possible to re -orientate the glazing to avoid e ntry of direct solar radiation. Alternatively, the 
interior layout can be suitably designed to eliminate glare.  
NOTE: Any fixed shading system reduces the amount of nature light entering the building throughout the year, irrespective 
of whether there is a glare problem at any particular time.  DRS 397 -3: 2018  
13                ©RSB 2018  - All rights reserved  
 4.8.3  Discomfort  glare  
Glazing products with light transmission lower than 50% can ameliorate discomfort glare. These products 
decrease the sky luminance components but permanently reduce the admission of daylight. Alternat ively, 
shading devices, internal or external, movable or fixed, may be used.  
Other methods of reducing the problems of glare should be considered, including:  
a) installation of windows in more than one wall to raise the general background illumination and, in so 
doing, to reduce the contrast between a window and its surrounding surfaces;  
b) use of light colored  matt finishes for the window frames and the surrounding surfaces;  
c) splayed reveals, to assist in reducing the contrast between the window and its surro undings;  
d) use of slender glazing bars and transoms of high reflectance; and 
e) lowering window sills to allow increased illumination to enter, which increases the adaptation level and 
reduces the likelihood of discomfort glare.  
4.9 Diffusion and obscuration  
The n ature of some glazing products (e.g., patterned or acid etched glass) can cause the direct incident solar 
beam to be scattered diffusely. Hence the window might assume an uncomfortable high brightness and 
become a discomfort glare source in its own right. Diffusing glazing used within the normal field of view should 
be used with caution.  
4.10 Fading  
Most materials can fade when subjected to either daylight (particularly direct sunlight) or artificial light. Fading 
is a complex phenomenon involving many chemical reactions, initiated or accelerated by light of different 
wavelengths. Generally, the better quality dyes and pigments fade relatively slowly and react only to the 
shorter wavelengths (ultraviolet and the blue end of the visible spectrum). Other materials can fade quickly 
and might do so under light of much longer wavelengths. It is the combination of wavelength, available light 
and transmission  which determines glass selection to minimize fading, not simply the UV transmission.  
4.11 Determination of characteris tic parameters   
4.11.1  General  
The characteristic parameters are determined for quasi -parallel, almost normal radiation incidence.  
4.11.2  Test setup  
The samples shall be irradiated by a beam whose axis is at an angle not exceeding 10° from the normal to the 
surface. The  angle between the axis and any ray of the illuminating beam shall not exceed 5°.   
©RSB 2018  - All rights reserved                  14 
 The characteristic parameters are as follows:  
a) spectral transmittance τ(λ), the spectral external reflectance ρo(λ) and the spectral internal reflectance 
ρi(λ) in the wavelength range of 300 nm to 2500 nm;  
b) light transmittance τv, the external light reflectance ρv,0 and the internal light reflectance ρv,i for 
illuminant D65;   
c) solar direct transmittance τe and the solar direct reflectance ρe;  
d) total solar energy transmittance (solar heat gain co efficient SHGC) g; and 
e) The UV -transmittance τUV  
When calculating the characteristic parameters of multiple glazing, the spectral dat a of each glass component 
is to be used instead of integrating data of the complete unit.  
4.11.3  Performance requirements of optical measurements  
4.11.3.1  Optical measurements in transmission and reflection require special care to achieve accuracy in 
transmittanc e and reflectance of about ± 0.01.  
4.11.3. 2 The wavelength calibration and the photometric linearity of commercial spectrophotometers shall 
be checked periodically using reference materials obtained from metrological laboratories.  
4.11.3. 3 The wavelength calibration shall be performed by measuring glass plates which feature relatively 
sharp absorption bands at specified wavelengths; the photometric linearity shall be checked using grey filters 
with a certified transmittance.  
4.11.3. 4 For reflectance measu rements, reference materials having reflection behaviour (i.e., reflectance 
level and ratio of diffuse and direct reflectance).  
4.11.3. 5 Thick samples (e.g. laminated glass or insulating units) can modify the optical path of the 
instrument’s beam as compa red to the path in air and therefore the sample beam hits an area of the detector 
having a different responsivity.   
4.11.3.6  A similar source of inaccuracy occurs in case of samples with significant wedge angles which 
deflect the transmitted (and reflected ) beams. It is recommended to check the reproducibility by repeating the 
measurement after rotating the sample.  
4.11.3. 7 Additionally, in the case of reflectance measurements, glass sheets cause a lateral shear of the 
beam reflected by the second surface,  causing reflectance losses (whose extent is particularly evident in the 
case of thick and/or wedged samples). This source of inaccuracy shall be taken into account particularly in the 
case of reflectance measurements through the uncoated side. In order to  quantify and correct systematic 
errors, it is recommended to use calibrated reflectance standards with a thickness similar to the unknown 
sample.  DRS 397 -3: 2018  
15                ©RSB 2018  - All rights reserved  
 
