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Energy 30 (2005)

An evaluation of the appropriateness of using overall thermal transfer value (OTTV) to regulate envelope energy performance of air-conditioned buildingsF.W.H Yik , K.S.Y WanDepartment of Building Services Engineering, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong SAR, China Received 18 March 2003

Abstract This paper inquires into whether overall thermal transfer value (OTTV) is an appropriate building envelope energy performance index for use in regulatory control. First, a historical review of the use of OTTV in American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) Standard 90 is presented, followed by a review of more recent work on its further development and application. The major deciencies of OTTV are then discussed, and simulation study results meant to highlight the impacts of such deciencies are presented. The study embraced air-conditioned oce buildings and air-conditioned high-rise residential buildings in Hong Kong. Results of this study clearly show that the OTTV calculated with the use of pre-calculated coecients may not truly reect the thermal performance of a building envelope. Therefore, a second thought should be given to the use of OTTV in building energy codes. # 2004 Elsevier Ltd. All rights reserved.

1. Introduction In the early 1970s, the oil crisis awakened industrialised countries to the fact that economic development can be highly vulnerable to instabilities in imports of energy resources. In addition to reducing reliance on imported fuels, environmental protection and sustainable development are nowadays the major impetus of energy conservation initiatives. Since buildings are a dominant energy consumer in modern cities, energy use in buildings has become a policy issue in many regimes worldwide [13].

Corresponding author. Tel.: +852-2766-5841; fax: +852-2774-6146. E-mail address: (F.W.H. Yik).

0360-5442/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/


F.W.H. Yik, K.S.Y. Wan / Energy 30 (2005) 4171

The American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) originated in 1975 the use of overall thermal transfer value (OTTV) as a thermal performance index for the envelope of air-conditioned buildings [4]. Thereafter, many countries included assessment of OTTV in their building energy codes [511]. OTTV was considered a better performance index than thermal transmittance (U-value) because it takes into account the impact of direct sun on the envelope of mechanically cooled buildings [12]. With all other things being equal, lowering the OTTV of a building should lead to less envelope heat gain and thus less cooling energy use. Despite that ASHRAE has ceased using OTTV in its Standard 90 [13] since 1989, use of OTTV in building energy codes continues outside the US, including those launched in the 1990s [11,14]. Much eort has also been made to enhance the method for OTTV calculation in Asia, including in Singapore and Hong Kong. The divergence between the US and the Asian countries in the use of OTTV warrants a more thorough evaluation of the appropriateness of using OTTV to regulate energy performance of building envelopes. 2. The introduction and abandonment of OTTV in ASHRAE Standard 90 Using OTTV to quantify the energy performance of envelopes of air-conditioned buildings was rst introduced by ASHRAE in Standard 90-75 [4], which was revised later into Standard 90A-1980 [15]. In the latter, OTTV was explicitly dened as the maximum thermal transfer permissible into the building through its walls or roof, due to solar heat gain and outdoorindoor temperature dierence, to be determined using Eqs. (1) and (2) below. The compliance criterion for the OTTV of roofs was a constant value of 26.8 W/m2, but that for OTTV of walls would vary with the latitude of the building site. OTTVw OTTVR Uw Aw TDEQ Af SF SC Uf Af DT Aw Af UR AR TDEQ 434:7 AS SC US AS DT AR AS (1)


where OTTVw is OTTV of a wall, W/m2; OTTVR is OTTV of a roof ( 26.8), W/m2; Uw, Uf, UR and US are thermal transmittance of the opaque part of a wall, a fenestration, the opaque v part of a roof and a skylight, respectively, W/m2 C; Aw, Af, AR and AS are area of the opaque part of a wall, a fenestration, the opaque part of a roof and a skylight, respectively, m2; TDEQ v is equivalent temperature dierence for the opaque part of a wall or a roof, C; SC is shading 2 coecient of a fenestration or a skylight; SF is solar factor, W/m ; DT is temperature dierence v between exterior and interior design conditions, C. As Eqs. (1) and (2) show, OTTV represents the total of three major components of envelope heat gain: conduction through opaque parts of walls and roofs; solar transmission through windows and skylights; and heat transfer through windows and skylights due to outdoorindoor temperature dierence. This resembles how envelope heat gains are determined in the CLTD/ CLF design cooling load calculation method rst introduced in 1977 by ASHRAE [16]. In the review for updating Standard 90A-80, the exterior envelope criteria were considered too simplistic to properly account for the interactions of the envelope with the complex energy

