The Thermal Performance of Building Form in the Urban Environment

PAUL HAY Capital Projects

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Topic:            Environment & Building Form

Author:          Paul Hay

e-mail:            paul.hay@phcjam.com

profile:           www.linkedin.com/in/phcjam

1.0     INTRODUCTION           

Heat input to buildings originates from internal & external sources.

1.1.1   External sources include (a) Solar Heat-gain, (b) Conduction through the envelope, and (c) Ventilation & Infiltration.

1.1.2   Internal sources include (a) Human occupation, (b) Lighting,  (c) Motors & appliances.

1.2       Heat energy is transferred from one body to another as the result of a temperature difference between them.

1.2.1   Sensible Heat transfers include (a) Solar heat-gain, and (b) Conduction through the envelope.

1.2.2   Latent Heat transfers include (a) 30% of heat from human occupation, (b) Ventilation & (c) processes (such as cooking);

1.3       First century architect, Vitruvius, wrote on the importance of wind to the urban environment and made recommendations for City Planning that are still currently relevant.

1.4       Wind should not be used as a generator of urban forms.

1.4.1.  Solar insolation should be considered

1.4.2.  Socioeconomic factors often prevail in the layout of city streets.

2.0     CLIMATIC PARAMETERS

2.1       Degree Days is the temperature difference above a threshold value that the average outdoor temperature attains on a day

2.1.1   Base temperature is typically 18 deg. C

2.1.2   Parameter is independent of orientation of building surface

2.2       Sol-air temperature is the theoretical temperature equivalent to the sum of outdoor air temperature, absorbed solar insolation, & long-wave heat exchange.

            Tsa = Ta + /h_o * I + /h_o * (MRT – Ta)                                                                           [2.1]

           

Where,

            Tsa = Sol-air temperature, deg. C                          Ta = Ambient air-temperature, deg. C

                  = Solar absorptance [ 0<<1]                         I   = Solar Insolation, W/m2

            h_{o    = External surface coefficient, W/m2-K}          = Emittance, W/m2-K

            MRT = Mean radiant temperature, deg. C

2.2.1   Long-wave exchange is generally neglected in calculations

2.2.2   Parameter is influenced by surface orientation.

3.0     NATURE OF WIND

3.1       Wind speed increases with altitude above the Earth=s surface.

3.1.1   Friction from the ground reduces wind speeds on the surface.

3.1.2.  The Gradient Height is the altitude above ground where the Earth=s friction has no effect on wind speed.

3.1.3   Gradient height varies from 270 m in open country to 450 m in urban areas.

3.2       Moderate gales, with speeds between 23 – 33 knots, are experienced as an annoyance when walking against the wind.

3.3       Strong gales, of speeds 41 - 47 knots, cause slight structural damage to buildings.

3.4       Whole gales, at 48 - 55 knots, can cause considerable structural damage.

4.0     VEGETATION

4.1       Deflected wind flow results in a zone of reduced pressure on the leeward side along with eddy currents.

4.2       Wind accelerates when blown beneath trees.

4.3       Wind accelerates more when shrubs or low walls are placed on the windward side of the trees, but is deflected upwards when these are placed on the leeward side of the tree.

5.0     LAYOUT OF STREETS

5.1       Orientation of streets have a major influence on velocity control in urban areas.

5.2       Current suggestions for ventilating streets vary according to climate.

5.2.1.  Wind speeds should be minimized in the hot dry tropics;

5.2.2.  Ventilation should be maximized in the humid tropics; and

5.2.3   Ventilation should be minimized in cold climates.

5.3       Wind entering narrow streets increase in speed because of the venturi effect.

5.4       Wind tunnel experiments can be used to verify wind effects.

6.0     BUILDING FORM

6.1       Building height and width also have a major influence on velocity control in urban areas.

6.2       Open structures split air streamlines such that one flows over the structure and the other through the structure.

6.3       Square buildings have the greatest volume of enclosed space relative to the exposed surface  area of their envelopes.

6.4       Victor Olgyay determined that elongating buildings along an east-to-west axis could improve their energy performance

6.4.1   Performance of a square building was used as a reference

6.4.2   Test buildings were insulated frame construction having 40% south glazing & 20% glazing on all other sides.

6.4.3   Tests were undertaken for four (4) different climates.

6.4.4   The influence of thermal mass was not evaluated.

 

Further Reading

Essentials of Physical Geography Today, Theodore M. Oberlander and Robert A. Muller;

Wind in Architectural and Environmental Design, Michele G. Melaragno;

Architectural Handbook, Alfred M. Kemper

Climate & Sensible Heat

PAUL HAY Capital Projects

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Sensible Heat & Climate

Author:          Paul Hay

e-mail:            paul.hay@phcjam.com

profile:           www.linkedin.com/in/phcjam

1.0   INTRODUCTION TO HEAT TRANSFER           

1.1_      The first law of thermodynamics states that energy cannot be created or destroyed, only transferred from one place to another, in one form or another.

