Thermal balance of the body

Heat is continuously produced in the body from most of our bio-chemical processes. Most of the bio-chemical processes involved in tissue building, energy conservation and muscular work are exothermal, i.e. heat producing . All energy and material requirements of the body are supplied from the consumption and digestion of food . The process involved in converting foodstuff in to living matter and useful form of energy are known as metabolism.

Body heat production in different activities

Activity                                                             Watts

Sleeping Minimum                                             70

Sitting, moderate movement, e.g. typing     130-160

Standing, light work at machine or bench     160-190

Sitting, heavy arm and leg movements               190-230

Standing, moderate lifting or pushing             220-290

Walking, moderate work, some walking     290-410

Intermittent heavy lifting, digging                     440-580

Hardest sustained work                                     580-700

Maximum heavy work for 30 minutes duration Max. 1100

(Average values of data published in many sources)

 The total metabolism heat production can be divided in to basal metabolism and muscular metabolism. The basal metabolism is meant by heat production of vegetative and automatic processes whish are ever-continuous in body and the muscular metabolism is meant by production of muscles whilst carrying out consciously controlled work. Only about 20% of all energy produced in the body is utilized. The remaining 80% is ‘surplus’ heat and must be dissipated to the environment from human body. This excess heat production varies with the overall metabolism rate and depends on the activity. The deep body temperature must remain balanced at around 37ºC. In order to maintain this body temperature at the steady level, all surplus heat must be dissipated to the environment. If there is some form of simultaneous heat gain from the environment (e.g. solar radiation or warm air) that must also be dissipated

Thermal balance of the body is the centre of focus in the concept of human comfort. The regularly mechanism of the body is such that it maintains constant body temperature from the automatic regulation of the heat and loss factors. The moment, this thermal balance is lost, discomfort will be experienced. Therefore, the importance of the factors that affect the heat dissipating process of the body, is obvious. As one of these factor changes, the condition of comfort changes as well. Therefore the sensation of comfort is relative.

Heat gains:

Met=Metabolism (basal and muscular)

Cv=Convection (if the air is warmer than the skin)

Cd=Conduction (contact with warm bodies) 

R=Radiation (from the sun, the sky and hot bodies)

Heat
losses:

E=Evaporation (of moisture and muscular) 

Cd=Conduction (contact with cold bodies) 

Cv=Convection (if the air is cooler than the skin) 

R=Radiation (to night sky and cold surfaces)

It can be expressed by an equation;

M-E±Cv±Cd±R=0

When this sum is more than zero, vasomotor adjustment will take place in side the body to reduce the temperature of the body. The blood circulation to the skin surface is increased, more heat is transported to the surface and the skin temperature is elevated and all forms of heat loss processes are accelerated .

Conversely, if the sum of the above equation is less than zero, the blood circulation to the skin is reduced , skin temperature is lowered and the heat loss processes are slowed down. If the vasomotor regulation is still insufficient, and overheating continuous, sweating will start . The rate of sweating may vary from about 20g / h to 3kg /h during periods of physical effort combined with hot environmental effects .

If in a cold environment, under-heating continuous in spite of vasomotor adjustments, violent shivering may occur, which can cause a ten-fold increase in metabolic heat production for short periods. Long-term endocrine adjustments constitute the acclimatization process .

These may involve the change in the basal metabolic heat production eventually increase in the quality of blood (to produce and maintain vaso-dilation) and an increase in sweat rate.

In a temperature climate, when the air temperature is around 18ºC in indoors, when the air is calm, i.e. air velocity does not exceed 0.25m/s, and when the humidity is between 40% and 60%, a person engaged in sedentary work will dissipate the surplus heat without any difficulty, in the following ways;

By radiation   45%

By convection   30%

By evaporation  25%

Assuming the temperature of bounding surface is approximately the same as the air temperature. The sensation of comfort or discomfort depends primarily on the climate variables, air temperature, wind, humidity and solar radiation.

Thermal preferences

Thermal preferences are however influenced by a number of subjective or individual factors. Following are these factors.

