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Glossary of Terms


An ultra-lightweight thermal insulation material – an extremely effective insulator with low thermal conductivity. Although it’s derived from a gas and a gel, the finished aerogel is a thin, low-density solid.

Calcium silicate
A white, free-flowing powder with a low bulk density and high water absorption. A much safer alternative to asbestos for high-temperature insulating projects.

Expanded polystyrene (EPS) / extruded polystyrene (XPS)
EPS is the familiar white plastic foam, often used as packaging. It features a closed-cell structure, formed by expanding a polystyrene polymer. XPS is also a rigid foam insulation formed from a polystyrene polymer, but it’s made using an extrusion process. Grey EPS and XPS are forms that incorporate graphite to increase the material’s U-value.

Glass wool insulation
Insulation made from layers of fiberised glass, bound and cured together. Air trapped between the glass layers boosts thermal performance.

Mineral wool insulation
Generic term used to refer to glass wool and / or stone wool.

Multifoil insulation
Insulation made from layers of aluminium covered with a plastic foil. It has high reflectivity and very low emissivity. The thermal performance comes from combining the multifoil with an insulant.

Phenolic insulation / phenolic foam insulation
Fire-resistant material created by mixing high solid plastics with phenolic resin. The resulting foam has a fine cell structure that resists compression as well as fire.

PIR/PUR insulation
Polyisocyanurate (PIR) and polyurethane (PUR) have a similar make-up and properties. They are both forms of rigid, closed-cell, low-density insulation. The low thermal conductivity of the gas trapped within the closed-cell structure provides excellent thermal performance. The big difference is between the catalysts and additives used in PIR and PUR formulations. PIR insulation products exhibit reduced combustibility and higher working temperature limits compared to PUR. Rigid foam insulation Types of insulation produced by mixing chemical resins with a surface-acting agent. PIR, PUR, and phenolic insulation are all types of rigid foam insulation.

Sheep wool insulation
Insulation made from sheep’s wool. The wool fibres can be held together mechanically or bonded using polyester adhesive to form insulating batts, rolls, and ropes.

Stone wool insulation
Insulation made from volcanic rock. It’s similar to glass wool insulation, but requires much higher temperatures to melt the rock. The resistance to heat is what gives it such excellent fire performance.



Another name for thermal conductivity.

Lambda λ (thermal conductivity)
The rate at which heat is transmitted through a material. It’s measured in watts per square metre of surface area for a temperature gradient of one degree kelvin per metre thickness, simplified to W/mK or λ. The lower the value, the more thermally efficient the material – which makes it a better insulator.

R-value (thermal resistance)
The ability of a material to resist the transmission of heat. It’s the thickness of the material (measured in metres) divided by its lambda value. The higher the R-value, the less heat that escapes through it. It’s a better insulator. 

Thermal bridge
A route through which heat can escape from an insulated area. The bridge could be a gap in the insulation material or a break filled by a material, such as a fixing, with a higher thermal conductivity. Thermal bridging will reduce the energy efficiency of an otherwise well-insulated building; it may also cause condensation.

Thermal conductivity
(Lambda λ) See Lambda λ above.

Thermal looping
A continuous circulating flow of air caused by temperature differences. On one side of the loop, warm air rises; on the other, cool air descends. Between the two, warm and cool air flow sideways (warm at the top, cool at the bottom) to fill the gaps.

Thermal resistance
See R-value above.

Total resistance or total thermal resistance
The sum of the resistance (R-value) of each individual element within a structure. Resistance can relate to the resistance of an insulation product that’s made of layers (board, foam, foil etc.), and the resistance of a structural element that’s made up of different materials (walls, windows, doors etc.).

U-value (thermal transmittance)
The rate of heat loss of a building material or component, expressed as the energy loss (in watts per square metre) divided by the temperature difference (in °K) between the inner and outer faces of the component. The U-value is also the inverse of the total resistance (1 divided by total resistance) of that component. The total thermal resistance of a component must incorporate every element within it, including thermal bridges such as gaps and fixings. The lower the U-value of a material or a component, the better insulation it provides.
U-value = 1/TR



Acoustic insulation
Inhibits the transmission of sound between neighbouring rooms – above, below, or to the side. Insulation has to reduce three types of sound transmission: airborne, flanking, and impact.

Airborne sound transmission
When airborne sound waves reach a solid element of a building, they cause it to vibrate. The vibrations travel through the element which then radiates them from the other side. Typical sources of airborne sounds are speech, music, and appliances.

Decibel (dB)
A measure of sound intensity widely used in electronics, signals, and communication. Decibels use a logarithmic scale to describe relative levels of sound. For every increase of 10 dB, the actual power of the sound multiplies tenfold.

Flanking sound transmission
A form of noise transmission in which vibrations from a noise source are transmitted to other rooms via elements of the building’s structure. Flanking sounds tend to take an indirect route of transmission.

