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CONTENTS Lesson 1 Calculation and Application of OTTV and U-value What is U-value? What is OTTV? Building Design and OTTV OTTV Requirements in Hong Kong Summary and Further reading

What is U-value? Lesson 1 - What is U-value?

U-value is often called the overall heat transfer coefficient. It is a factor for consideration in the design of buildings and choose of building materials. Lesson 1 - What is U-value?

U-value is a measure of the rate of heat loss through a material over 1 square meter of area for every temperature degree difference, under *a standardized condition. *The usual standard is at a temperature gradient of 24 °C, at 50% humidity with no wind. *The usual standard is at a temperature gradient of 24 °C, at 50% humidity with no wind is the heat loss in Watt W is the area calculated in square metre m² is the degree difference in Kelvin scale K Lesson 1 - What is U-value?

The lower the U-value, the slower the rate of heat transfer per unit square and the higher its insulting value, which is desirable in building construction. Lesson 1 - What is U-value?

Light-weight Concrete (1800kg/m³) 1.13 Reinforced Concrete Brick Hardwood Softwood Double glazing MATERIAL U-VALUE Softwood 0.13 Hardwood 0.18 Brick 0.77 Light-weight Concrete (1800kg/m³) 1.13 Normal Concrete (2400kg/m³) 1.93 Reinforced Concrete 2.3 Low-E Double Glazing 2.8 Single Glazing 5.7 Source: www.puravent.co.uk Lesson 1 - What is U-value?

Lesson 1 - What is U-value?

[Example 1: Single Glazing] U-value of a standard single glazed window = 5.7 W/m²K For each degree difference (K) and per 1 m² of the surface area of glass, 5.7W of thermal energy is transferred through from its either side. Lesson 1 - What is U-value?

[Example 2: Low-E Double Glazing] U-value of a standard double glazed window = 2.8 W/m²K For each degree difference (K) and per 1 m² of the surface area of glass, 2.8 W of thermal energy is transferred through from its either side. Therefore low-E double glazing has better insulation performance. Lesson 1 - What is U-value?

Low-E glass (E = thermal emissivity) Metal oxide to reduce solar radiation entering indoor by reflecting the radiant heat In summer, heat radiation from the sun is reflected away In winter, heat originated from indoors can be reflected back inside, resulting in better heat insulation. Lesson 1 - What is U-value?

Veterinary Laboratory at Tai Lung, Sheung Shui has used low-e glass on its external façade to reduce solar heat gain. © Architectural Services Department Lesson 1 - What is U-value?

What is OTTV? Lesson 1 - What is OTTV?

OTTV is the Overall Thermal Transfer Value. It is a value which indicates the average heat gain into a building through the building envelope. Lesson 1 - What is OTTV?

[2 major components in OTTV] Heat conduction through opaque wall Qwc Solar radiation through window glass Qgs Lesson 1 - What is OTTV?

Basic principles of OTTV Calculation The amount heat transfer through a building envelope can be expressed as: where Q = total heat transfer through envelope (W) A = gross area of building envelope (m2) Lesson 1 - What is OTTV?

1 Peking Road uses a triple-glazed active wall system, combining 3 layers of low-E clear glass with a ventilated cavity that results in high light transmission and a low OTTV. Lesson 1 - What is OTTV?

Calculations of heat transfer through external walls in Hong Kong: Lesson 1 - What is OTTV?

The OTTV equation for external walls in Hong Kong: Lesson 1 - What is OTTV?

External shading of IFC 2 is not considered as part of the roof in OTTV calculation. Lesson 1 - What is OTTV?

[Extended Knowledge] The general OTTV equation is used in the United States and other countries. Lesson 1 - What is OTTV?

The glass conduction term Qf is introduced. [Extended Knowledge] Three major difference between the general OTTV equation and the equation in Hong Kong: The glass conduction term Qf is introduced. A solar absorptivity term α is omitted. External shading multiplier EMS is not calculated. Lesson 1 - What is OTTV?

How does the design of building affect the OTTV value in Hong Kong? The OTTV value of a building is affected by the following factors: • Building Orientation (Temperature Difference) • Material of wall and roof (U-value) • External finish and colour of walls (Solar Absorptivity) • Type of Glass (Shading Coefficient) • Shading of windows (External Shading Multiplier) Lesson 1 - What is OTTV?

Comparing different dimensions of fenestration Study the above wall sections and the following information: a. All 3 walls are south-facing b. Assume the width of the walls are 1 m c. All 3 windows use tinted glass with a shading coefficient (SC) of 0.7 d. Solar Factor = 191 e. External shading multiplier (ESM) = 1 f. All walls and concrete beans have the same external surface and colour g. Solar absorptivity of all walls and beams = 0.58 h. Equivalent temperature difference (K) =1.4 K Given that the U-value of a 600mm concrete beam = 1.51 the U-value of a 100mm concrete panel = 2.32 Lesson 1 - What is OTTV?

Calculation of OTTV of the 3 walls per storey: Lesson 1 - What is OTTV?

Calculation of OTTV of the 3 walls per storey: Lesson 1 - What is OTTV?

Calculation of OTTV of the 3 walls per storey: Lesson 1 - What is OTTV?

[Discussion] By observing the drawings and the calculations, what conclusion can you draw regarding the relationship between the OTTV and the areas of the wall and the fenestration? Lesson 1 - What is OTTV?