4.11.3. 8 In the case of diffusing samples (or samples with a non -negligible diffusing component or wedged 
samples), transmittance and reflectance measurements shall be performed using integrating spheres whose 
openings are sufficiently large to collect the entire diffusely transmitted or reflected beam. The sphere 
diameter shall be adequate and the internal surf ace adequately coated with a highly diffusing reflectance 
material, so that the internal area can provide the necessary multiple reflections. Reference materials with 
characteristics similar to the unknown sample as specified above shall be used. If the tr ansmittance or 
reflectance curve recorded by  the spectrometer exhibits a high level of noise for some wavelengths, the 
values to be considered for those wavelengths should be obtained after a smoothing of the noise.  
4.11.4  Light transmittance  
4.11.4.1  Light trans mittance is the measure of the amount of light passing through the glazing. The visible 
light lies between the wavelength 380 nm to 780 nm. The light transmission of the glazing depends on the 
substrate type and coating layer done on the base glass.  
The l ight transmittance τv of glazing shall be calculated using the following formula:  
 
 
 
 
 
where,  
Dλ  is the relative spectral distribution of illuminant D65,  
τ (λ)  is the spectral transmittance of glazing;  
V(λ)  is the spectral luminous efficiency for ph otopic vision defining the standard observer for 
photometry   
Δλ  is the wavelength interval  
 
Table 5 indicates the values for Dλ V (λ) Δλ for wavelength intervals of 10 nm. The table has been drawn up in 
such a way that Σ Dλ V (λ) Δλ = 1.  
4.11.4. 2 In the case of multiple glazi ng, the spectral transmittance τ(λ) shall be obtained by calculation 
from the spectral characteristics of the individual components. Alternatively measurements on non -diffusing 
multiple units may be performed using an integrating sphe re. This should  be achieved after reducing the 
interspaces under conditions that allow the collection of the whole transmitted beam.  
4.11.4. 3 The calculation of the spectral transmittance τ(λ) shall be performed using methods such as 
algebraic manipulatio n. Any algorithm that  can be shown to yield consistently the correct solution is 
acceptable.   
©RSB 2018  - All rights reserved                  16 
 4.11.4. 5 For the calculation of τ(λ) as well as for the calculation of spectral reflectance the following 
symbols for the spectral transmittance and spectral refl ectance of the individual components are used:  
τ1(λ) is the spectral transmittance of the outer (first) pane;  
τ 2(λ) is the spectral transmittance of the second pane;  
τn(λ) is the spectral transmittance of the nth (inner) pane (e.g. for triple glazing n = 3);  
ρ1(λ) is the spectral reflectance of the outer (first) pane measured in the direction of incident radiation;  
ρ′1(λ) is the spectral reflectance of the outer (first) pane measured in the opposite direction of incident 
radiation;  
ρ2(λ) is the spectr al reflectance of the second pane measured in the direction of incident radiation;  
ρ′2(λ) is the spectral reflectance of the second pane measured in the opposite direction of incident 
radiation;  
ρn(λ) is the spectral reflectance of the nth (inner) pane m easured in the direction of incident radiation;  
ρ′n(λ) is the spectral reflectance of the nth (inner) pane measured in the opposite direction of incident 
radiation.  
4.11.4. 6 For the spectral transmittance τ(λ) as a function of the spectral characteristics  of the individual 
components of the unit, the following formulae are obtained . 
 
