OTTV (SNI 03-6389-2011) and ETTV (BCA 2008) Calculation for Various Building’s Shapes, Orientations, Envelope Building Materials: Comparison and Analysis

: The Indonesian National Standard SNI 03-6389-2000 adapted the 1983 Singapore’s Handbook on Energy Conservation and limited the Overall Thermal Transfer Value (OTTV) of the building envelope to 45 Watt/m 2 . In 2008, the Singapore’s Building and Construction Authority (BCA) shifted to Envelope Thermal Transfer Value (ETTV) value of 50 Watt/m 2 , while SNI 03-6389-2011 continues to use OTTV. This paper reviewed the new SNI 03-6389-2011 and compared it with BCA by calculating OTTV and ETTV of prismatic buildings with eight different shapes and building orientations, 11 Window to Wall Ratio, and 27 building envelope materials. This study also tested those variables to find the best building shape and orientation for an energy saver building. The result shows that ETTV (BCA) is stricter than OTTV (SNI 03-6389-2011) except the OTTV with black building envelope, while parallelogram shape building with North-South orientation is the best combination of energy saver building.


Introduction
People started to pay attention on energy conservation due to increasingly high demand of energy, especially in residential and commercial building sectors. Specifically in Singapore and Indonesia, air conditioning uses most energy in commercial buildings. Therefore, it is essential to understand the condition of air system when designing a building since the amount of energy usage must be established early on [1].
Energy consumption in Indonesia is regulated by limiting the Overall Thermal Transfer Value (OTTV) [2,3], adopting the Handbook on Energy Conservation in Buildings and Building Services published by the Development & Building Control Division of Public Works Department of Singapore in 1983 [4]. In the meantime the new regulation in Singapore published by the Building and Construction Authority (BCA) [5] uses Envelope Thermal Transfer Value (ETTV) instead of OTTV. According to BCA [5] the later research works revealed that OTTV did not reflect accurately the relative performance of the different elements in an envelope system.
The difference between OTTV [2,3] and ETTV [5] is in the limiting value and the level of sun radiation absorption. The OTTV in SNI 03-6389-2011 is limited to 35 W/m 2 and considers sun radiation absorption (α) due to the color of building envelope in the formulation. While the limiting ETTV value according to BCA is 50 W/m 2 , and no sun radiation absorption coefficient α. in the formulation.

Concept of OTTV
The foundation of an energy efficient building starts with its design process. The main issue in creating an energy efficient building comes from the absorption of the building's solar heat load through its air conditioning system. Aligning the direction of the building's façade to East and West and choosing light colors for wall finish are some examples of the common design practice to reduce solar heat input. Limiting OTTV is one of the energy efficiency strategies. OTTV takes into account the elements of heat gain through the external wall of a building, such as: heat conduction through opaque walls and glass windows as well as solar radiation through glass windows. OTTV value is measured by taking the average measurement of these three elements over the whole envelope area of the building.

OTTV based on SNI 03-6389-2011[3]
To calculate the OTTV of an external wall, the following basic equations shall be used: Where more than one type of material and/or fenestration is used, the following equation shall be used: For the purpose of energy conservation, the maximum permissible OTTV is set to 35 Watt/m 2 .

ETTV based on BCA of Singapore [5]
Since 1979, the Building Control Regulations had prescribed an envelope thermal performance standard known as OTTV. The OTTV standard applied only to air-conditioned non-residential buildings. A major review of the OTTV formula was carried out in the early 2000 to provide a more accurate measure of the thermal performance of building envelope. The new formula is named Envelope Thermal Transfer Value (ETTV) to differentiate from the original OTTV formula. The ETTV requirement does not apply to non air-conditioned buildings.
The ETTV formulas are given as follows: To calculate the ETTVfor the envelope of the whole building, the following equation shall be used: For the purpose of energy conservation, the maximum permissible ETTV is set to 50 Watt/m 2 .

Buildings Considered
This study uses data and building specification from Singapore Reference Office Building Description [6].
The buildings that will be analyzed are hypothetical buildings with eight different building shapes and eight different building orientations.
In this study, the Authors calculated OTTV [3] and ETTV [5] of prismatic buildings with eight different shapes, eight building orientations, 11 Window to Wall Ratio (WWR), and 27 alternative for building envelope material for OTTV as shown in Table 1, and nine alternative for building envelope material for ETTV as shown in Table 2.
The Authors then compared the results to find the OTTV/ETTV value that fits the energy-saver building standard. The eight hypothetical building shapes; circular, equilateral octagon, equilateral triangle, parallelogram, square, rectangle, ellipse, and trapezoid with eight orientations (N, NE, E, SE, S, SW, W, NW) are shown in Figure 1. Each hypothetical building is ten story with four meters floor to floor height. The overall floor area is 625 m 2 with 100 m 2 core, leaving the total floor that requires cooling as 525m 2 . Total area of the whole building is 5250 m 2 .
There are three materials considered for the building envelope.   Table 3 shows a typical result of OTTV calculation on prismatic building, with circular shaped [7] using27 different materials and WWR from 0% to 100% with 10% interval. The shaded cells denote buildings with OTTV satisfying SNI 03-6389-2011 Table 4 shows a typical result of ETTV calculation on prismatic building, with circular shape. This building was built with nine different materials with WWR from 0% to 100% with 10% interval. The shaded cells denote buildings with ETTV satisfying BCA 2008     Interpolating other OTTV and ETTV for the rest of the buildings gives WWRmin and WWRmax as presented in Table 5.

ETTV Calculation
Based on The comparison of WWR maximum that satisfied OTTV and ETTV requirement is presented in Table  6.

Conclusions
The requirements of ETTV for outdoor paintings with medium  2) and the smallest  3) are stricter than OTTV requirement. But for outdoor painting with the largest  1), OTTV has stricter regulation than ETTV.
From this study, the Authors concluded that the Parallelogram shape building with building orientation JG5/JG1 is the best combination of energy saver building. This combination created the biggest WWR value that satisfies the requirement of OTTV/ETTV regulation.
This study also highlighted the need for further analysis to determine the  value needed for creating a stricter ETTV value.