F.W.H. Yik, K.S.Y. Wan / Energy 30 (2005) 4171


ow within commercial buildings [17]. One major criticism was about the use of the equivalent temperature dierence (TDEQ) in the OTTV equation to account for the thermal storage eects of envelope elements [18]. The impact of the building envelope on cooling energy use is dependent on climate, building operation schedule, and three characteristics of the perimeter wall: the orientation, thickness and position of the insulation relative to the mass [19]. Furthermore, the contribution of conduction to the cooling energy is not as consistent as other heat gains, such as solar and lighting, and it can be very small and can be either positive or negative. Conduction loss can occur during some of the cooling hours for some buildings located at high latitudes [17]. The envelope performance requirements were also considered restrictive, as the envelope and the HVAC systems were treated independently, and the few number of factors considered limited design exibility [2024]. The use of OTTV was nally abandoned since ASHRAE launched Standard 90.1-1989 Energy Ecient Design of New Buildings Except New Low-rise Residential Buildings [13]. Instead, the prescriptive criteria put limits to the percentage of fenestration relative to the gross external wall area; the thermal transmittance (U-value) of envelope elements and fabric elements separating conditioned and unconditioned spaces; and the thermal resistances of slabs-on-grade and walls below grade. The permissible limits were dependent on the local weather conditions; shading coecient of fenestration; characteristics of shading device; and heat capacitance of wall and position of insulation [13,24,25]. Alternative compliance paths had also been introduced in Standard 90.1, which included performance-based criteria that were based on the cumulative heating and cooling energy ux to allow trade-os among dierent envelope assemblies, and the Energy Cost Budget approach, which oered even greater exibility for meeting the Standard requirements. In 1992, the US Energy Policy and Conservation Act required every state in the US to certify, before October 1994, its energy codes would meet or exceed the requirements of the ASHRAE Standard 90.11989 [3,26,27]. Ten years later, a new version of ASHRAE Standard 90.1-1999 [28] was issued. Since then, Standard 90.1 will be re-issued on regular three-year cycles, for incorporating changes resulting from continuous maintenance proposals from the public. The latest version of Standard 90.1 has been published in 2001 [29].

3. Use of OTTV for regulating building energy performance in Asian countries Among Asian countries, Singapore was the rst to have regulatory control over the OTTV of external walls of air-conditioned buildings (since 1979). Details of the control were stipulated in a Singaporean Standard [5,30]. Four years later, the Handbook was revised [5], which included the introduction of a new standard on OTTV for roofs with skylights and a new method for determining the coecients in the OTTV equation to account for the eects of external shading devices at exterior walls. Turiel et al. [31,32] reviewed the OTTV standard and showed that the term for solar gain through windows in the OTTV formulation understated, but the conduction terms for walls and windows exaggerated the rates of heat transfer through the respective paths. They recommended the OTTV formulation be revised into a single term equation, accounting only for the solar heat gain.


F.W.H. Yik, K.S.Y. Wan / Energy 30 (2005) 4171

Later, Chou and Lee [33,34] reviewed both the OTTV formulation in the Standard as well as that proposed by Turiel et al. In their attempt to derive an OTTV equation for Singaporean buildings, they dened OTTV as the annual heat gain of the air-conditioned spaces in a building from the envelope during both air-conditioned and non-air-conditioned periods, averaged over the total air-conditioned hours throughout the year and normalised by the envelope area enclosing such spaces, as Eq. (3) depicts. This was based on the consideration that the heat gain would ultimately contribute to the cooling load on air-conditioning systems [35]. OTTV Total heat gain through building envelope W=m2 Total air-conditioned hours Envelope area (3)

Note that Eq. (3) is for evaluation of OTTV based on predicted heat gains from detailed computer simulations. The OTTV predictions can then be used in regression analyses or other methods to evaluate the coecients