Code on Envelope Thermal Performance for Buildings

The aim of this Code is to assist architects and professional engineers to comply with the envelope thermal performance standards prescribed in the Building Regulations.

This Code covers the following Envelope Thermal Performance Standards:

1. Envelope Thermal Transfer Value (ETTV) for air-conditioned non residential buildings

2. Roof Thermal Transfer Value (RTTV) for air-conditioned non residential buildings (with skylight)

3. Residential Envelope Transmittance Value (RETV) for residential buildings

4. Roof insulation for air-conditioned non-residential buildings (without skylight) and residential buildings

Since 1979, the Building Control Regulations had prescribed an envelope thermal performance standard know as Overall Thermal Transfer Value (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 given the name ‘Envelope Thermal Transfer Value’ (ETTV) to differentiate it from the original OTTV formula.

A similar review of the OTTV formula for roof was also conducted and a new formula, known as ‘Roof Thermal Transfer Value’ (RTTV), replaces the Roof OTTV formula.

The ETTV requirement does not apply to non air-conditioned buildings such as residential buildings that are designed to be naturally ventilated. However, as it becomes increasingly common for residential buildings to be air-conditioned, there is a need to regulate the design of their envelopes so that heat gain into the interior spaces and hence air-conditioning energy used can be minimised.

Based on the results of a NUS study commissioned by BCA, the ETTV concept was extended in 2008 to cover residential buildings. As the air conditioners in residential buildings are usually turned on in the night, the envelope thermal performance standard for residential buildings is given the name Residential Envelope Transmittance Value (RETV) so as to differentiate it from ETTV, which is meant for buildings that operate the air conditioning system during the day.

The ETTV takes into account the three basic components of heat gain through the external walls and windows of a building. These are:

heat conduction through opaque walls,

heat conduction through glass windows,

solar radiation through glass windows.

These three components of heat input are averaged over the whole envelope area of the building to give an ETTV that represents the thermal performance of the whole envelope. For the purpose of energy conservation, the maximum permissible ETTV has been set at 50 W/m².

The ETTV formula is given as follows:

ETTV = 12(1-WWR) U_w + 3.4(WWR)U_f + 211(WWR)(CF)(SC)

where

ETTV : envelope thermal transfer value (W/m²)

WWR : window-to-wall ratio (fenestration area / gross area of exterior wall)

U_w : thermal transmittance of opaque wall (W/m² K)

U_f : thermal transmittance of fenestration (W/m² K)

CF : correction factor for solar heat gain through fenestration

SC : shading coefficients of fenestration

Where more than one type of material and/or fenestration is used, the respective term or terms shall be expanded into sub-elements as shown:

where

A_w1, A_w2, A_wn : areas of different opaque wall (m²)

A_f1, A_f2, A_fn : areas of different fenestration (m²)

A_o : gross area of the exterior wall (m²)

U_w1, U_w2, U_wn : thermal transmittances of opaque walls (W/m² K)

U_f1, U_f2, U_fn : thermal transmittances of fenestrations (W/m² K)

SC_f1, SC_f2, SC_fn : shading coefficients of fenestrations

CF : correction factor for solar heat gain through fenestration

As walls at different orientations receive different amounts of solar radiation, it is necessary in general to first compute the ETTVs of individual walls, then the ETTV of the whole building envelope is obtained by taking the weighted average of these values. To calculate the ETTV for the envelope of the whole building, the following formula shall be used:

where

A_o1, A_o2, A_on : gross areas of the exterior wall for each orientation (m²)

The solar correction factors for eight primary orientations of the walls have been determined for the Singapore climate. They are given in Table C1.

If the wall is curved, the eight primary orientations are segmented as follows:

If the roof of an air-conditioned building is provided with skylight, the ETTV concept is also applicable to its roof. To differentiate between the walls and the roof, the term Roof Thermal Transfer Value (RTTV) is used instead. Similarly, RTTV takes into consideration the three basic components of heat gain through the opaque roof and skylight. These are:

heat conduction through opaque roof,

heat conduction through skylight,

solar radiation through skylight.

The maximum permissible RTTV has also been set at 50 W/m².

