Building Envelope - Group B (ETTV)

Calculation of ETTV Group B

Alan, Jaz, Xuan Xuan, Si yu, Fang Cong

Content ETTV equation

Computing steps of ETTV

Work example 1

Work example 2

Sensitivity analysis

Quiz

ETTV Equation

ETTV Equation • Where more than one type of material and/or fenestration is used:

ETTV Equation • ETTV of the whole building envelope is obtained by taking the weighted

average of individual walls at different orientations.

Computation of ETTV

• First step: calculation of different areas with different material

• Second step: computation of U-value for different types of building envelope

• Third step: find out the shading coefficient (SC) for different types of glass

• Forth step: find out the correction factors (CF) for solar heat gain through fenestration in different orientation

• Last step: use ETTV equation to calculate the total ETTV

1st step: Areas • South-west elevation

Beam

Glass window

Glass window

Glass window

Brick wall

60000

1800

2400

350

3150

1st step: Areas

• Area of beam:

– Aw1=350x60000

• Area of glass windows:

– Af1=2400x1800

• Area of brick wall:

– Aw2= 3150x60000-Af1

2nd step: U-value • Thermal conductivity, k

– Material’s ability to conduct heat

• Thickness, b

• Thermal resistance, R

– R=b/k

• Thermal transmittance, U

– U=1/R

3rd step: SC • A measure of the total amount of heat passing through the

glazing compared with that through a single 3mm clear glass

• SC=solar heat gain of any glass and shading combination / solar heat gain through a 3mm unshaded clear glass

• Between 0 to 1

• The lower the better

• Provided by manufacturer

• Affected by external shading devices

Shading Coefficient

4th step: CF • The solar correction factors for eight primary orientations of the walls

have been determined for the Singapore climate.

• the eight primary orientations are segmented as follows:

4th step: CF Solar correction factors (CF) for walls

5th step: ETTV • Find out individual ETTV of walls of different orientations

• Calculate the overall ETTV

Work example 1

• An single-storey office has the internal layout plan shown in Fig.1. Section detail of the walls are shown in section A-A and B-B. The office is orientated in the North, East, South and West directions with the front façade facing the south.

• Calculate the ETTV for the building envelope.

Fig. 1

Section A-A

250mm r. c beam

Single glazing

250 mm r. c beam

200mmr.c wall

Section B-B

Answer (i) South façade: (a) Single glazing Af1 : 3.6 x 45= 162.0m2

(b) 250mm r.c beam Aw1:1.1 x 45 = 49.5m2

(ii) East façade: (a) Single glazing Af1 : 3.6 x 25 = 90m2

(b)250mm r.c beam Aw1 : 1.1 x 25 = 27.5m2

(iii) North façade: (a) 200mm r.c wall Aw2 : 4x45=180 m2

(b) r.c beam Aw1 :0.7x45 = 31.5m2

(iv) West façade: (a) 200mm r.c wall Aw2 : 4x25 =100m2

(b) r.c beam Aw1 : 0.7x25 =17.5m2

U-Value Calculation • (a) 8mm single glazing (South & East Facades)

R = 0.044+0.008/1.053 +0.12 =0.172m K/W U=1/R=5.82W/m2

SCg = 0.61(by manufacturer)

U-Value Calculation

• (b) 250mm r.c beam

R=0.044+0.012/1.298+0.25/1.442+0.012/0.053+0.12=0.369m2

U=1/R=2.71W/m2 K

U-Value Calculation

• (c) 200mm r.c wall(North and west facades)

R=0.044+0.012/1.298+0.2/1.442+0.012/0.533+0.12=0.334m2 K/W

U=1/R=2.99W /m2 K

ETTV Calculation General Equation:

ETTV=1/Ao [12(Aw x Uw )+3.4(Af xUf )+211xCF(Af xSC)]

• South façade(CF=0.83)

ETTVs =1/211.5[12(49.5x2.71)+3.4(162x5.82)+211x0.83(162x0.61)=104.59W/m2

• East facade (CF =1.13)

ETTVe =1/117.5[12x(27.5x2.71)+3.4(90x5.82)+211x1.13x(90x0.61)=134.17W/m2

• North façade(CF=0.80)

ETTVn =1/211.5[12x(180x2.99+31.5x2.71)=35.38W/m2

ETTV Calculation • West façade

ETTVw = 1/117.5[12x(100x2.99+17.5x2.71)]= 35.38W/m2

• Overall for Whole Building

ETTV= (104.57x211.5+ 134.17x 117.5+ 35.38x 211.5+35.38x117.5)/(211.5+117.5+ 211.5+117.5)

= 75.26W/m2

Work Example 2

The internal lay-out plan of an single-story office building is shown in Fig(A). Typical section details of wall are shown in section A-A, section B-B, section C-C. Calculate the ETTV for the building envelope.

