Knowledge Center


A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z


A = area, square feet

Btu = British thermal unit

C = conductance, Btu/hr, sq.ft., F

°C = degrees centigrade

cuft = cubic feet or cubic foot

e = emissivity, a dimensionless number expressing the efficiency with which a surface exchanges radiant energy

°F = degrees Fahrenheit

h = rate of heat transfer from surface to air (heat transferred per unit time per unit area F) Also “f” by some and “E” by others

IPS = Iron pipe size

ID = Inside diameter

k = thermal conductivity (heat transferred per unit of time for unit area for a temperature gradient of 1° F per unit length of heat path) Btu/hr, sq. ft. (F/in.).

lb = pound

loge N = natural logarithm = 2.303 log10 N; also ln

cal = calorie

OD = Outside Diameter

psi = pounds per square inch

psia = pounds per square inch absolute pressure

psig = pounds per square gauge pressure

Q = total quantity of heat transferred in unit time from an object or system

q = heat transferred per unit of time from 1 sq.ft.

R = thermal resistance; a number denoting the resistance of a material or system offers in the flow of heat, (F/in.)sq.ft.,hr./Btu.

r = radius

sq ft = square foot or square feet

sq in = square inch or square inches

t = temperature, °F or °C

U = overall coefficient of heat transfer (heat transferred per unit time for unit area for 1 °F overall) Btu/hr, sq.ft., °F


  • Absolute temperature is the temperature expressed in degrees above absolute zero.

  • Absolute Zero is the lowest temperature theoretically attainable. The temperature at which there is no more heat and no longer molecular movement. The zero point of the scale is 459.6 degrees below the zero of the Fahrenheit scale and 273.2 degrees below the zero of the centigrade scale.

  • Absorption is the action of a material in extracting another substance from an atmosphere when there is a chemical and/or physical change in the absorbent.

  • Absorptivity is the ratio of radiant heat absorbed by a body to the heat absorbed by a black body under the same conditions.

  • Adsorption is the action, associated with surface adherence, of a material in extracting another substance from an atmosphere without a chemical or physical change of the adsorbent.

  • Ambient Air - Generally, the air surrounding an object or a surface.

  • Ambient Temperature is the average temperature of the medium, usually air, surrounding the object under consideration.

  • ASTM International provides a global forum for the development and publication of international voluntary consensus standards for materials, products, systems, and services.

  • Atmospheric pressure is the pressure due to the weight of the atmosphere as indicated by a barometer. Standard atmosphere is 14.7 psi.

  • Attenuation - The limiting of sound propagation from one area to another.


  • Black Body is a theoretical body that would absorb all radiation falling on it, reflecting and transmitting none.

  • Boiling point is that temperature at which a liquid is vaporized into a gas without a change in temperature. The boiling point of a liquid is dependent upon the absolute pressure at the vapor-liquid surface.

  • Bond Strength - The force in tension, compression, cleavage or shear required to break an adhesive assembly.

  • Bonding Time - The time required for an adhesive to reach its optimum bonding strength.

  • Btu - Btu is the abbreviation of British thermal unit and is the amount of heat needed to raise the temperature of 1 pound of water 1°F. The Btu is used to measure the amount of heat just as the inch or foot is used to measure length or as the minute or hour is used to measure time. When fuels burn, the approximate heat generated is 13,000 Btu from a pound of coal, 141,000 Btu from a gallon of oil, and 1,000 Btu from a cubic foot of natural gas.


  • Calorie (Gram Calorie) is a unit of heat or energy. It is the amount of heat necessary to raise the temperature of 1 gram of water 1°C.

  • "C" - Represents the conductance of a material and is used to show the amount of heat (Btu's) that wilpass per hour through 1 square foot of a homogeneous or non-homogeneous material or a combination of materials for the thickness or type under consideration for a difference in temperature of 1°F between the two surfaces. The average "C" value of an 8 inch hollow concrete block is 0.90.