Impact sound transmission
Sound transmitted between two adjacent rooms by direct impact upon the dividing element. A typical example would be footsteps on an internal floor.

In an enclosed space, sound bounces off walls – sometimes many times over. The collective effect of multiple reflected sounds is known as reverberation. Reverberation continues after the original source of the sound has stopped.

Robust Details
A method of achieving compliance with Part E of the Building Regulations (England and Wales) that avoids precompletion testing. A Robust Detail is a building element that meets the acoustic standards set by Robust Details Limited. 

Pre-completion testing (PCT)
An acoustic test stipulated within Part E of the Building Regulations that applies to new flats, houses, and flat conversions. The owner or builder must show that the dwelling achieves its stated acoustic rating and that it complies with Building Regulations. At least 10% of all new dwellings must be PCT-tested on site. Sound absorption The absorption of sound waves by a material. A material or surface that absorbs sound waves does not reflect them. Absorption of a given material is frequency-dependent and affected by the size, shape, location, and method used.

Sound attenuation
The gradual loss in intensity of sound as it passes through a medium or a material. Attenuation is a combination of a material’s ability to scatter and absorb sound.



Class 0
The top classification for fire rating or fire resistance. A product or system with a Class 0 fire rating protects against the surface spread of flames, and limits the amount of heat released from the surface during a fire.

Euroclass rating
The new European fire-rating standard that replaces the UK’s Class 0. The ratings are A1, A2, B, C, D, E, and F. A1 is the best and safest rating, E is the least safe, and F is unclassified.

Flaming droplet
Burning droplets or particles can be a source of skin burns and the further spread of fire. There are three classes of burning droplets – d0, d1 and d2. d0 is the best, d2 is the worst.

The rapid transition to a state of total surface involvement in a fire. The time to flashover determines the limits of the reaction-to-fire classification.

A material that will not support a fire. Building construction materials are not always required to be rated as noncombustible. However, fire-rated materials are much more likely to be specified as non-combustible.

Surface spread of flame
The speed with which a flame will spread from one area to another. A requirement within Part B2 of the Building Regulations specifies that all internal linings should adequately resist the surface spread of flame. 



Part B
The section of the building regulations that deals with construction requirements for fire safety.

Part E
The section of the building regulations that deals with construction requirements for resistance to the transmission of sound.

Part L
The section of the building regulations that deals with construction requirements for the conservation of fuel and power. Part L stipulates minimum U-values for each element of construction, such as walls, roof, and floor. The requirements vary depending on whether the project is a new-build or a renovation and whether it’s for domestic or commercial use.



Clear-cavity wall
A cavity wall without any insulation in the cavity.

External wall insulation (EWI)
An exterior cladding system that’s thermally insulated, protective, and decorative. EWI involves expanded polystyrene (EPS), mineral wool, PIR, PUR, or phenolic insulation overlaid with a reinforced cement-based, mineral or synthetic finish.

Full-fill cavity
A wall cavity that has been fully insulated and prevents air from passing between the top and bottom of the cavity. The insulation leaves no uninterrupted air paths that allow the flanking escape of energy from either end of the wall.

Internal wall insulation (IWI)
Insulation created by fitting rigid insulation boards to the internal face of a wall. The insulation can be fixed using dot-and-dab or mechanical techniques. An alternative is to build a stud wall filled with insulation material such as mineral wool fibre.

Partial-fill cavity
A wall cavity that has not been insulated to the full width of the cavity. Partial-fill cavities typically have a residual width of 30mm to 50mm. Although the amount of insulation can be increased, the savings are likely to be less than for a standard cavity wall.

Party wall
A structural wall separating two adjoining dwellings. The mid-point of the wall forms the boundary between the two dwellings. The wall may not be structural if it separates dwellings within an apartment complex, but it will still have to meet specified criteria for sound insulation and fire protection.

Solid wall insulation (SWI)
Many older houses – especially those built before 1930 – have solid masonry walls. The heat loss through their external walls is around 35%. As well as reducing energy loss and improving weatherproofing, external solid wall insulation can strengthen the structure and improve the look of an old house.

Steel framing system (SFS)
A building technique involving a skeleton frame of vertical steel columns and horizontal T-beams. The frame forms a rectangular grid to which the floors, roof, and walls are attached.

Window reveals
A timber surround fitted to the fin of the window, used to fit the window to timber framing. The reveal covers the stud and becomes the window frame.



Dew point 
The air temperature below which water droplets begin to condense and form dew. Dew formation often leads to condensation problems. The dew point varies from day to day and location to location; it depends on the pressure and the humidity of the air.

Moisture ingress
Water that penetrates the fabric of the building. This can lead to dampness and mould. Over time, it may affect the integrity of the building structure.