Insulation glossary

Short definitions of the terms and values you meet in product datasheets

Thermal insulation

Thermal conductivity λ, k: How readily heat flows through a material, in W/m·K. The lower the value, the less heat the material conducts. Values are only comparable when measured at the same mean temperature. Read the article
Thermal resistance (R-value) R: How well a layer of a given thickness resists heat flow. For a uniform layer, R = thickness ÷ λ. The SI unit is m²·K/W, which differs from the imperial unit used in the US — check the unit before comparing. Read the article
Thermal transmittance (U-value) U: The rate of heat transfer through a whole building element such as a complete wall or roof, in W/m²·K. Lower means less heat passes. It describes the whole construction, not a single material. Read the article
Reflective foil / radiant barrier: A shiny metallic sheet that reduces radiant heat transfer. It works only with an air space facing the reflective surface, and it works differently from fibre or foam insulation, which reduce conducted heat. Read the article
Emissivity: How strongly a surface emits heat radiation, on a scale from 0 to 1. Shiny metal surfaces have low emissivity, so they emit and absorb little radiant heat. Dust or dirt on the surface changes it. Read the article
Thermal bridge: A spot where heat passes more easily than through the surrounding area — metal framing, joints or uninsulated gaps. It lowers overall performance and, on cold work, can cause condensation at that spot. Read the article

Sound and acoustics

Sound absorption: Reducing the sound energy reflected from surfaces inside a room. It reduces echo and makes speech clearer in the room, but it does not stop sound travelling through the wall to another room. Read the article
Sound insulation: Reducing the sound that passes from one side of a wall, floor or ceiling to the other. It depends on the whole construction — the mass of the linings, the cavity, the cavity insulation and the sealing of leaks. Read the article
Noise Reduction Coefficient NRC: A single number summarising a material's sound absorption across the mid frequencies. Higher means more absorption. It depends on the thickness and mounting used in the test, and it says nothing about blocking sound. Read the article
Sound Transmission Class STC: An ASTM single-number rating of how well a complete tested wall or floor blocks airborne sound in the laboratory. It belongs to the assembly, not to one material. Read the article
Weighted sound reduction index Rw: An ISO single-number rating, in decibels, of the airborne sound insulation of a complete wall or floor tested in the laboratory. Similar in purpose to STC but calculated differently, so the two should not be compared directly. Read the article
Reverberation time RT: The time it takes for sound in a room to fall by 60 decibels after the source stops. Rooms with many hard surfaces and little absorption have a long reverberation time. Read the article
Flanking transmission: Sound that does not go straight through the separating wall but travels around it — over the ceiling void, through the floor, ducts or leaks. It makes on-site results lower than laboratory ratings. Read the article

HVAC and ductwork

Dew point: The temperature at which air with a given moisture content starts to form water droplets. If a pipe or surface is colder than the dew point of the surrounding air, water forms on it. Read the article
Condensation: Water vapour in the air turning into droplets on a surface colder than the dew point — the reason cold AC pipes and ducts drip. Read the article
Vapour barrier / vapour retarder: A layer that limits water vapour moving into the insulation. It matters most on cold work and must be continuous with sealed joints — at a tear or open joint, vapour gets in and condenses inside the insulation. Read the article
Duct wrap: Insulation wrapped around the outside of a duct, usually with a foil facing that acts as a vapour retarder. It reduces heat gain and condensation on the duct surface. Read the article
Duct liner: Insulation fixed inside a duct. It helps with heat and reduces noise travelling along the duct; the airstream face must be a type designed for use in moving air. Read the article

Industrial and high temperature

Ceramic fibre: Fibre insulation for very high-temperature industrial work such as furnaces and boilers. Installers need protective equipment and must follow the manufacturer's safety data sheet. Read the article
Service temperature: The temperature range the manufacturer states the material can be used at continuously. It is not the same as fire resistance or non-combustibility; check the conditions in the manufacturer's documents. Read the article

Fire protection

Reaction to fire: How a material behaves when exposed to fire — how easily it ignites, how fast flame spreads, whether it makes smoke or burning droplets. Classified under standards such as EN 13501-1. It is a material property, not a wall's fire-resistance rating. Read the article
Non-combustible: A material that has passed a specified test showing it does not burn, or contributes almost nothing to a fire. Being non-combustible does not by itself give a wall or system an hourly fire-resistance rating. Read the article
Fire resistance: How long a complete wall, floor or fire-stopping system keeps its fire-separating function under test criteria — for example 1 or 2 hours. It belongs to the system installed as tested, not to one material. Read the article
Passive fire protection: Dividing a building into compartments that hold back fire and smoke, using tested walls, floors and seals for openings — as opposed to active systems such as sprinklers. It must be installed as tested by qualified installers. Read the article

Materials

Closed-cell structure: Foam whose internal bubbles are sealed from each other, so water and vapour pass through it less easily than through fibrous materials — for example elastomeric foam and PE foam. Read the article
Elastomeric foam: Flexible closed-cell synthetic rubber foam, widely used on refrigerant, chilled-water and refrigeration pipes. Read the article
Glass wool: Fibre insulation made from molten glass. Used for heat, sound absorption and ductwork, in many densities and forms — rolls, boards and pipe sections. Read the article
Stone wool: Fibre insulation made from molten rock — not the same thing as asbestos. Used for sound, heat, high-temperature work and fire-protection systems. Read the article
Density: Mass per volume, in kg/m³. It tells you how firm the insulation is, but not its thermal or acoustic performance directly — check the property you actually need. Read the article