  • Cellular Elastomeric - Insulation composed principally of natural or synthetic elastomers, or both, processed to form a flexible, semi-rigid or rigid foam that has a closed-cell structure.

  • Cellular Insulation - Insulation composed of small, individual cells separated from each other. The cellular material may be glass or plastic such as polystyrene, polyurethane, polyisocyanurate or elastomeric.

  • Centigrade (C) temperature scale is a thermometric system in which 0 degrees denotes the freezing and 100 degrees the boiling point of pure water at standard atmospheric pressure.

  • Cladding - Jacketing installed over insulation.

  • Clearance - Adequate space allowed for installation of insulation materials.

  • Closed Cell Foam - A material comprised predominantly of individual non-interconnecting cellular voids.

  • Coating - A liquid or semi-liquid that dries or cures to form a protective finish, suitable for application to thermal insulation or other surfaces in a dry thickness of 30 mils or less per coat.

  • Cooler - In cold-storage practice, an insulated room maintained at 30°F or above.

  • Compressive Strength - The property of an insulation material that resists any change in dimensions when acted upon by a compaction force.

  • Condensate Drain - Piping carrying condensed water from air conditioning or refrigeration drip pans to a point of discharge.

  • Condensate Return - The liquid formed by condensation of vapor. In steam heating it is water condensed from steam. In air conditioning it is the water extracted from the air by cooling.

  • Condensation - The act of water vapor turning into liquid upon contact with a cold surface.

  • Conduction (Thermal) is the transfer of energy by virtue of a temperature difference. It is the process of heat transfer through a substance in which energy is transmitted from particle to particle without gross displacement of the particles. If you sit on a cold stone step or a cold metal chair, you instantly feel the discomfort that comes from the contact of your warm body with the colder surface. This way of transferring heat by contact is called Conduction.

  • Convection is the motion resulting in a fluid from the differences in density and the action of gravity. A warm-air furnace transfers heat to the rooms of the house by moving air. This transfer of heat by moving air is called Convection.

  • Contact Adhesive - An adhesive that when tacky to the touch will adhere to itself instantaneously on contact.

  • Cryogenic Insulation - Insulation for extremely low-temperature processes surfaces from -100° F to -459° F (absolute zero).


  • Decibel (Db) - A logarithmic measure of the ratio of like power quantities as used in describing levels of sound pressure or sound power.

  • De-humidification is the removal of water vapor from a gas and can be accomplished by physical, chemical, or thermal means.

  • Dehydration is the removal of water from all forms of matter. Liquid water, hygroscopic water, water of crystallization and water of hydration are included.

  • Density of a material is the mass or weight per unit volume.

  • Dew point - The temperature at which the condensation of water vapor begins for a given condition of humidity and pressure as the temperature of the water vapor is reduced. The dew point temperature corresponds to 100 percent relative humidity for a given absolute humidity at constant pressure.

  • Duct - A passageway made of sheet metal or other suitable material used for conveying air or other gas.


  • Elastomeric - A closed-cell foam insulation containing elastomers that provide the property of high elasticity.

  • Emissivity is a characteristic of a surface which determines its ability to emit or give off heat by radiation. Its value is the ratio of heat radiated by a body to the heat radiated by a black body under the same conditions. Values range from 0 to 1.

  • Enthalpy is a term used in lieu of "total heat" or "heat content," e.g., when a change occurs at constant pressure, as when water is boiled, the change in enthalpy is equal to the heat added; in this case, latent heat.

  • Entropy is the ratio of the heat added to a substance to the absolute temperature at which it is added.


  • Fahrenheit (F) temperature scale is a thermometric system in which 32 degrees denotes freezing and 212 degrees the boiling point of pure water at standard atmospheric pressure.

  • Fibrous Glass - A synthetic vitreous fiber insulation made by melting predominantly silica sand and other inorganic materials, and then physically forming the melt into fibers. To form an insulation product, there are often other materials applied to the mineral wool such as binders, oils, etc. Commonly referred to as either fiber glass or fiberglass.

