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Glossary
Air Infiltration
Air ingress into a building through cracks or gaps.
Blown Wool
Loose mineral wool insulation applied by blowing, usually into cavity walls or lofts.
Breather Membrane
A
breather membrane repels water that penetrates an outer cladding
material but is permeable to water vapour escaping from the structure.
It is defined as a material with a vapour resistance between 0.1 and
2.0 MNs/g, although the norm is for breather membranes to have a
maximum vapour resistance of 0.6 MNs/g.
Cavity Tray
A
damp-proof membrane or pre-formed tray crossing the cavity of a wall at
interruptions in the cavity. Rises from the outside of the cavity
upwards at least 150mm.
Condensation
When water vapour from the air comes into contact with cold surfaces, the vapour condenses on the cooler surface.
Conduction
The transmission of heat through, along or from one material to another material in contact with it.
Convection
The transfer of heat by movement of air.
Dewpoint
A calculated temperature at which water vapour will condense.
Emissivity
Emissivity
is a measure of the radiation emitted from a surface. 'Black bodies'
(materials with matt black surfaces) have a high emissivity & emit
large amounts of radiation. In contrast shiny surfaces, such as
polished aluminium, have a low emissivity. This characteristic can be
exploited in buildings by providing aluminium foil facings on
insulation materials, facing out towards cavities to reduce the
radiated heat loss. Matt black has a typical emissivity of 0.95,
compared to 0.08 for polished aluminium. But most building surfaces,
even white painted surfaces, have high emissivities, of around 0.90 to
0.95.
Fire Resistance
The
ability of a building element or component to act as a barrier to the
spread of fire and confine it to the area in which it started. Contains
flames and inflammable gas and prevents not only the transmission of
flames and smoke but also of heat to the other side of the construction.
Fire Resistance: Heat Transmission
Contains
flames and inflammable gas for a short period of time, but does not
prevent the transmission of heat to the other side of the construction
(example: wired glass).
Frequency of Sound
The
frequency of sound is measured in Hz (hertz). The higher the value, the
lighter the tone (bass - treble). The frequency of speech lies
primarily between 125 and 8000 Hz, while audible sound lies between 20
and 20 000 Hz.
Heat Loss
Heat transfer from internal spaces to external areas through conduction, convection and radiation.
Interstitial Condensation
Interstitial
condensation is condensation that occurs within a building element. It
can be very harmful if interstitial condensation occurs on surfaces
within the structure, as this can lead to the rot of timber or
oxidation of metal surfaces, and other problems. There are many
instances when condensation can form on the surface or interface of two
materials without causing problems i.e. the cavity side of an external
brick or block leaf.
Kelvin
An
absolute temperature scale in which the temperature intervals
correspond to those of the Celsius scale and the freezing point of
water is 273 Kelvin (written 273K). When referring to temperatures on
the Kelvin scale, refer to "Kelvin" and not "degrees Kelvin". Zero
Kelvin (0K) is known as "absolute zero".
Noise Reduction Coefficient (NRC).
A single value for sound absorption, derived as the mean value of 4 frequencies in the range 250-2000 Hz.
Noise Reduction Coefficient (NRC).
A single value for sound absorption, derived as the mean value of 4 frequencies in the range 250-2000 Hz.
Radiation
The process by which heat is emitted from a body through open space, for example, sunlight.
Sound Absorption
Sound
energy converted into mechanical vibration energy and/or heat energy.
Sound absorption is usually expressed as a sound absorption coefficient.
Sound Insulation
The
ability of a building element or building structure to reduce the sound
transmission through it. The sound insulation is measured at different
frequencies, normally 100-4000 Hz. Airborne sound insulation is
expressed by a single value, Dn,c,w , Rw or R'w. Impact sound
insulation is expressed by a single value Ln,w or L' n,w.
Sound Strength (dB)
Measured in dB (deciBel). dB is measured at different frequencies.
Surface Resistance
Surface resistance (Rs) is the reciprocal of surface coefficient. Its units are m2/K/W.
Temperature
Temperature
levels are commonly measured in degrees Celsius (°C) whereas
temperature differences are measured in Kelvin (K). Each unit Celsius
is the same size as each unit Kelvin, so that, if the outside
temperature was 3°C and the internal temperature was 18°C the
difference in temperature would be 15K. Note that the degree symbol °
is used with degrees Celsius but not with Kelvin.
Thermal Conductivity
The
rate at which heat is transmitted through a material, measured in watts
per square metre of surface area for a temperature gradient of one
Kelvin per metre thickness, simplified to W/mK.
Thermal Mass
The mass in a structure which is used to absorb solar heat during the day and then release the heat in the evening.
Thermal Resistance
A
measure of the thermal resistance of a particular thickness of
material. It is measured in m2K/W and is equal to the thickness of the
material (in metres) divided by the conductivity of that material.
Surfaces and cavities also provide thermal resistance and there are
standard figures for these resistances that must be taken into account
when calculating U-values. The resistances of each material within an
element are added together to determine the overall resistance of the
element. The reciprocal of the overall resistance is the U-value.
Thermal Resistivity
Resistivity
is a measure of a material's thermal resistance and is the reciprocal
of its conductivity. It is a measure of the thermal characteristic of a
material and is independent of its thickness.
U-value (Thermal Transmittance)
Commonly
known as the U-value, it is a measure of the rate of heat loss of a
building component. It is expressed as watts per square metre, per
degree Kelvin, W/m2K. The U-value is calculated from the reciprocal of
the combined thermal resistances of the materials in the element , air
spaces and surfaces, also taken into account is the effect of thermal
bridges, air gaps and fixings.
Vapour Control Layer
A
vapour control layer is defined as a material that substantially
reduces the water vapour transfer through a building element into which
it is incorporated i.e. polythene sheet materials or foil backed
plasterboard. Vapour control layers are sometimes required on the warm
side of the insulation, to reduce the possible risk of interstitial
condensation within the construction element.
Vapour Permeable Underlay
A
vapour permeable underlay repels water that penetrates a roofing finish
but is permeable to water vapour escaping from the structure. It is
usually defined as a material with a vapour resistance of not more than
0.25MNs/g.
XPS - Extruded Polystyrene
A
cellular plastics insulation material that has been extruded, with or
without a skin and which has a closed cell structure, for example,
Polyfoam