Answer (Areas)

1. South-west façade

250mm RC Beam: Aw1= 0.5x20=10m2

Single glazing wall: Af1 = 3x20=60m2

2. West-north façade

250mm RC Beam : Aw1= 1x30=30m2

200mm RA Wall : Aw1= 2.5x30=70m2

Areas 3. North-east façade

250mm RC Beam : Aw1= 0.5x20=10m2

Double glazing window: Af1 = 2x20=40m2

200mm RC Wall : 1x20=20m2

4. East-south façade

250mm RC Beam : 1x30=30m2

200mm RC Wall : 2.5x30=70m2

U-value calculation 1. 250mm RC Beam

Rt=0.044+0.012/1.298+0.012/0.533+0.25/1.422+0.12=0.3691m2

k/w

U=1/Rt=2.71w/m2k

2. 8mm single glazing wall

Rt=0.044+0.008/1.053+0.12=0.1716m2k/w

U=1/Rt=5.83w/m2k

SC=0.5(By manufacturer)

U-value calculation 3. 200mm RC Wall

Rt=0.044+0.02/1.154+0.02/0.37+0.2/1.442+0.12= 0.3741m2k/w

U=1/=2.67w/m2k

4. Double glazing window

Rt=0.044+0.008/1.053+0.008/1.053+0.118+0.12= 0.290m2k/w

U=3.448w/m2k

SC=0.5(By manufacturer)

ETTV calculation General equation:

ETTV=1/Ao [12(Aw * Uw )+3.4(Af *Uf )+211*CF(Af *SC)]

1. South-west façade(CF=1.06)

ETTVW=1/70[12(10*2.71)+3.4(60*5.83)+211*1.06(60*0.7)]=155.83w/m2

2. West-north façade

ETTVN=1/100[12(30*2.71+70*2.67)]=32.18w/m2

ETTV calculation 3. North-east façade(CF=0.97)

ETTVE=1/70[12(10*2.71+20*2.67)+3.4(40*3.448)+211*0.97(40*0.5)]=78.98w/m2

4. East-south façade

ETTVS=1/100[(12(30*2.72+70*2.67)]=32.18w/m2

Total ETTV:

ETTVT=(155.83*70+32.18*100+78.98*70+32.18*100)/(70+100+70+100) = 67.27w/m2

Sensitivity Analysis of ETTV

Sensitivity Analysis • ETTV can be calculated by summing up three basic components of heat

gain through building envelope. These are

– Heat conduction through opaque walls

– Heat conduction through transparent window

– Solar radiation through transparent window

• Therefore, analysis can be done based on each component.

Sensitivity Analysis

Heat conduction through opaque walls and transparent windows •Thermal transmittance ( U- value)

– 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

– The smaller value of thermal transmittance of the material of being used, the smaller amount of heat will be transmitted.


Heat conduction through opaque walls and transparent windows • Surface air film resistance ( Façade orientation and inclination)

– The transfer of heat to and from a surface of a body through air is reduced by the presence of a thin layer of relatively motionless air at the surface of the body

– Surface air film resistance is affected by wind velocity and therefore different resistance values for outside and inside air films are given.

– If you want to increase the thermal resistance, then you should use high emissive surface and try to reduce the slope of the roof as much as possible.


Heat conduction through opaque walls and transparent windows • Air space resistance

– Air is a relatively poor conductor of heat.

– The concept is the same as the air film resistance

– In addition, reflective material can be inserted in an air space to increases the thermal resistance of the air space.


Solar Radiation through transparent window. • Shading coefficient of fenestration (SC)

– The ratio of solar hear gain through the fenestration system having combination of glazing and shading device to the solar heat gain through an unshaded 3mm clear glass.

– The factors affecting SC are

• Inter – block shading

• Direct shading

• Diffused shading

• Reduced surface temperature

Sensitivity Analysis Solar Radiation through transparent window Shading coefficient of fenestration (SC)

– Inter – block shading

• Neighboring buildings overshadowing another

• This will inadvertently affect the heat gains through façade

• Singapore’s ETTV methodology does not consider this factor

• Because assumption changes when the adjacent building changes


Solar Radiation through transparent window Shading coefficient of fenestration (SC)

– Direct shading

• Shading devices such as perforated screens


Solar Radiation through transparent window. •Shading coefficient of fenestration (SC)

– Indirect shading

• The sky factor was used as a benchmark figure

• Code’s standard procedures ignores this

– Reduced conduction heat gain

• Relates to heat gain through the opaque building envelope

• due to direct and diffused shading


Solar Radiation through transparent window. •Solar data

– Solar geometry

• The position of the sun

– Shadow angel

– Intensity of solar radiation


ETTV • Area of opaque wall or transparent windows (Window-to-wall

ratio) – The bigger the area, the more heat will be transmitted

Quiz 1 (matching)

1st step U – value

2nd step CF

3rd step Areas

4th step ETTV

5th step SC

Answer

1st step Areas

2nd step U – value

3rd step SC

4th step CF

5th step ETTV

Quiz 2 (matching)

Thermal transmittance Reflective material can be inserted

Air space resistance Shading devices

Surface air film resistance Assumption changes when the adjacent building changes

Direct shading

The quantity of heat that flows through a unit area of a building section

Inter – block shading

Affected by wind velocity

Answer Thermal transmittance The quantity of heat that flows through a

unit area of a building section

Air space resistance reflective material can be inserted

Surface air film resistance affected by wind velocity

Direct shading

Shading devices

Inter – block shading

assumption changes when the adjacent building changes

Documents

Building Envelope - Group B (ETTV)