  • Finishes - Jackets, mastics or strong films used for aesthesis or to protect insulation from at least one or more of the following: weather, mechanical, and/or personnel abuse.

  • Fire Resistance - The property of a material or assembly to withstand fire or give protection. It is characterized by the ability to confine a fire and to continue to perform a given structural function.

  • Fire Retardance (FR) - The property of a material that retards flame spread across its surface.

  • Firestopping - Furnishing and installing a material or a combination of materials to form an effective barrier against the spread of flame, smoke, gases and moisture. It is to maintain the integrity of the fire-rated construction.

  • Fitting Cover - The insulation for a pipe fitting composed of the specified thickness of insulation material, which may be preformed. Also, a preformed jacketing.

  • F-Rating - A rating usually expressed in hours indicating a specific length of time that a fireresistive barrier can withstand fire before being consumed or permits the passage of flame through an opening in the assembly, as determined by ASTM E 814 (UL 1479).

  • Freezer - In cold-storage practice, an insulated room where the temperature is kept below 30°F.


  • Hanger (Pipe) - Devices used to support piping.

  • Heat - A form of energy that is transferred by virtue of a temperature differential.

  • Heat Capacity is the amount of heat required to raise the temperature of a body 1 degree. Numerically, it is the mass multiplied by the specific heat.

  • Heat Transmission is any flow of heat and usually refers to conduction, convection and radiation combined.

  • Hertz (Hz) - A measurement of sound frequency measured in cycles per second.

  • Humidity-Relative - is the ratio of the weight of water vapor in a mixture of water vapor and air to the weight of water vapor in dry saturated air at the same temperature. The ratio is usually expressed as a percent of relative humidity.

  • Humidity-Absolute - is the weight of water vapor per unit volume (lb/cu.ft.).

  • Humidity-Specific - is the weight of water vapor per pound of dry air in a mixture of water vapor and air.

  • Humidification is the process of increasing, by any means, the density of water vapor in an atmosphere.


  • Insulation (Thermal) is a material having a relatively high resistance to heat flow, and is used principally to retard the flow of heat.


  • "k" - Thermal conductivity, the amount of heat (Btu's) transferred in 1 hour through 1 square foot of a homogeneous material 1 inch thick for a difference in temperature of 1°F. For example, the average "k" for Armaflex is 0.25. This means that for a 1 inch thickness, there is a heat transfer of 0.25 Btu per hour per square foot for each degree difference in temperature between its two surfaces. Usually expressed in Btu/hr, sq.ft.(F/in.) in the insulation field.

  • Kelvin (K) temperature scale is sometimes called centigrade absolute. Its zero is at the lowest attainable temperature or 273.15° below the zero on the centigrade scale.


  • Latent Heat is that heat involved in a change of state without a change in temperature, i.e., the heat necessary to change 212°F water to 212°F steam.


  • Mean Temperature - The arithmetic mean of inner and outer surface temperatures of insulation. Mean temperature is used to select a value for conductivity in heat-loss calculations. When a conductivity curve is not a straight line, a theoretical error in selection is involved, which, however, is not generally so much as 1 percent.

  • Mold and Mildew Resistance - The property of a material that enables it to resist the formation of fungus growth.


  • Noise Reduction Coefficient (NRC) - A single number rating that is the arithmetic average of the individual sound absorption coefficients at 250, 500, 1000 and 2000 Hz to the nearest 0.05.

  • Noncombustible - A material that will not contribute fuel or heat to support a fire to which it is exposed.

  • Nonflammable - A material that will release very little heat when exposed to fire or flame.


  • Perm - A measure of vapor transmission rate. Defined as 1 grain of water vapor per hour for 1 square foot area for 1 inch of mercury-pressure difference. Other units are also used to express vapor transmission rates. Grains/hr., sq.ft., in.Hg.

  • Permeability - A rating of a material giving the amount of water vapor that passes through 1 inch thickness of the material. Grains/hr., sq.ft. (in.Hg/in.)

  • Permeance - Same as permeability except that it is a rating of the material at the thickness tested. Grains/hr., sq.ft., (in.Hg.)