• Clothing

• Acclimatization 

• Age and sex

• Body shape

• Subcutaneous fat

• State of health

• Food and drink 

• Skin color

Thermal qualities

It is established fact that human sense of comfort is dependence with the thermal conditions. The qualities related to the thermal phenomena have to be defined before going through the analysis . A thermal phenomenon is basically the heat and its movement . It is obvious to give light on the physical facts regarding the nature of heat and the ways of its propagation . It is tried here to define frequently referred qualities in the solar building design.

1. Temperature is actually not a physical quantity but it can be thought of as a symptom- as the outward appearance of the thermal state of a body.
2. Heat is a form of energy, appearing as molecule movement in substances or as ‘radiant heat’, a certain wavelength band of electromagnetic radiation in space.
3. Specific heat of a surface is the amount of heat energy necessary to cause unit temperature increase of a unit mass of the substance. It is measured in: J/kg degC.
4. Latent heat  of a substance is the amount of heat energy absorbed by unit mass of the substance at change of state (from solid to liquid or liquid to gaseous) without any change in temperature. It is measured in : J/kg.
5. Thermal capacity of a body is the product of its mass and the specific heat of its material. It is measured as the amount of heat require to cause unit temperature increase of the body, in units of J/degC.

Heat flow 

Heat energy tends to distribute itself until a perfectly diffused uniform thermal field is achieved. It tends to flow from high temperature to lower temperature zones, by any or all the following ways: Conduction, Convection, Radiation.

Conductivity(k): In conduction through a body or bodies in direct contact, the spread of molecule movement constitutes the flow of heat. The rate at which such molecule movement spreads varies with different material- its thermal conductivity (or ‘k-value’). It is measured as the rate of heat flow (flow of energy per unit time) through unit area of unit thickness of the material, when there is a unit temperature difference between the two sides.

Thermal diffusivity (K)=k/(d*c) Where, 

k=conductivity (W/m degC)

d= density (kg/m3) 

c=specific heat (J/kg degC)

Resistivity(R) is the reciprocal of this quantity (1/k) measured in units of: m deg C/W. Better insulators will have higher resistivity values.

Conductance(C) whilst conductivity and resistivity are properties of a material, the corresponding properties of a body of a given thickness are described as conductance (c ), or its reciprocal, resistance (R ): C=1/R. Conductance is heat flow rate through a unit area of the body when the temperature difference between the two surface is 1 degC.

Surface conductance

In addition to the resistance of a body to the flow of heat, resistance will be offered by its surfaces, where a thin layer of air film separates the body from the surrounding air. A measure of this is the surface or film -conductance, denoted thus: 1/f(m2 deg C/W). f being the surface or film-conductance (w/m2 deg C).

The overall, air-to-air resistance ( Ra) is the sum of the body’s resistance and the surface resistances:

Ra=1/fi +Rb +1/fo

Where Rb =internal surface resistance 1/fi =resistance of the body

1/fo = external surface resistance 

Transmittance(U) is the reciprocal of this air-to-air resistance or the air-to-air transmittance. U=1/ Ra

Its unit of measurement is same as for conductance-W/m2 degC-the only difference being that here the air temperature difference will be taken into account.

• Absorptivity, Emissivity, Reflectivity

• Time log ,Detriment factor 50

Thermal properties of building components and materials

Buildings in their own modify the climate and their main function is to provide comfortable living conditions for their inhabitants. It also maintains its indoor climate relatively constant for this purpose. The choice of building materials is of prime importance along with other factors regarding the thermal performance of the building . The choice of material is also responsible for reduction of the energy loads required to maintain comfortable levels with in the closed environment. While dealing with the thermal performance of building, understanding of thermal properties of the material is must. The solar radiation as a greatest source of thermal phenomena should be dealt before going through the material and their thermal properties . Material as such are responsible for absorption, storage and distribution of heat . This can be understanding the different modes of heat transfer.