The RTTV formula is given as follows:

RTTV = 12.5(1-SKR)U_r + 4.8(SKR)U_s + 485(SKR) (CF) (SC)

where

RTTV : roof thermal transfer value (W/m^₂)

SKR : skylight ratio of roof (skylight area / gross area of roof)

U_r : thermal transmittance of opaque roof (W/m² K)

U_s : thermal transmittance of skylight area (W/m² K)

CF : solar correction factor for roof

SC : shading coefficient of skylight portion of the roof

Similarly, when more than one type of material and or skylight is used, the respective term or terms shall be expanded into sub-elements, such as;

A_r1, A_r2, A_rn : areas of different opaque roof (m²)

A_s1, A_s2, A_sn : areas of different skylight (m²)

A_o : gross area of roof (m²)

U_r1, U_r2, U_rn : thermal transmittances of opaque roofs (W/m² K)

U_s1, U_s2, U_sn : thermal transmittances of skylights (W/m² K)

SC_s1, SC_s2, SC_sn : shading coefficient of skylights

If a roof consists of different sections facing different orientations or pitched at different angles, the RTTV for the whole roof shall be calculated as follows:

where

A_o1, A_o2, A_on : gross areas of the roof for each section (m²)

The solar correction factors for roof are given in Table C2.

The RETV is similar to ETTV in that it takes into consideration the three basic components of heat gain through the external walls and windows of a building. These are:

1. heat conduction through opaque walls,

2. heat conduction through glass windows, and

3. solar radiation through glass windows.

These three components of heat input are averaged over the whole envelope area of the building to give an RETV that represents the thermal performance of the whole envelope. For the purpose of energy conservation, the maximum permissible RETV has been set at 25 W/m².

The RETV formula is given as follows:

RETV = 3.4(1-WWR)U_w + 1.3(WWR)U_f +58.6(WWR)(CF)(SC)

where

RETV : residential envelope transmittance value (W/m²)

WWR : window-to-wall ratio (fenestration area/gross area of exterior wall)

U_w : thermal transmittance of opaque wall (W/m² K)

U_f : thermal transmittance of fenestration (W/m² K)

CF : correction factor for solar heat gain through fenestration

SC : shading coefficients of fenestration

Where more than one type of material and/or fenestration is used, the respective term or terms shall be expanded into sub-elements as shown:

where

A_w1, A_w2, A_wn : areas of different opaque walls (m²)

A_f1, A_f2, Afn : areas of different fenestrations (m²)

A_o : gross area of the exterior wall (m²)

U_w1, U_w2, U_wn : thermal transmittances of opaque walls (W/m² K)

U_f1, U_f2, U_fn : thermal transmittances of fenestrations (W/m² K)

SC_f1, SC_f2, SC_fn : shading coefficients of fenestrations

The external façades of the living, dining, study and bedrooms are considered in the RETV computation. As walls at different orientations receive different amounts of solar radiation, it is necessary in general to first compute the RETVs of individual walls, then the RETV of the whole building envelope is obtained by taking the weighted average of these values. To calculate the RETV for the envelope of the whole building, the following formula shall be used:

where

A_o1, A_o2, A_on : gross areas of the exterior wall for each orientation (m²)

The solar correction factors for walls are given in Table C3. For an example of the orientation of the wall, please refer to section 5.4.1.

Should the building’s WWR and SC¹ fall within any one of the following sets of criteria, the building is deemed to have complied with RETV and; hence, is exempted from computing RETV.

WWR_Bldg < 0.3 and SC¹ _facade < 0.7

or

WWR_Bldg < 0.4 and SC¹ _facade < 0.5

or

WWR_Bldg < 0.5 and SC¹ _facade < 0.43

where

WWR : Window to wall ratio

SC : Shading coefficient of fenestration = SC_Glass X SC_{shading device}

Note: This is applicable to buildings with external masonry walls.

¹Each SC of the facade must not exceed the prescribed value.

Otherwise detailed RETV computation is required.

Solar heat gain into a building through an uninsulated roof increases air temperature indoor. In all buildings, directional radiation received on the roof can be one of the main causes of thermal discomfort.

For an air-conditioned building, solar heat gain through the roof also constitutes a substantial portion of the cooling load. From on-site solar radiation measurements taken in Singapore, the intensity of radiation on a horizontal surface can be as much as 3 times of that on a vertical surface.

The purpose of roof insulation is therefore to conserve energy in airconditioned buildings. The building regulations require that the average thermal transmittance (U-value) for the gross area of the roof shall not exceed the limit in the Table C4 for the corresponding weight group.

If the roof of an air-conditioned non-residential building is provided with skylight then RTTV computation is required.