         For double glazing;  
 
           For triple glazing :  
 
4.11.4. 7 For multiple glazing with more than three components, relationships similar to above expressions 
are found to calculate τ(λ) of such glazing from the spectral characteristics of the individual components.  
As an exa mple for calculating τ(λ) according to the procedures of this standard, a glazing composed of five 
components may be treated as follows:  
a) first consider the first three components as triple glazing and calculate the spectral characteristics of 
this combinat ion;  
b) next, run the same procedure for the next two components as double glazing;  
c) then calculate τ(λ) for the five component glazing, considering it as double glazing consisting of the 
preceding triple and double glazing.  τ(λ) =    τ1 (λ) τ2 (λ)  
1 - ρ′1(λ) ρ2(λ)   
τ(λ) =    τ1 (λ) τ2 (λ) τ3 (λ) 
[1 - ρ′1(λ) ρ′2(λ)]*[1 - ρ′2(λ)  ρ3(λ) ] -τ22(λ)ρ′1(λ)ρ3(λ)   DRS 397 -3: 2018  
17                ©RSB 2018  - All rights reserved  
 
The light transmission requireme nt for the glazing should be selected in accordance with relevant Rwandan  
standard specifications.  
5 Light Reflectance  
5.1 External Light Reflectance of Glazing  
External light reflectance corresponds to the amount of sunlight reflected by the external glass sur face. The 
amount of light reflection depends on the type of base glass (substrate tint) and the position of coating on the 
glass surface.  
The external light reflectance of glazing ρv,o shall be calculated using the following formula:  
 
 
 
 
 
 
Where,  
ρo(λ) is the spectral external reflectance of glazing, and  Dλ, V(λ), Δλ and the integration procedure 
are defined in 4.11.4 . 
For multiple glazing, the calculation of the spectral external  reflectance ρo(λ) shall be  performed using the 
same methods as given in 4.11.4 for the calculation o f the spectral transmittance τ(λ).  
For the  spectral external reflectance ρo(λ) as a function of the spectral characteristics of the individual 
components of the unit, the following formulae are applied.  
a)  For double glazing:  
 
b) For triple glazing:  
For multiple glazing with more than three components, relationships similar to above 
Equations are fo und to calculate ρo(λ) of such glazing from the spectral characteristics 
of the individual components.  ρo(λ) = ρ1(λ) +        τ 21 (λ) ρ2 (λ)  
1 - ρ′1(λ) ρ2(λ)    
©RSB 2018  - All rights reserved                  18 
 
As an example for calculating ρo(λ), a glazing composed of five components may be 
treated in the same way as described in 4.11.4  
 
 
5.2 External Light Reflectance of Glazing  
Internal light reflectance corresponds to the amount of light reflected by the inner glass surfa ce. The amount 
of light reflection depends on the type of base glass (substrate tint) and the position of coating on the glass 
surface. The internal light reflectance of glazing ρv,i shall be calculated using the following formula:  
 
 
 
 
where,  
ρi(λ) is th e spectral internal reflectance of glazing, and  
Dλ, V(λ), Δλ and the integration procedure are as defined in light transmittance  4.11.4  
For multiple glazing, the calculation of the spectral internal reflectance ρ i(λ) shall be performed using the same 
meth ods as given in 4.11.4  for the calculation of the spectral transmittance τ (λ).  
For the spectral internal reflectance ρ i(λ) as a function of the spectral characteristics of the individual 
components of the unit, the following formulae are applied.  
 
a) For double glazing:  
 
b) For triple glazing:  ρo(λ) = ρ1(λ) +    τ1 2(λ)  ρ2 (λ) [1 -  ρ′2(λ) ρ3(λ) ] + τ12(λ) τ22(λ) ρ3(λ) 
[1 - ρ′1(λ) ρ 2(λ)]*[1 - ρ′2(λ)  ρ 3(λ) ] -τ22(λ)ρ′1(λ) ρ3(λ)   
ρi(λ) =   ρ′2(λ) +        τ22 (λ)  ρ′1 (λ)  
1 - ρ′1(λ) ρ2(λ)   
ρi(λ) = ρ′3 (λ) +    Τ3 2(λ)   ρ′2 (λ) [1 -  ρ2(λ)  ρ′1 (λ) ] + τ32(λ) τ22(λ)  ρ′1(λ) DRS 397 -3: 2018  
19                ©RSB 2018  - All rights reserved  
 