1.2     Heat is energy transferred from one body to another as the result of a temperature difference between them:

1.2.1   Sensible Heat is heat energy that can be measured by a thermometer;

1.2.2   Latent Heat is stored energy that cannot be directly measured by a thermometer.

1.3       Heat transfer takes place in four different modes:

1.3.1   Radiation is the transfer of heat by electromagnetic waves which does not affect the medium through which it passes;

1.3.2   Convection is the transfer of heat through a fluid medium;

1.3.3   Conduction is the transfer of heat from one solid body to another through physical contact;

1.3.4   Phase Change is the transfer of heat which takes place when the medium undergoes a change of state.

2.0 SOLAR RADIATION INCIDENT ON THE EARTH VARIES

2.1       Solar Radiation is the short-wave heat input emitted from the sun;

2.2       Extra-terrestrial Radiation is solar radiation outside the Earth's atmosphere;

2.3       The intensity of extra-terrestrial radiation is constant over time;

2.4     The intensity of solar radiation incident on the Earth's surface varies during the course of a day because of  atmospheric attenuation and rotation of the Earth.

3.0   THE EARTH ROTATES ABOUT A SKEWED AXIS

3.1       The Earth's atmosphere is classified by temperature distribution into five layers:

3.1.1   The upper atmosphere comprises two cold layers having 90% of the Earth=s ozone layer.

3.1.2   A large portion of ultra-violet rays is removed by the ozone layer, thus changing the solar spectrum.

3.1.3   The lower atmosphere comprises three warm layers which represents 86% of the atmosphere by mass:

3.1.1   Clouds reflect 28% of solar radiation out of the lower atmosphere;

3.1.2   The atmosphere absorbs 25% of solar radiation;

3.1.3   Remaining Solar Radiation is separated into beam and diffuse components as it passes through the lower atmosphere.

3.2       Rotation of the Earth causes the angle of incidence with the surface to vary throughout the day.

3.3    Rotation of the Earth also causes attenuation to increase at sunrise and sunset because the path through the atmosphere is increased;

3.4       The Earth's surface absorbs 47% of the solar radiation;

3.5       The Cosine Law states that the intensity of an electromagnetic wave (such as solar radiation) on any surface is proportional to the cosine of the angle of incidence to the normal of the surface:

3.5.1   Higher latitudes receive less intense solar radiation;

3.5.2   Solar radiation is least intense at sunrise and sunset;

3.5.3   Solar radiation is more intense at noon; and

3.5.4   Solar radiation is most intense when the Sun's rays are perpendicular to the Earth's surface.

3.6       Ground cover shades or reflects a portion of the available solar radiation.

3.7       Thermal storage is primarily influenced by the oceans, which cover 75% of the Earth's surface.

4.0 TERRESTRIAL RADIATION IS RESPONSIBLE FOR COOLING

4.1       Terrestrial Radiation is the long wave heat output emitted from the Earth.

4.2       Atmospheric Turbidity inhibits transmission of terrestrial radiation.

4.3       Accumulation of carbon dioxide in the atmosphere also inhibits terrestrial radiation and is referred to as the AGreen House Effect@.

5.0 TEMPERATURE FLUCTUATES ACCORDING TO INCIDENT SOLAR RADIATION, TERRESTRIAL RADIATION AND THERMAL STORAGE BY EARTH

5.1       The temperature of the Earth's surface rises as solar radiation is absorbed:

5.1.1   Higher latitudes are colder when sunlight hours are reduced, because the intensity of  solar radiation is less;

5.1.2   Temperature increase is less over bodies of water because the heat capacity of water is 2-3 times higher than that of dry soil.

5.1.3   Heat Capacity is the ability of a given volume of material to store or absorb heat while undergoing a temperature change.

V = ρ x C_v                                                                                                             5.1

where.