Three fundamental modes of heat transfer 

• By direct interaction with neighboring particles (collisions): this is conduction

• By mixing different parts of a fluid with different temperature: this is convection

• By absorption or emission of electromagnetic radiation: this is radiation

The steady state

• From a thermal point of view every material is constantly seeking equilibrium between the heat absorbed on a part or over all of its surface and the heat losses at it’s periphery.
• For each level of thermal equilibrium, there is a corresponding level of heat and temperature of equilibrium. This state of static condition or of thermal equilibrium is called the steady state.
• For the purpose of simplification in the analysis, the assumption of steady state condition is made .
• The results thus may be taken as reliable if the fluctuations in the conditions do not exceed ± 3º C.
• If the fluctuation in temperature exceeds this tolerable limit, there is a need of establishing the heating or cooling installation.

 

Thermal insulation: thermal resistance and thermal capacity

Insulation may be defined as materials or features of construction provided to minimize the flow of heat between the spaces separated. Insulation as such reduces heat flow

Functions of insulation

• Conserve energy used for heating and cooling.

• Reduce temperature fluctuations and increase comfort with in the enclosed space.

• Protect buildings and other structures from thermal damage, freezing damage, frost heaving, and damage for condensation of water vapor.

• Control surface temperatures of building components for economy in operation, comfort of occupants, or safety.

• Prevent water vapor condensation on cold surfaces.

• Reduce water vapor transmission, properly seen as the separate but related topic of moisture control.

• A significant contribution of most types of insulation is also in reducing airborne sound transmitted through walls, partitions, floors and ceilings.

Insulation and human thermal comfort 

Insulation is the measure to achieve human thermal comfort. Wherever the occupants feel discomfort in a space inside building in functioning their activities, the question of controlling heat flow comes in the front . The insulation of the surface is the ultimate solution for this.

Thermal comfort

The daily life cycle of human being comprises state of activities, fatigue and recovery. It is essential that the mind and the body recover through recreation, rest and sleep to counter balance the mental and physical fatigue resulting from the activities of the day . The cycle can be and is often impeded by unfavorable climatic conditions and the resulting stress on the body and mind causes discomfort, loss of efficiency and may eventually lead to a break down of health . The effect of climate on human is therefore a factor of considerable importance.

 

Thermal factors

The factor affecting comfort may be divided into followings:

• Personal variables

1. Human activities 

2. Clothing

• Environmental factors

1. Air temperature 

2. Wind

3. Humidity

4. Solar radiation

The fluctuation on these factors cause discomfort to the human being. They must be simultaneously considered to predict the comfort range.

 Building Science (Damp/water proofing DPC in building)

 General terminology of water proofing

1. Below grade water proofing

2. Above grade water proofing

3. Flashing

4. Positive and negative side water proofing system

General methods of water proofing

1. Surface treatment

2. Integral water proofing

3. Membrane damp proofing

4. Guniting

5. Cavity wall construction

6. Thick and dense wall

DPC in buildings

Methods to make building damp proof

Horizontal DPC: It is provided in;

1. Plinth

2. Basement floor

3. Roof

4. Parapet wall covers

Vertical DPC: It is provided in;

1. Basement wall

2. Outer surface of wall of superstructure

3. Surface of parapet wall

Material used for damp proofing

1. Flexible materials like; bitumen, Polythene sheets, Lead, copper etc.

2. Semi rigid materials like mastic asphalt.

3. Rigid materials as brick, stone, cement concrete, slates, etc.

Requirements of DPC materials

1. Completely impervious

2. Durable-having longer life

3. Comparatively thin sheet-prevents disfigurement of building

4. Flexible enough to entertain settlement if any

Water proofing in basement

Generally, the moisture control in basement is done from following methods;

1. Surface drainage

2. Subsurface drainage

3. Damp proofing or water proofing on the walls and floor surfaces

Sealants

a. Sealants are the flexible materials applied in the joints or other places of any structure for the purpose of water proofing and environment control.

b. Sealants are exterior applications using high performance materials (e.g. silicones), which are typically colored and are primarily applied for water proofing .

c. As the sealant prevents the infiltration of air from/ to in or out of a building, its secondary role may be the environment control in the interior of the building.