THERMAL TRANSMITTANCE (U-VALUE) CALCULATION

The ability of a material to transmit heat is measured by its thermal conductivity or k-value. The k-value of a material is defined as the quantity of heat transmitted under steady-state conditions through unit area of the material of unit thickness in unit time when unit temperature difference exists between its opposite surfaces. It is expressed in W/m K. Table C5 gives the k-values of some commonly used building materials.

The thermal resistivity of a material is the reciprocal of its thermal conductivity, i.e.

It may be defined as the time required for one unit of heat to pass through unit area of a material of unit thickness when unit temperature difference exists between opposite faces. It is expressed as m K/W.

Thermal conductance refers to specific thickness of a material or construction. It is the thermal transmission through unit area of a material per unit temperature difference between the hot and cold faces. It is expressed in W/m2 K and is given by:

where b is the thickness of the material (m)

The thermal resistance of a material or construction is the reciprocal of its thermal conductance. It refers to the thermal resistance of any section or assembly of building components and is particularly useful in computing the overall transfer of heat across the building section. It is expressed as m² K/W and is given by:

The thermal transmittance or U-value of a construction is defined as the quantity of heat that flows through a unit area of a building section under steady-state conditions in unit time per unit temperature difference of the air on either side of the section. It is expressed in W/m² K and is given by:

where R_T is the total thermal resistance and is given by:

where

R_o : air film resistance of external surface (m² K/W)

R_I : air film resistance of internal surface (m² K/W)

k₁, k₂, k_n : thermal conductivity of basic material (W/m K)

b₁, b₂, b_n : thickness of basic material (m)

The transfer of heat to and from a surface of a body through air is impeded by the presence of a thin layer of relatively motionless air at the surface of the body. This offers resistance to the heat flow and results in a temperature drop across the layer of air.

Surface air film resistance is affected by wind velocity and therefore different resistance values for outside and inside air films are given. These are defined as follows:

R_o : outside surface air film resistance (moving air)

R_i : inside surface air film resistance (still air)

Table C6 gives the values of surface resistances for walls and roofs at different positions of surface and for different surface emissivity values.

Air is a relatively poor conductor of heat. Its presence as a gap between two layers of materials contributes further thermal resistance to the whole construction. The U-value of a building section can therefore be modified as follows:

where

R_a : thermal resistance of air space

Reflective materials such as aluminium foil have high surface reflectivity and low surface emissivity. If a reflective foil is inserted in an air space with its reflective surface facing the space and against the direction of heat flow as shown below, approximately 95% of the radiation will be reflected. This increases the thermal resistance of the air space.

If the heat flow is reversed as shown below, the result would be the same, as in this case, the low emissivity of the reflective surface emits only about 5% of the absorbed heat as radiant energy.

Table C7 gives the values of air space resistances for walls and roofs at different positions and for different surface emissivity values in the air space.

SHADING COEFFICIENT OF SUN SHADING DEVICES

The position of the sun can be specified by the angles illustrated below:

These angles are (i) altitude (α, angle above the horizon) and (ii) azimuth (z, compass orientation of a vertical plane through the sun, measured clockwise from north). The orientation of a wall is the angle measured clockwise from north of a plane normal to the wall and the wall-solar azimuth (τ) is the angle between the two planes.

For the purpose of finding the shading effect of horizontal projections, fins, louvers, or canopies, the vertical shadow angle (VSA) is required. This is the angle (θ₁) between two planes viz, the horizontal plane and an inclined plane projected through the sun as illustrated in the diagram below:

The vertical shadow angle is given by:

tan θ₁ = tan α sec τ

where

θ₁ : the vertical shadow angle

α : the altitude of the sun

τ : the wall-solar azimuth

To calculate the shading coefficient of vertical fins and projections, the horizontal shadow angle (HSA) has to be determined and it is given by the wall-solar azimuth angle (τ), i.e.

θ₂ = τ

where

θ₂ : the horizontal shadow angle

To facilitate the calculation of the effective shading coefficient of external shading devices, the intensities of diffuse, direct and total radiation transmitted through a standard 3mm clear glass sheet are tabulated in Tables C8 to C11 together with the horizontal and vertical shadow angles for March, June, September and December.