  • Phenolic Foam - A foamed insulation made from resins of phenols condensed with aldehydes.

  • Pin Weld (Pinning) - Attachment of insulation anchor pins to ductwork or equipment usually by capacitor discharge welding.

  • Pipe - A circular conduit for the conveyance of liquids or semisolids.

  • Pipe Insulation - Insulation in a form suitable for application to cylindrical surfaces.

  • Plenums - Enclosures for the collection of air at the termination or origin of duct systems. They may be a space below floors, above ceilings, a shaft or a furred area.

  • Polyethylene - A closed-cell, thermoplastic material used for insulation.

  • Polyimide - See cellular polyimide.

  • Polyisocyanurate - A closed-cell, thermoset, plastic foam formed by combining isocyanurate, polyol, surfactants, catalysts and blowing agents.

  • Polymer - A long chain molecule resulting from the chemical attachment of short molecules (monomers) of the same product. For example, when ethylene (a gas) is polymerized, the synthetic resin polyethylene is produced.

  • Polyolefin - A closed-cell thermoplastic material used for insulation.

  • Polyvinyl Chloride (PVC) - A polymerized vinyl compound using chloride.

  • Polyvinyl Fluoride (PVF) - A polymerized vinyl compound using fluoride.

  • Potential Heat - Approximate amount of Btu's required to fully burn or incinerate 1 lb. Armaflex -- 9,000 Btu's for 4.5 lb. of Armaflex.

  • Psychrometer - An instrument for determining the humidity or hygrometric state of an atmosphere. The instrument is usually a form of hygrometer with means for obtaining true wet and dry bulb temperatures.

  • Psychrometric Chart is a graph drawn to represent the thermodynamic properties of moist air. A psychrometric chart has been included with these notes and noted on it are the various temperature, and humidities that you should learn to read.

  • Psychrometry is that branch of physics relating to the measurement or determination of atmospheric conditions, particularly regarding the moisture mixed with the air.


  • "R" - Represents resistivity or resistance which is the reciprocal of conductivity or conductance:

  • Radiation is the transmission of energy by means of electromagnetic waves. Radiated heat moves at high speed through the air without heating the air and flows in direct lines from a warm surface to a cooler one. Sun heat is radiated heat. Heat going from your body to a nearby colder surface, such as a wall, ceiling or floor, is also radiated heat.

  • Rankine (R) temperature scale is sometimes called Fahrenheit absolute. Its zero is at the lowest attainable temperature or 459.67 (460) degrees below the zero on the Fahrenheit scale.

  • Reflectivity is the fraction of the incident radiation reflected by a surface. (No radiant heat is reflected by a perfect black body.) With an opaque non-black body: Emissivity = Absorptivity = 1 – Reflectivity

  • Refrigeration, ton of -- The removal of heat at a rate of 12,000 Btu per hr. This figure is obtained from the commercial unit of refrigerating capacity, which is the heat required to melt 2,000 lbs. of pure ice to water at 32°F in 24 hours. The latent heat of ice is 144 Btu per lb. Therefore, a ton of refrigeration is 144 x 2,000 = 288,000 Btu per 24 hrs. or 12,000 Btu per hr.

  • Resiliency - The property of a material that enables it to recover its original shape and thickness after compression.

  • Retrofit - The application of additional insulation over existing insulation, new insulation after old insulation has been removed, or new insulation over existing, previously uninsulated surfaces.


  • Saddle - Rigid support for piping or equipment with allowance for insulation.

  • Saturation is the condition of co-existence in stable equilibrium of a vapor and a liquid or a vapor and a solid of the same substance.

  • Self-Extinguishing - The property of a material that enables it to stop its own ignition after external ignition sources are removed.

  • Sensible Heat is that heat which changes only the temperature of a substance.

  • Sharp freezer - In cold-storage practice, a freezer room generally operating at -10°F or lower.

  • Solar Resistance - The property of a material to resist decomposition by the ultraviolet rays from the sun or the passage of radiant heat from the sun.