Process of installation of sealants

Followings are the process for the successful installation of the sealant;

1. Joint design

2. Material selection

3. Substrata preparation

4. Sealant application

Building Science Moisture and its movement

 

Moisture and its movement through building component

1. Moisture or damp is water particles that appear on the surface of the materials.

2. If we talk this in the building, it is one of the most important elements affecting the living condition inside and deteriorating the strength and durability of the building components .

3. Damp appears in any surface of the building ; on the ceiling, on the wall, on the floor and on the other surfaces inside or outside.

Sources of moisture

1. Rain water

2. Ground water

3. Water from condensation

Phenomenon of condensation

                The air that exists everywhere in the atmosphere is a mixture of dry gasses and water vapor Therefore, moisture is present normally in the atmosphere in the form of water vapor. This water vapor exists in the form of finely divided particles of superheated steam at the given temperature (dry-bulb temperature). The amount of water vapor that a given quantity of air can hold increases with the temperature. If the air temperature is increased, it would take more water particles and if the air temperature is lowered the water particles diffuse each other and at a definite temperature, the air can no longer hold molecules in vapor state.

        When air at any particular temperature contains as much as water vapor as it can hold, the air is said to be saturated and the condition of air is said to be moisture saturation state. The particular temperature at which the air is at the state of moisture saturation is known as its dew point. If the temperature of air is increased from dew point, the excess vapor can no longer be held by the air will be deposited on the surface as condensation.

Building Science Moisture and Vapor management

Surface condensation and interstitial condensation

        When the temperature of any surface within a building is at temperature below the dew point of adjacent air, some of the water particles in the air will condense on that surface, this is called surface condensation. Depending on the nature of the surface , the moisture may either be absorbed by the material (perhaps remaining unnoticeable) or it may appear as water on the surface.

Surface condensation will not occur if,

1. The temperature of the surface is kept above the dew point of the adjacent air by adequate heating or by sufficient insulation behind the surface.

2. The humidity of the air is limited so that its dew point is below the temperature of the surface. With the occasional or intermittent surface condensation, an absorbent surface is advantageous, as it can retain a limit quality of moisture until condensations change and re-evaporation can take place.

        Condensation in buildings is not necessary confined to exposed surfaces, but may, under certain conditions occur within a material or on a surface within the thickness of a wall, roof or floor construction. This is called interstitial condensation.

Vapor diffusion

        The flow of water vapor through a porous building material or composite slab is analogous to the flow of heat through the structure. Convection current transfers heat and moisture at the fluid-solid boundary. Conduction heat transfer is similar to vapor diffusion through a porous material and its resistance to moisture flow varies with density as does thermal resistance but in the opposite sense.

The sources of water vapor in an occupied buildings are as follows:

• People (transpiration releasing 0.7kg per 24 h).

• Cooking

• Washing, bathing, drying clothes

• Humidifiers and open water surfaces

• Animals (domestic animals exhale than people produce)

• Combustion of paraffin

 The effects of the moisture

1. Efflorescence (disintegration of bricks, stones, tiles etc. to power like material)

2. Softening and crumbling of plaster.

3. Bleaching and flanking of paints with formation of patches.

4. Warping, buckling and rotting of timber.

5. Corrosion of metals (particularly ferrous metals)

6. Damage of electrical fittings

7. Growth of fungus and termites

8. Unhygienic condition to occupant in the building

9.  Damaging sound and thermal insulation.

Movement of Moisture

        Almost all construction materials used in the buildings absorb water to some extend. There are definite phenomena for the absorption and movement of this water in the building components. Following are different forces governing the movement of moisture through the building components;

1. Capillary action

2. Wind loads (momentum of water particles)

3. Surface tension of the building component

4. Gravity forces

5. Air pressure

6. Diffusion (occurs due to difference in vapor pressure)

Moisture control

        The ground water and the rain water percolating in to the ground causes great problem to the building. This is very sensitive work and due attention has to be paid to control the entry of moisture. Besides, condensation is also quite frequent in ground floor and basements.

        Under damp proofing, It is meant by the application of simple damp proofing paints or membrane to control the capillary infiltration .