In the RETV formula, the solar factor has been derived from the annual average of solar radiation transmitted through a 3mm clear glass window. For other system of fenestration, the rate of solar heat gain is modified by the shading coefficient of the fenestration system which is defined as the ratio of solar heat gain through the fenestration system having combination of glazing and shading device to the solar heat gain through an unshaded 3mm clear glass. This ratio is a unique characteristic of each type of fenestration system and is represented by the equation:

In general, the shading coefficient of any fenestration system can be obtained by multiplying the shading coefficient of the glass (or effective shading coefficient of glass with solar control film where a solar control film is used on the glass) and the shading coefficient of the sun-shading devices as follows:

SC= SC₁ x SC₂

where

SC : shading coefficient of the fenestration system

SC₁: shading coefficient of glass or effective shading coefficient of glass with solar control film where a solar control film is used on the glass

SC₂ : effective shading coefficient of external shading devices

Note: 1. For the purpose of RETV calculation, the shading effect offered by internal blind and curtain should be ignored.

The shading coefficient of the glass or effective shading coefficient of glass with solar control film should be based on the manufacturer’s recommended value.

The effective shading coefficient of external shading devices as given in Tables C12 to C23 shall be used unless the type of shading device in question is not included in the tables. In that case, the effective shading coefficient shall be calculated from the basic solar data given in Tables C8 to C11 in accordance with the method specified in Section B2.2.

Method of calculating effective shading coefficient of external sun shading device

When a window is partially shaded by an external shading device, it is assumed (for simplicity and for the purpose of RETV computation) that the exposed portion receives the total radiation, I_T, and the shaded portion receives only the diffuse radiation, I_d.

The instantaneous heat gain due to solar radiation can then be expressed as follows:

Q = A_e x I_T + A_s x I_d

= A_e x I_D + (A_e + A_s ) x I_d

where

Q : solar heat gain

A_e : exposed area of window

A_s : shaded area of window

I_T : total radiation

I_D : direct radiation

I_d : diffused radiation

Since

A = A_e + A_s

Therfefore

Q=A_e x I_D + A x I_d

For an unshaded 3mm clear glass, the solar heat gain is given by A x I_T. By definition, the hourly Shading Coefficient, SC, of a shading device can be expressed as:

where

To calculate the shading coefficient (SC) of a shading device for the whole day, the hourly solar heat gain shall be computed and summed up for the 12 daylight hours. The total solar heat gain is then divided by the sum of the total radiation, I_T, through an unshaded 3mm clear glass for the same hours of the day, to obtain the SC for the day. Mathematically, the computation can be expressed as follows:

For simplicity, the SC of a shading device for a particular month can be worked out basing on the solar data for a representative day of the month.

To determine the effective SC of a shading device, theoretically, the computation has to be carried out for 12 months of the year. However, as the computation involved is rather tedious and the degree of accuracy required is not a critical factor, it is deemed sufficient to base the SC computation on 4 representative months of the year, viz. March, June, September and December. The representative days of these 4 months are March 21, June 22, September 23 and December 22.

Further, since the solar data for March 21 and September 23 are almost identical, it suffices to compute the solar heat gain for March and double it to take account of the heat gain for September. Mathematically, the effective SC of a shading device is given by:

Where

M denotes March

J denotes June

S denotes September

D denotes December

The relevant solar data are given in Tables C8 to C11.

The fraction of window area exposed to the sun (G) at any time for a given orientation can be determined using solar geometry. With the VSA and HSA given, the G factor can be worked out graphically. For simple design, the G factor can also be calculated using plane trigonometry. In the following examples of calculating the G factor for simple horizontal overhangs, vertical fins and egg-crate sun-shades using trigonometry, the following convention is used:

θ1 = VSA (always positive)

θ2 = HSA (positive, if to the right of wall orientation; if to the left of wall orientation)

Φ₁ = projection angles of horizontal projections with respect to horizontal plane (assume positive for practical reason).

Φ₂ = projection angle of vertical fin with respect to wall orientation (positive, if to the right of wall orientation;negative, if to the left of wall orientation)

For continuous horizontal projection fixed at window head level

Where

Notes:

1. G₁ ≥ 0

2. Table C12 to Table C15 give the SC of horizontal projections for a range of R₁ values with Φ₁ ranging from 0º to 50º.

Notes:

1. G2 ≥ 0

2. Table C16 to Table C19 give the SC of vertical fins for a range of R2 values with / Φ₂ / ranging from 0^o to 50^o. Φ₂ is chosen for the situation which gives the lower SC of the two possible values, viz positive or negative Φ₂ .