 
 
For multiple glazing with more than three components, relationships similar to above expressions are found to 
calculate ρi(λ) of such glazing from the spectral characteristics of the individual components.  
6 Total Solar Energy Transmittance (Solar Heat Gain Coefficient)  
6.1 General  
Solar heat gain coefficient (SHGC) / solar factor (SF) is the measure of amount of heat transfer from outside 
to inside by direct transmission (Short Wave Radiation) and internal remitted radiation (Long Wave Radiation). 
The SHGC or SF depends on the tint of the substrate and also the type of coating that is done on the base 
glass. The amount of solar energy gained through glass is between 0 and 1. Multiplying the SHGC or SF by 
100 gives the percentage of solar energy allowed into the building.  
 
 
             R = Reflection  
               A = Absorption  
                              T = Direct Transmission  
                             E1 = Outdoor Emission  
                                   E2 = Indoor Emission  (Re-emitted Energy)  
            SHGC =T +E  
                                              Solar Energy (100%) = R+A+T  
                           E1 + E2 = Absorption  
 
 
Figure 1: Solar Heat Gain Coefficient  
The total solar energy transmittance g is the sum of the solar direct  transmittance τe and the  secondary heat 
transfer factor qi towards the inside, the latter resulting from heat transfer by convection and long -wave IR -
radiation of that part of the incident solar radiation which has been absorbed by the glazing:  
g = τe +qi 
 
6.2 Division of Incident solar radiation flux  
The incident so lar radiant flux per unit area φe is divided into the following three parts (see figure below):  
a) The transmitted part τe φe;  
b) The reflected part ρe φe;  [1 – ρ3(λ) ρ′2 (λ)]*[1 - ρ2(λ)   ρ′1 (λ) ] -τ22(λ) ρ3(λ)  ρ′1(λ)    
©RSB 2018  - All rights reserved                  20 
 
c) The absorbed part αe φe;  
Where;  
τe is the solar direct transmittance  
ρe is the solar direct reflectance  
αe is the solar direct absorptance   
 
          Figure 8:  Incident solar radiant flux per unit area  
 
The relationship between the three characteristics is  
τe + ρ e + αe = 1 
The absorbed part αeφe is subsequently divided into two parts qiφe and qeφe, which are energy transferred to 
the inside and outside respectively:  
αe = qi+ qe  
where,  
qi is the secondary heat transfer factor of the glazing towards the inside;  
qe is the secondary heat tran sfer factor of the glazing towards the outside  
6.3 Solar Direct Transmittance  
The solar direct transmittance τe of glazing shall be calculated using the following formula:  
 
 
 
 
 
where,  
 Sλ is the relative spectral distribution of the solar radiation;  DRS 397 -3: 2018  
21                ©RSB 2018  - All rights reserved  
 
τ(λ) is the spectral transmittance of the glazing;  
Δλ and the integration procedure are the same as in 4.11.4 except that the data points shall be chosen at 
the wavelengths given in Table 5 . 
6.4 Solar Direct Reflectance  
The solar direct reflectance ρe of the glazing  shall be calculated using the following formula:  
 
 
 
 
 
Where;  
      Sλ =     is the relative spectral distribution of the solar radiation ( see 6.3 Solar direct transmittance);  
  ρo(λ) =  is the spectral external reflectance of the glazing;  
    Δλ = and the integration procedure are the same as in light transmittance except that the data points shall 
be chosen at the wavelengths given in Table 6 .  
In the case of multiple glazing, the spectral external reflectance ρo(λ)  is calculated in accordance with 6.1. 
6.5 Solar direct absorptance  
The solar direct absorptance αe shall be calculated from the formula;  
τe + ρ e + αe = 1 
6.6 Secondary heat transfer factor  towards the Inside  
6.6.1  Boundary Conditions  
For the calculation of the secondary heat transfer factor toward s the inside, qi, the heat transfer coefficients of 
the glazing towards the outside, he, and towards the inside, hi, are needed. These values mainly depend on 
the position of the glazing, wind velocity, inside and outside temperatures and, furthermore, on the 
temperature of the two external glazing surfaces.   
©RSB 2018  - All rights reserved                  22 
 