V =  Heat Capacity, W/m³*K

ρ  =  Density, Kg/m³

C_v = Specific Heat, W/Kg*K

5.2       The Earth's surface transmits heat to adjoining materials:

5.2.1   A portion of heat at the surface of dry soil is conducted to deeper layers of soil;

5.2.2   A portion of heat at the surface of bodies of water is convected to greater depths;

5.2.3   Adjacent air is heated by conduction; and

5.2.4   Air currents transfer heat by convection.

5.3       Diurnal temperature results from daily storage of absorbed solar radiation and cooling by terrestrial radiation:

5.3.1   The amplitude of temperature fluctuations is greater over continental land masses;

5.3.2   The time lag of diurnal temperature is more delayed over continental land masses.

6.0   WIND RESULTS FROM DIFFERENTIAL HEATING ON THE EARTH'S SURFACE AND ROTATION OF THE EARTH

6.1       Differential heating gives rise to convective air-currents:

6.1.1   Land and Sea Breezes result from differential heating of land and sea;

6.1.2   Between latitude 30EN and 30ES, two opposite eddy currents develop, known as Hadley Cells.

6.2       The Earth's rotation sets up a Coriolis Force to turn the eddy currents.

7.0  SEASONS RESULT FROM ROTATION OF THE EARTH ABOUT THE SUN

7.1       The Earth is divided into 6 declinations because its axis of rotation is not perpendicular to the plane in which it rotates about the sun.

7.2       Seasons are produced by the revolution of the Earth about the Sun:

7.2.1   The sub-tropics (between 23.5EN and 23.5ES) generally receive the greatest solar insolation because the surface is inclined towards the sun;

7.2.2   Daylight hours and solar insolation are more variable outside the tropics, so there is greater fluctuation of temperature. 

Acoustics: Human hearing, room acoustics, noise control & sound reinforcement

PAUL HAY Capital Projects

www.phcjam.com

Acoustics

Author:          Paul Hay

e-mail:            paul.hay@phcjam.com

profile:           www.linkedin.com/in/phcjam

1.0   INTRODUCTION TO ACOUSTICS            

1.1_      Sound is an electro-magnetic wave consisting of a series of pressure variations, or vibrations, in an elastic medium.

1.2       Noise is unwanted sound.

1.3       Hertz is the frequency of sound wave cycles per second

1.4       The human voice has a frequency range of 100 to 7,500 Hz.

1.5       Wave-lengths are longer for lower frequencies:

λ = C/f                                                                                                                   1.1

where,

λ =  Wave-length, m

C = Velocity of sound, m/s

f = Frequency of sound, Hz

1.6       The velocity of sound in air is 344 m/s.

1.7       Sound originates at a source and takes a path to a receiver.

1.8       The Inverse-Square Law states that the intensity of an electromagnetic wave is inversely proportional to the square of the distance of path from the source.

2.0   HUMAN HEARING

2.1       Sound audible to humans is within the frequency range 20 - 20,000 Hz.

2.2       Audible frequencies have wavelengths from 17 - 15,200 mm long.

2.3       The threshold of hearing occurs at a sound level of 10^-20 W/m².

2.4       Decibel is the intensity level (IL) equivalent to 10 times the logarithm of a measured sound level (I) over the threshold of hearing (I_o):

IL = 10log(I/I_o)                                                                                                      2.1

2.5       The human ear is less responsive to frequencies at the extremes of the audible range at low sound levels, but this improves as the volume is increased:

2.6       The apparent loudness of sound is also exponential:

2.6.1   A-weighed decibels [db(A)] only measures audible sound with the frequency responsiveness of the human ear at the respective sound level;

2.6.2   0 db(A) is the threshold of hearing;

2.6.3   10 db(A) is an apparent loudness twice that of the threshold of hearing;

2.6.4   60 db(A) is loudness of the human voice; and

2.6.5   130 db(A) is the threshold of pain.          

3.0   ROOM ACOUSTICS

3.1       The acoustics within a room depends on interior noise control; reverberation and room shape.

3.2       Reverberation is the repetition of sound at reduced levels of loudness after sound has ceased from the source.

3.3       Reverberation time (T_r) is the time taken for the sound to reduce to -60db:

            T_r = 0.16 V/3A                                                                                                      3.1

where,

            T_r = Reverberation time, sec.

A = Total absorption at the respective frequency, m²

3A = α_avg x S

α_avg = S₁ α₁ + S₂ α ₂ + ............. + S_nα_n

S₁ +S₂ + .........................S_n

S = Total Surface Area, m²

α_n = Coefficient of absorption at the n^th surface

3.4       The units of total absorption are also called Sabins, in honour of W. C. Sabine, a pioneer in architectural acoustics.

3.5       The coefficient of absorption (α) is the ratio of intensity levels of an absorbed sound and an incident sound:

α  = I_a/I_i                                                                                                                   3.2

where,

I_{a =} Absorbed wave intensity, W/m²

I_{i =} Incident wave intensity, W/m²

3.6       Rooms with Alive@ sound have highly reflective surfaces [i.e. α_avg < 0.2].

3.7       Rooms with highly absorptive surfaces are Adead@ [i.e. α_avg > 0.4].

3.8       Auditoriums and Quiet Rooms require treatment of absorptive material be placed on all surfaces, but other spaces generally require only ceiling treatment.