For egg-crate and combination fins made up of horizontal and vertical components for which the horizontal component may be sloped.

G₁ = 1-R₁ (cosΦ₁ tanΦ₁ - sinΦ₁)

G₂ = 1-R₂ | tan Φ₂ |

Since G1 and G2 are independent of each other, the combined effect of the two components can be expressed as follows:

G₃ =G₁ x G₂

Notes:

1. G3 ≥ 0

2. Table C20 to Table C23 give the SC of combination fins for a range of R₁ and R₂ values with Φ₁ ranging from 0º to 40º.

The following examples are meant to illustrate how the SC of a shading device is calculated from first principle.

Find the effective shading coefficient (SC) of a sloping horizontal projection 1m in length, inclined at 15^o and located over a window of 2m in height, in a North-East facing direction.

Find the effective SC of an egg-crate shading device having R₁ = 0.4, Φ₁ = 0, R₂ = 0.4 in the North-facing direction.

The same procedure is repeated for March 21, September 23 and December 22 in order to work out the effective SC for the whole year.

Find the effective SC of a shading device parallel to the wall as shown in the diagram below. It is installed in a North-facing wall.

The glass window is shaded by a panel parallel to the wall. The shadow cast on the window varies according to the time of the day depending on the sun’s position and its vertical shadow angle (θ₁).

For 68.2º < θ₁ < 90º, the shading device is ineffective as sun rays strike the window directly. See figure (b). For θ₁ = 45º, the window is totally shaded. See figure (c).

For θ₁ < 45º, the window is partially shaded by the lower portion of the strip. See figure (d).

The shadow patterns for figure (c) and figure (d) can be worked out by simple geometry.

Given : Window on South-West facing wall with a 0.3m horizontal overhang.

Find : The effective shading coefficient if (a) height of window is 0.6m; (b) height of window is 0.75m with the overhang inclined at 30º to the horizontal.

Solution : Refer to Table C15

a) R₁ = 0.5 SC = 0.698

b) R₁ = 0.4 SC = 0.669

Given : Window on West facing wall with a 0.3m horizontal overhang and height of window is 0.75.

Find : The effective shading coefficient if the window is located 0.2m below the overhang.

Solution : Assuming the window has a height h and extends to the underside of the overhang, the solar heat gain into the window can be expressed as follows:

SCHEDULES OF TABLES

Note:

1. The correction factors for other orientations and other pitch angles may be obtained by interpolation.

Note:

1. The correction factors for other orientations and other pitch angles may be obtained by interpolation.

Notes:

1. Ordinarily, high emissivity is assumed for surfaces of building materials with reasonably smooth finishing. Low emissivity applies only to internal surface if the surface is very reflective, such as that of an aluminium foil.

2. Interpolation between the angle of slope from horizontal to 45° is permitted. For angle beyond 45°, the value for 45° can be used; no extrapolation is needed.

Notes:

1. Ordinarily, high emissivity is assumed for air spaces bounded by building materials of moderately smooth surfaces. Low emissivity only applies where one or both sides of the air space are bounded by a reflective surface such as that of an aluminium foil.

2. Interpolation with the range of pitch angles from horizontal to 45° is permitted. For angles beyond 45°, the value for 45° can be used; no extrapolation is needed.

3. Interpolation within the range of thickness from 5mm to 100mm is permitted. For air spaces less than 5mm, extrapolation basing on R_a = 0 for zero thickness is allowed; otherwise R_a is assumed to be zero. For air spaces greater than 100mm, the R_a for 100mm should be used; i.e. extrapolation is not permitted.

4. In the case of air space in roof, reflective foil used should be installed with the reflective surface facing downward as dust deposit will render an upward-facing surface ineffective after a while.

Orientation: North & South

Note:

1. For the purpose of calculating shading coefficient, the solar data for the North orientation can be used for the South orientation.

Orientation: East & West

Note:

1. For the purpose of calculating shading coefficient, the solar data for the East orientation can be used for the West orientation.

Orientation: North-East & North-West

Note:

1. For the purpose of calculating shading coefficient, the solar data for the North-East orientation can be used for the North-West orientation.

Orientation: South-East & South-West

Note:

1. For the purpose of calculating shading coefficient, the solar data for the South-East orientation can be used for the South-West orientation.
   

Orientation: North & South

Orientation: East & West

Orientation: North-East & North-West

Orientation: South-East & South-West

Orientation: North & South

Orientation: East & West