As the purpose of this section is to provide basic information on the performance of glazing, the following 
conventional conditions have been stated for  simplicity:  
a) position of the glazing: vertical;  
b) outside surface: wind velocity approximately 4 m/s; corrected emissivity 0.837;  
c) inside surface: natural convection; emissivity optional;  
d) air spaces are unventilated.  
Under these conventional, average conditions, standard values for he and hi are obtain ed:  
              he = 23 W/(m2·K) 
  
where,  
Ɛi is the corrected emissivity of the inside surface [for soda lime glass,  Ɛi = 0.837 and hi = 8 W/(m2K)]. 
If other boundary conditions are used to meet special requirements they shall be stated in the test report.  
Values for Ɛi lower than 0.837 (due to s urface coatings with higher reflectance in the far infrared) should only 
to be taken into account if condensation on the coated surface can be excluded.  
6.6.2 Single glazing    
The secondary heat transfer factor towards the inside, qi, of single glazing sh all be calculated using the 
following formula:  
 
where,  
αe is the solar direct absorptance obtained from the in accordance with part 5 of this standard  
he, hi are the heat transfer coefficients towards the outside and inside, respectively, in accordance with part 5 
of this standard . 
 
6.6.3 Double glazing  DRS 397 -3: 2018  
23                ©RSB 2018  - All rights reserved  
 
The secondary heat transfer factor towards the inside, qi, of double glazing shall be calculated using the 
following formula:  
 
 
 
Where;  
αe1  =  is the solar direct absorptance of the outer (first) pane within the double glazing;  
αe2  =  solar direct absorpt ance of the second pane within the double glazing; and  
Λ  =  thermal conductance between the outer surface and the innermost surface of the 
double glazing ( see Fig. 9 ), in watts per square metre Kelvin (W/m2.K); and  
he, hi  =  are the heat transfer coe fficients towards the outside and the inside respectively in 
accordance with 6.6.1 .  
 
 
 
 
 
 
 
 Figure 9: Illustration of the meaning of thermal conductance Λ 
 
Characteristics αe1 and αe2 are calculated as follows:  
 
 
  
©RSB 2018  - All rights reserved                  24 
 
 
 
 
 
Where;  
α1(λ)  =  spectral direct a bsorptance of the outer pane, measured in the direction of the 
incident radiation, given by the relationship:  
α1(λ) = 1 – τ1(λ) − ρ1(λ )  
α′1(λ)  =  spectral direct absorptance of the outer pane, measured in the opposite 
direction to the incident radiati on, given by the relationship:  
α1′(λ) = 1 – τ1(λ) − ρ’1(λ )  
α2(λ)  =  spectral direct absorptance of the second pane, measured in the direction of 
the incident radiation, given by the relationship: α2(λ) = 1 – τ2(λ) − ρ2(λ ) 
The thermal conductance Λ shall be determined for a temperature difference of Δ T = 15 °C across the sample 
and a mean temperature o f the sample of 10 °C, or by measuring methods using the standard guarded hot 
plate method, or the standard heat flow meter method.  
If another temperature difference Δ T across the sample and/or another mean temperature of the sample is 
used for the determ ination of the thermal conductance Λ to meet special requirements, this shall be stated in 
the test report . 
6.6.4 Multiple Glazing with n > 2 Components  
The secondary heat transfer factor towards the inside, qi, of a multiple glazing with more than two com ponents 
shall be calculated using the following formula:  
 
 
 
 
Where;   Δλ      =  and the integra tion procedure are the same as in 4.1.11.4 except that the 
data points shall be chosen at the wavelengths given in Table .   DRS 397 -3: 2018  
25                ©RSB 2018  - All rights reserved  
 
αe1 = solar direct absorptance of the outer (first) pane within the n-fold glazing;  
αe2 = solar direct absorptance of the second pane within the n-fold glazing;  
αen = solar direct absor ptance of the nth (inner) pane of the n-fold glazing;  
he, hi = heat transfer coefficients towards the outside and towards the inside respectively in accordance 
with 6.6.1 ; 
Λ12 = thermal conductance between the outer surface of the outer (first) pane and the centre of the 
second pane ( see Fig. 9);  
Λ(n−1)n  = thermal conductance between the centre of the ( n−1)th pane and the outer surface of the 
nth(inner) pane ( see Fig. 9);  and 
Λ23 thermal conductance between the centre of the second pane and the centre o f the third pane  
 
