Understanding Heat Transfer and Material Properties in Building Design

Understanding the fundamental principles of heat transfer and material properties is crucial in building design and energy efficiency. These principles not only influence the thermal comfort of a building's occupants but also play a significant role in energy consumption and sustainability efforts. This article will explore several key concepts, including reflectance, absorptance, transmittance, emittance, and the basic equations governing heat transfer.

Optical Properties in Building Design

Reflectance: The ability of a surface to reflect light. It's the ratio of the amount of light that is reflected from a surface to the amount of light that falls on it. Reflectance is often expressed as a percentage, where a higher percentage indicates more reflection and less absorption by the surface.

Solar Reflectance, Solar Reflectivity SR [Albedo] : The proportion of sunlight that a surface like an exterior wall or roof reflects over a year. It indicates how effectively a surface can reflect solar radiation, typically ranging from 0 to 1. For instance, black paint has a solar reflectance of 0, while white paint has a solar reflectance of 1.

Solar Reflectance Index SRI : is a measure of the constructed surface’s ability to stay cool in the sun by reflecting solar radiation and emitting thermal radiation. A standard black surface has an initial SRI of 0, and a standard white surface has an initial SRI of 100.

Cool Roof and the SRI : A “ Cool Roof “ should have both:

?????? High “ Solar Reflectance”

?????? High “Thermal Emissivity”

The higher the SRI, the “cooler” the surface

Absorptance: The ability of a material to absorb electromagnetic radiation, such as light, sound, or heat when it passes through or interacts with the material. It represents the fraction of incident radiation that is absorbed by the material rather than transmitted or reflected. Absorptivity is often expressed as a dimensionless value between 0 and 1, where 0 indicates complete transparency (no absorption) and 1 indicates complete absorption (no transmission or reflection). This property is important in various fields, including optics, thermal engineering, and materials science.

Transmittance: The ability of a material to allow light to pass through it. It's the ratio of the amount of light that passes through a material to the amount of light that strikes it. Transmittance is also usually expressed as a percentage, where a higher percentage indicates more transparency and less opacity of the material.

Emittance: The ability of a surface to emit radiation. It's the ratio of the amount of radiation emitted by a surface to the amount of radiation it would emit if it were a perfect emitter (a blackbody) at the same temperature. Emittance is often expressed as a value between 0 and 1, where a value of 1 indicates a perfect emitter (a blackbody) and lower values indicate less efficient emission relative to a blackbody.

Reflectance + Transmittance + Absorptance = 1

This equation states that the total amount of incident light energy is accounted for by these three components: the light that is reflected, transmitted, and absorbed. Since these are the only possible outcomes for incident light, their sum must equal 1 (or 100% if expressed as percentages).

Solar Heat Gain : In buildings with large areas of exposed glazing, solar radiation is considered a major source of the cooling load Impacts depends on a number of factors such as Solar radiation intensity, Time of day, Orientation, Availability of shading, and Type of glass.

?Solar Heat Gain Coefficient SHGC : The ratio of transmitted solar radiation to incident solar radiation “less is better”

Q = SHGC . A . I (W)

where:

A : is the window’s surface area (m2)

I : is the incident solar radiation intensity (w/ m2)

Glass Visible Transmittance VT :?A measure of the amount of visible light that passes through a piece of glass or glazing material.

Glare: Excessive brightness that causes visual discomfort. It can be caused by poorly controlled natural light or improper artificial lighting.

Color Rendering Index: CRI measures the ability of a light source to reveal colors accurately in comparison to a natural light source.

Thermal performance characteristics

Heat Transfer in buildings occurs through three primary mechanisms: conduction, convection, and radiation.

Conduction : The transfer of heat through materials without movement. The rate of heat conduction is typically measured in units of watts (W), which represent the amount of heat energy (Whr) transferred per unit of time (hr).

Heat Conduction Equation??

Q = (Area .? ?T)/ΣR? (W)

Where:

A : is the surface area (m2)

L : is the thickness (m)

?T : is the temperature difference across the material surfaces (?C)

ΣR : total thermal resistance of the roof, wall and insulation materials, which is calculated from the individual thermal resistance of each component/layer of them.

Thermal resistance [R] is a measure of how much heat loss is reduced through the given thickness of a material, (m2 ?C /W).

If you cannot find the R value for specific thickness… Use Thermal Conductivity [K] (W/ m ?C ) which can be used for any thickness (L)

Convection : The transfer of heat through the movement of a fluid (liquid or gas). This movement can occur through natural convection, or through forced convection, Convection heat transfer rates are also typically measured in watts (W).

Heat Convection Equation??

We also have to account for the convection that occurs on the material surface that depends on many variables such as surface geometry, the fluid motion, etc.

convective heat transfer coefficient is often used [h] represents the rate of heat transfer per unit area per unit temperature difference (W/(m2 · ?C).

However, for basic geometries (Wall and roofs) under typical air velocities, recommended values R can be used for simplicity.

Radiation : The transfer of heat through electromagnetic waves. Unlike conduction and convection, radiation does not require a medium to propagate; it can occur through a vacuum. The rate of heat transfer by radiation is often expressed in terms of power flux density (W/ m2).

Overall Heat Transfer Coefficient – U Value : Measures how well a material or assembly of materials insulates against heat transfer. A lower U-value means better insulation.

Q = U . A . ?T (W)

The value of UA is often referred to as the “ Building Load Coefficient” BLC?as an indication of the role of heat conduction through the envelope of a building.

Remember

U = 1/ΣR? (W/ m2 ?C )

Degree Days : An “approximation” for geographic weather, used to predict the amount of heating and cooling needed.

HDD Heating Degree-days and CDD Cooling Degree-days are separate values and are specific to a particular geographic location and assumes:

?????? Average building has a desired indoor temperature of 21.1 ?C

?????? 2.8 ?C of this is supplied by internal loads (Lighting, Occupancy, Equipment)

?????? The base of computing Degree-days is 18.3 ?C

Hence , Seasonal Energy Losses through the envelope

Q = 24 . U . A . DD (Whr/Year)

Air Infiltration: The unintended or uncontrolled entry of outside air into a building through cracks, gaps, and other openings.

Vapor Permeability: Material's ability to allow moisture vapor to pass through it. It's mandatory for managing condensation and maintaining indoor air quality.

Thermal Mass :? Refers to certain materials' ability to absorb and store heat, helping to regulate indoor temperatures efficiently.

?????? Thermally Light – A building whose heating and cooling requirements are proportional to the weather driven outside temperatures, e.g., homes, shopping centers, and commercial office building

?????? Thermally Heavy – A building whose indoor temperature remain fairly constant in the face of significant changes in the outdoor temperature, e.g., plastic injection molding facility or building with a high heat generation device or area inside.