  • Sorption is adsorption or absorption.

  • Specific Heat is the heat absorbed (or given up) by a unit mass of a substance when its temperature is increased (or decreased) by 1 degree. Or the ratio of the amount of heat required to raise unit mass of a material 1 degree to that required to raise unit mass of water 1 degree at some specified temperature.

  • Sprayed-on Insulation - Insulation of the fibrous or foam type that is applied to a surface by means of power spray devices.

  • Surface Conductance (h) is the amount of heat transmitted by radiation, conduction, and convection from a surface to the fluid surrounding it, or vice versa, in one hour for each square foot for a temperature difference of 1 degree between the surface and the fluid. Usually expressed as Btu/hr., sq.ft., F.


  • Tear Strength - The property of a material that enables it to resist being pulled apart by opposing forces.

  • Temperature - the thermal state of matter as regards its tendency to communicate heat to matter in contact with it. If there is no difference in temperature, no heat will flow on contact.

  • Temperature: Dry-bulb  - is sometimes called ambient or sensible temperature and it is the temperature of a gas or mixture of gases as measured by a transducer that remains dry. The transducer may be a thermometer, thermocouple, resistance bulb or any other temperature measuring device. To be truly accurate, it should be shielded from radiation or corrected for it.

  • Temperature: Wet-bulb -  is the temperature indicated by a wet-bulb transducer when used according to accepted standards. Technically, the thermodynamic wet-bulb temperature is the temperature at which liquid or solid water, by evaporating into air, can bring the air to saturation adiabatically at the same temperature.

  • Thermal Insulation System - Applied or installed thermal insulation complete with any accessories, vapor retarder, and facing required.

  • Thermal Properties of Insulation - Usually expressed as C-value, K-value, R-value and U-value.

  • Thermocouple is a device for measuring temperatures utilizing the effect that an electromotive force (emf) is generated whenever two junctions of two dissimilar metals in an electric circuit are at different temperature levels. In practice, the two dissimilar metals are usually in the form of wires. The two wires are joined together at one point by welding, soldering, or by mechanical means and this thermojunction is placed in or on the item whose temperature is to be determined. The other ends of the wire are usually connected to an instrument in such a way that the temperature at the instrument junction is known and the emf produced in the circuit can be measured. By the use of tables of calibration, the emf measured can be related to temperature difference between one thermojunction and the other, and with one junction temperature known, the other can be determined.

  • Thermometer is an instrument for measuring temperature usually consisting of a glass tube of capillary bore partly filled with mercury or colored alcohol, the degree of the expansion and contraction of which, due to temperature change, is read on a scale.

  • Thermostat is an instrument which responds to changes in temperature, and which directly or indirectly controls temperature.


  • "U" - Designates the total or overall transmission of heat (Btu's) in 1 hour per square foot of area for a difference in temperature of 1°F between the air on one side to air on the other side of a structure. The "U" value of an uninsulated frame wall consisting of wood siding, wood sheathing, 2 x 4 studs, gypsum lath and plaster is 0.24 Btu. If insulated with 2 inches of typical blanket insulation, the "U" value becomes 0.087 Btu, and if insulated with 3 inches, it would be 0.065 Btu.

  • UL - Underwriters Laboratories. An independent materials testing company. UL provides testing, evaluation, and listing services for products having specific safety-related features. UL test standards generally are similar to ASTM International standards when both exist.


  • Vapor pressure (in psychrometry) is that part of the atmospheric pressure which is exerted by the water vapor present in the air.

  • Vibration Resistance - The property of a material that indicates its ability to resist mechanical vibration without wearing away, setting or dusting.


  • Water vapor is a gas. It is the gas phase of water and is colorless, odorless, and tasteless.

  • Water Vapor Retarder (Barrier) - A material or system that adequately impedes the transmission of water vapor under specified conditions.

  • Wicking - Action of absorbing by capillary action. (some terms courtesy of the National Mechanical Insulation Committee (NMIC)

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