 
 
 
 
 
 
 
  
Figure 9: Illustration of the meaning of thermal conductance Λ12 Λ23 …… Λ(n−1)n  
 
The thermal conductances Λ12, Λ23……… Λ(n-1)n shall be determined by iteration technique. The calculation of 
the direct solar absorptance  αe1, αe2 …...... αen shall be performed using the methods given in 6.6.3.  
As an example for the calculation of the direct solar absorptance  the following procedure is given which 
consist of the following ( n-1) steps for a glazing consisting of n components.  
a) First step - calculate the spectral characteristics of a unit consisting of ( n-1) components 2,3,....n 
according to what has been prescribed in 4.11.4 and in accordance with part 4 of this standard . Then 
combine this unit with the first (outer) pane as a doubl e glazing.  
b) Second step - calculate the spectral characteristics of a unit consisting of the ( n-2) components 3,....n and 
furthermore, those of a double glazing consisting of pane 1 and pane 2. These units are then combined as  
©RSB 2018  - All rights reserved                  26 
 
a double glazing. The sum αe1 + αe2 is then obtained. This procedure is continued up to the last ( n-1)th 
step.  
c) (n-1)th step - combine the ( n-1) panes 1,2,... ( n-1) and determine the spectral characteristics of this unit. 
This unit is then combined with the n th (inner) pane as a do uble glazing. From the second equation in 
6.6.1 , the sum  αe1, αe2 ……. αe(n-1) is obtained, that is with the known values αe1, αe2 ……. αe(n-2) from the 
previous steps αe(n-2) is determined. αen is obtained acc ording to equation 3 given in part 5 of this sta ndard.  
In the case of triple glazing for the solar absorbtances αe1, αe2 and αe3 as a function of the spectral 
characteristics of the individual components of the unit, the following formulae are obtained:  
 
 
 
 
 
 
 
 
 
 
 
 
Where;  
τ1(λ), τ2(λ), τ3(λ), ρ1(λ), ρ′1(λ), ρ2(λ), ρ′2(λ), ρ3(λ) are as defined in  4.11.4 ;  
α1(λ), α′1(λ) and  α2(λ) are as defined in 6.6.1  double glazing;  
α′2(λ)  = spectral direct absorptance of the second pane, measured in the opposite direction to the incident 
radiation, given by the r elationship:  
α′2(λ)  = 1− τ2(λ) − ρ′2(λ)  
α3(λ) = spectral direct absorptance of the third pane, measured in the direction of the incident radiation, given 
by the relationship:  
α3(λ) = 1− τ3(λ) − ρ3(λ) 
Δλ and the integration procedure are the same as in 4.11.4 except that the data points shall be chosen at the 
wavelengths given in Table 5.   DRS 397 -3: 2018  
27                ©RSB 2018  - All rights reserved  
 
For a glazing with more than three components, the formulae for the solar absorptances , αe1, 
αe2……,αen as a function of the individual components.  
6.6.5 Mean radiant t emperature (MRT)  
MRT can be calculated by various methods, among which measuring with globe thermometer is the most 
common and is given by:  
 
 
 
Where;  
Ta  =  air temperature, in Kelvin;  
Tg  =  black globe temperature, in Kelvin;  
εg  =  emissivity of bl ack globe;  
σ  =  Stefane -Boltzmann constant; and  
hcg  =  convective heat transfer coefficient of the black globe.  
 
7 Total Solar Energy Transmission   
The total solar energy transmitted into the room per unit area of glazing φei is given by the relation ship:  
φei  = φeg 
where;  
φei  =  incident solar radiation flux per unit area;  
G  =  total solar energy transmittance of the glazing; and  
φe  =  values can be obtained from appropriate tables in meteorological literature.  
NOTE:  The SHGC/SF requirement for the glazing should be selected based on the prescriptive requirements are 
mentioned in Annex A.  
Additional Heat Transfer  
If the room temperature Ti differs from the outside temperature To, an additional heat transfer occurs in 
addition to φ ei. This ad ditional heat flow qz can be calculated as follows:   
©RSB 2018  - All rights reserved                  28 
 
qz  = U (To−Ti)  
Where; U is the U-value (thermal transmittance) of glazing.  
8 UV-Transmittance  
The UV -transmittance of glazing is the fraction of the incident solar radiation transmitted by the glazin g in the 
300 nm to 380 nm range (UV-B range s from 300 nm to 315 nm and UV -A range from 315 nm t o380 nm). The 
UV-transmittance T UV is calculated as follows:  
 