3.9       The optimum reverberation time for speech can be determined by the relationship:

            T_r = 0.30 logV/10 = 0.35 sec.                                                                             3.3

3.10    Typical volumes for drama and speech are 2.3 - 4.2 m³/seat.

3.11    Concert Halls adjudged excellent for music have reverberation times of 1.6<T_r<1.8.

3.12    Typical volumes for concerts are 6.2 - 11.0 m³/seat.

3.13    Recommended proportions for Music Rooms are 3:4:5.

3.14    Undesirable qualities can result from poor room design:

3.14.1 An echo is a sound reflected at sufficient volume to be heard with a delay greater than 50 ms;

3.14.2 Flutter is a buzzing or clicking sound comprised of repeated echoes between non-absorbing parallel surfaces;

3.14.3 Standing Waves are like flutter but accentuate specific frequencies that have wave-lengths twice as long as the distance between the parallel surfaces.

3.14.4 Standing Waves are only important in rooms that are small relative to the wave-length of the frequency [i.e. <9.1 m for music, and <4.6 m for speech.

3.14.5 Focusing is primarily reflection from concave surfaces that converge at one area, so round buildings should be avoided.

3.14.6 Creep is the reflection of sound along a concave surface from a source at one end to a receiver at the other.

4.0    NOISE CONTROL

4.1       Noise control is exercised by (a) reflecting and absorbing sound within a room, (b) intrusion and attenuation of sound from sources outside the room, and (c) reducing equipment and airflow noises.

4.2       Sound waves are reflected by objects larger than their wave-length.

4.3       Noise Reduction Coefficient (NRC) is an area-weighed average of sound absorption coefficients at frequencies 250, 500, 1000, and 2000 Hz.

4.4       NRC is increased by greater material thickness, composition, and resilience of the installation.

4.5       Effective reduction of outside noise involves keeping sound from going around, under and through a room=s walls, roof and floor.

4.6       Sound waves go around objects smaller than their wave-length.

4.7       Sound Transmission Coefficient (STC) is the measure of sound absorbed in transmission through a building element:

4.7.1   STC measurements are restricted to airborne sound at frequencies of 125 - 4,000 Hz and is therefore relevant to evaluation of speech privacy;

4.7.2   STC does not measure impact noise, low frequency noise (eg. traffic & VAC sources) or amplified music.

4.8       Impact (Structural borne) Isolation Class (IIC) should be equal or greater than STC ratings.

4.9       An architect analyzes and solves a noise control problem in a four stage process:

4.9.1   Establish Permissible Noise Criteria (PNC) for background noise in the respective occupancy;

4.9.2   Identify and measure intensity level of noise;

  

4.9.3   Calculate the Noise Reduction (NR):

NR = IL of noise source - PNC                                                                                    4.1

4.9.4   Specify building element with transmission loss (TL) curves that exceed the required NR curve.

4.10    NR requirements can be reduced by using heavier construction and locating noisy space next to rooms having high PNC.

4.11    Machinery (eg. compressors) which vibrates due to rapidly moving parts should be isolated from the building=s structure.

4.12    Rapid air movement in ducts are also sources of noise:

4.12.1 Noise is reduced by creating smooth transitions from one duct to the next; and

4.12.2 Noise is reduced by increasing the cross-sectional area of the duct and reducing the air-velocity.

5.0   SOUND REINFORCEMENT

5.1       Sound Reinforcement Systems augment sound that would otherwise be inadequate:

5.1.1   Systems are required in spaces having volumes greater than 1,400 m³; but

5.1.2   These systems cannot achieve complete correction of poor room acoustics.

5.2       Sound Systems comprise (a) input from sources like microphones, (b) amplifiers and controls (such as equalizers), and (c) output from loudspeakers.

5.3       Loudspeakers should give the impression that the sound is originating from the source:

5.3.1   Time delay is necessary because electrical input travels at the speed of light while sound is slower;

5.3.2   Equalization may need to be applied to create a frequency response comparable to the original sound.

5.4       Distributed loudspeaker systems comprise a number of low-level loudspeakers located overhead which may be used as public address systems, where directional realism is not essential.

5.5       The sound system operator must be able to hear the sound as the audience.

 

FURTHER READING

            Mechanical and Electrical Equipment for Buildings, 8^th edition, Benjamin Stein, John S. Reynolds, John Wiley & Sons Inc., USA, 1992

            Ramsey/Sleeper Architectural Graphic Standards, AIA, Robert T. Packard (ed), John Wiley & Sons Inc., USA, 1981