        Under water proofing,it is meant by deliberate application of impervious layer under all adverse subsurface water condition.

       Subsurface drainage is the method of diverting underground drain may be directed towards the low land or a drywell of adequate capacity. Subsurface drainage is also be directed to a sump pump and the water collected may be pumped out. After this, there must be proper water proofing treatment in the foundation and walls of the basement.

Rain control

        Rain is the most important factor to control in order to construct a durable building. Rain may enter in building in many ways. Rain is more dependent on the climate and varies from place to place. It is difficult to forecast the intensity of the rain in time and place. A general idea could be drawn from the amount of annual rainfall. Besides, the wind substantial effect on the intensity, strength and the direction of rain. Rain penetration into and through building surfaces is governed by capillary action, momentum, surface tension, gravity force and air pressure. Capillary forces draw rain water into the pores and tiny cracks, while the remaining forces direct rain water into larger openings.

Following are the means of rain penetration control in practice:

1. Capillary breaks

2. Obstruction of horizontal openings

3. Drip edge or kerfs on horizontal openings

4. Use of flashings

5. Creation of pressure equalization

Vapor control

         In general, moisture vapor moves from warm side to cold side of the building components. As the temperature depends on the climate there must be clear idea of climate. Water vapor moves in two ways; by vapor diffusion and by air transport. The mechanism differs for both the case. It may happen that the means of effective control of the vapor diffusion may not be effective for air transport. Vapor diffusion is the movement of moisture in the vapor state through a material as a result of vapor pressure difference (concentration gradient) or a temperature difference (thermal gradient). It is not the movement of moisture as a result of air movement. Vapor diffusion moves moisture from an area of higher vapor pressure to an area of lower vapor pressure, as well as from the warm side of the building component to the cold side. Therefore the moisture will migrate by diffusion from where there is more to where there is less. The movement of the moisture from warm side to the cold side of the building component is called ‘thermally driven diffusion. The moisture condenses on cold surfaces acting as dehumidifiers pulling more moisture towards them. The air transport is the process of movement of moisture present in the air from the area of higher air pressure to the area of lower air pressure.

Vapor diffusion barrier

        There are the materials that control the entry of the water vapor into the building components by the mechanism of vapor diffusion. The vapor diffusion barrier may be required to control the diffusion entry of water vapor into the building components from the interior, from the exterior or from both the interior and exterior of the building. Vapor diffusion barriers are permeable to water vapor. These material allow water vapor to pass through them. Materials which are generally classed as permeable to water vapor and act as vapor diffusion barrier are: unpainted gypsum board and plaster, fiberglass insulation, cellulose insulation, dimensional lumber and board lumber, unpainted stucco, some latex-based paints, masonry, bricks, light weight asphalt-impregnated building papers, asphalt-impregnated fiber board sheathings, house wraps etc.

        Some materials are semi-permeable to water vapors. Following materials fall under this category; plywood, expanded polystyrene (EPS), extruded polystyrene (XPS), fiber based insocyanurate, heavy asphalt impregnated building papers, most latex based paint etc. depending on the design, specific condition of the building component and climate. All of these materials may not be considered to act as vapor diffusion barrier. Most often, these materials are treated as vapor permeable. There are many materials which are generally considered as impermeable to water vapor.

        Followings are some of these materials, rubber membranes, polyethylene film, glass, aluminum foils, oil paints, bitumen impregnated Kraft paper, almost all types of wall coverings and their adhesives, foil faced insulating non-insulating materials etc.

Precaution in use of vapor barriers

        The main purpose of the installation of the vapor barrier is to establish overall resistance and make the surface able to keep the dew point gradient below the thermally produced temperatures. The designer must be very serious on the proper installation of the vapor barriers.  Followings are some of the point that have to be considered while performing installation of the vapor barriers.

1. Use the materials with low thermal resistance but a high vapor resistance, such as aluminum foil, plastic sheets, roofing felt, gloss paint etc.

2. Providing vapor barriers on or near warm sides

3. Providing vapor barriers after the insulation

4. Vapor release on cooler sides

5. No barriers on both sides.