OTTV1 > OTTV2 > OTTV3 [Result] OTTV1 > OTTV2 > OTTV3 The above calculations show that the OTTV is mainly governed by the ratio of opaque wall area to fenestration area. OTTV will increase the area of fenestration in the wall, meaning the extensive use of glass will lead to high OTTV (poor insulting performance). Lesson 1 - What is OTTV?

2. Comparing different fenestration materials Referring to Fig. 1, in wall section 2, if the tinted glass (SC = 0.7) is substituted by reflective glass (SC = 0.4), and other conditions remaining the same, how will the result change? Lesson 1 - What is OTTV?

OTTV2 in case 1 = 63.55W/m² > OTTV2 in case 2 = 36.57W/m² [Result] OTTV2 in case 1 = 63.55W/m² > OTTV2 in case 2 = 36.57W/m² From the above calculation, with the same proportion of wall and fenestration area, the OTTV of wall section 2 is reduced when reflective glass is used instead of tinted glass. Compared to the previous case where tinted glass (SC=0.7) is employed, reflective glass (SC=0.4) achieves a lower OTTV and hence provides a better insulting performance. Therefore, the type of glass for the fenestration used is an important factor for OTTV. Lesson 1 - What is OTTV?

OTTV requirement in Hong Kong The Building (Energy Efficiency) Regulation (B(EE)R) aims at reducing heat transfer through the building envelope and minimising electricity consumption for air-conditioning by requiring the external walls and roofs of commercial buildings to be designed and constructed to have a suitable Overall Thermal Transfer Value (OTTV) Lesson 1 – Application of OTTV?

OTTV requirement in Hong Kong The OTTV code was subsequently amended (for all buildings except residential buildings) in 1995 as follows: In the case of a building tower, the OTTV should not exceed 24W/m² In the case of a podium, the OTTV should not exceed 56W/m² open-front shops or the like on ground level may be exempted from the OTTV calculations upon applications Lesson 1 – Application of OTTV?

Exercise 1 Study the drawing and figures below and calculate the OTTV of this building. Assume that the four elevations of the building are identical. Calculate the OTTV of the building. Determine if this building follows the OTTV code in Hong Kong (24W/m²). U-value of a wall Uw = 1.9 W/m2K Outdoor temperature = 28.4oC Indoor temperature = 27oC Solar absorptivity of wall α = 0.58 External shading multiplier ESM= 1 Shading coefficient of window glass SC= 0.4 Solar factor SF = 191W/m2 Lesson 1 – Application of OTTV?

Reference answer Considering the tower part of the building: Total area of all windows on one façade Af = 2m x 2m x 18 = 72m2 Area of the wall Aw = (32m x 10m)-72m2= 248m2 Equivalent temperature difference TDeq = 28.4°C-27°C = 1.4°C Lesson 1 – Application of OTTV?

Exercise 2 [Discussion] Q1 What are the limitations of OTTV standard? Please study the article “Limitations of the OTTV Standard in Hong Kong ”, and information about OTTV Code [Discussion] Q1 What are the limitations of OTTV standard? Q2 What are the difficulties of implementing OTTV controls in Hong Kong? Q3 How can the government improve the standard of OTTV controls on buildings? Lesson 1 – Application of OTTV?

Q1 What are the limitations of OTTV standard? In controlling the energy efficiency of a building, OTTV has the following limitations: Cannot fully reflect how other parts of the building affect the heat gain Not precise and cannot ensure the efficient use of energy Limits the freedom of building design Shading from surrounding buildings affects the value measured OTTV might not be a valid standard in locations with extreme climate Lesson 1 – Application of OTTV?

Manual checking and calculation of OTTV value are not reliable. Q2 What are the difficulties of implementing OTTV controls in Hong Kong? Manual checking and calculation of OTTV value are not reliable. Lack of professional engineer responsible for OTTV submission. No control measures for the design and operation of existing buildings. Lesson 1 – Application of OTTV?

Q3 How can the government improve the standard of OTTV controls on buildings? Adopt a performance-based approach. Incorporate both mandatory and voluntary requirements for OTTV controls to accommodate greater flexibility in design. Promote the use of compliance software to calculate and check the OTTV. Research support on building energy standards. Study of energy data and design practices will help setting up criteria which are tailored according to local conditions. Provide education and training for designers and professional engineers on the calculation and evaluation process of OTTV. Regularly review the standard and set up an information network for information exchange with other countries. Lesson 1 – Application of OTTV?

Summary U-value is called the overall heat transfer coefficient. The lower the U-value, the higher the insulating value of the material. OTTV stands for the ‘Overall Thermal Transfer Value’ . The lower the OTTV, the higher the insulating performance of the building envelop. Size of the fenestration and the type of glass used are important factor for OTTV. OTTV has limitations to address the energy efficiency of a building at a certain context. A combined testing and tailored building energy standard can improve the reliability of OTTV.

Further Reading Sam C. M. Hui. Introduction to OTTV and Building Energy Simulation. http://www.arch.hku.hk/~cmhui/teach/65256-X.htm Wong Wah-sang and Chan Hon-wah Edwin, eds. Building Hong Kong: Environmental Considerations. Hong Kong University Press, 2000. Building (Energy Efficiency) Regulation Hong Kong Government, 1995. Building (Energy Efficiency) Regulation, Cap. 123 sub. Leg. M (L.N. 144 of 1995). http://www.legislation.gov.hk/blis_pdf.nsf/4f0db701c6c25d4a4825755c00352e35/25C2868DA2669A12482575EE003F079B/$FILE/CAP_123F_e_b5.pdf Hong Kong Green Building Technology Net. EMSD. http://gbtech.emsd.gov.hk/tc_chi/minimize/green_windows.html