 
 
Where;  
Sλ  =  relative spectral distribution of UV -radiation;  
τ(λ) =  spectral transmittance of t he glazing;  
Δλ  =  and the integration procedure are the same as in light transmittance except that 
the data points shall be chosen at the wavelength given in Table 6 .  
This average extends over the defined UV -portion of the solar spectrum. It may not b e correlated with solar 
radiation damage of materials and skin.  DRS 397 -3: 2018  
29                ©RSB 2018  - All rights reserved  
 Annex  A 
(normative)  
 
Minimum Performance Requirement of Glazing for Exteriors  
A.1 General  
The panel size design of the exterior glass for the building is done in accordance with this standard 
considering  the effect due to wind load, other imposed load, seismic design (as per part 3 of this standard). 
The type of glazing to be installed is determined considering human safety in accordance with part 4 of this 
standard.  
In the determination of the energy an d thermal performance of the glazing, the three compliance approaches 
prescriptive in nature, defines the minimum glazing/glazing system requirement to be met.   
The prescriptive values are specified with respect to the geographic climatic zone and glazing area of the 
building.  
A.2 Building Fenestrations  
The fenestration shall comply with the maximum glazing area weighted on U -value and solar factor as given in 
Table  7. The Window Wall Ratio (WWR) is defined by the total fenestration area to the total gross area of the 
building envelope which is limited to 70%. For fenestration area greater than 70% refer other specialist 
literatures.  
 
 
 
 TABLE 7 : U - VALUE IN W/SQMK REQUIREMENT FOR GLAZING SYSTEM  
Sl  
No.  Climate  Maximum U -factor  
W/m2 - °C  Maximum SHGC  
WWR<40%  40% <WWR<60%  
(1)  (2)  (3)  (4)  (5)  
i)  Composite  3.30  0.25  0.20  
ii)  Hot and Dry  3.30  0.25  0.20  
iii)  Warm and 
Humid  3.30  0.25  0.20  
iv)  Moderate  6.90  0.40  0.30  
v)  Cold  3.30  0.51  0.51  
WWR  Minimum Visible Light Transmi ssion  
0 - 0.3 27% 
0.31 - 0.4 20% 
0.41 - 0.5 16% 
0.51 - 0.6 13% 
0.61 - 0.7 11%  
©RSB 2018  - All rights reserved                  30 
 Annex  B 
(normative)  
 
Test Requirements f or Coated Glass  
B.1 General  
The coated glass that is used for the exterior building of commercial and non commercial (residential building) 
should meet the test requirement listed in this annexure.  
B.2  Measurements o f Performance  
The performance factors and spectral details of the coated glass shall be determined using standard accepted 
spectrophotometer equipment. The actual size shall be dependen t on the type of equipment being used for 
the measurements. The transmittance of the measurement sample shall be measured with radiation of normal 
incidence at the following wavelengths:  
 550 nm (representative wavelength for light and solar transmittance) ;  
 900 nm (representative wavelength for solar transmittance).  
For glass claiming to have a low emissivity coating, a measurement of the reflectance shall be made at 8 μm 
using radiation of nearly normal incidence.  
B.3 Condensation Resistance Test  
This test consists of subjecting the coated glass to a water saturated atmo sphere at constant temperature. 
The samples have condensation continually forming on them and it is this condensation that may cause 
surface degradation.  
Test apparatus should have a provision of four test pieces and the materials used for the inner walls shall be 
corrosion -resistant and shall not affect the test pieces.  
B.4 Procedure  
The water tank shall be filled with demineralized water, having conductivity lower than 30 μS and a pH higher 
than 5. The internal temperature of the cabinet shall be controlled by means of the reference thermocouple 
keeping a  
 DRS 397 -3: 2018  
ICS 81.040.20  
 
 
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