Heat flows inwards and outwards in relation to fluctuating internal and external temperatures. Heat flow is Convective, Conductive and Radiative. Two qualities of the construction determine its overall thermal performance. The first is its Thermal Resistance and the second is its Thermal Capacity.
This is the resistance to the flow of heat across a material or at an interface between two materials. Where the resistance to heat is through a material it is the product of its thickness and a property known as its thermal resistivity. The thermal resistance of a construction is the sum of all the thermal resistances of its elements and is usually known as the R-value.
This is the rate of flow of heat across a specific construction. It is the inverse of the Thermal Resistance of all the elements that make up the construction.
Thermal resistivity is a property of a given material to resist the flow of heat across it per unit thickness. It in turn is the inverse of its thermal conductivity (lambda value). Metals have a very low thermal resistivity, whereas insulants are defined by high resistivity.
This is the rate of flow of conductive heat per unit area per unit temperature drop. Metals have very high thermal conductivity and insulants very low thermal conductivity.
As heat flow is convective as well as conductive, surfaces have an inherent resistance to heat flow. Where the air flow past the surface is high convective losses are higher and the surface thermal resistance is low and vice versa.
The property of surface resistance is exploited in the use of cavities trapping still air and having two surfaces to resist heat flow. Since heat flow is also radiative, shiny surfaces (with low emissivity) have a higher thermal resistance than matt surfaces with (high emissivity). Plasterboards and other sheet materials are often specified with a aluminised polyester backing which increases the thermal resistance of the adjacent cavity.
Insulation is designed to trap still air (or other gas). Its properties and performance depends on its ability to do this. The Thermal Resistance of the insulation is by far the most significant factor in any construction. Cavities, surface resistances etc. are contributive but are far less significant. Some materials are themselves good insulants - in particular wood-based products, which can add significantly to the overall thermal resistance.
This is the property of a material to retain heat. It is a product of its density and specific heat. It does not affect the rate of flow of heat under steady conditions but under fluctuating conditions acts as a damper - soaking up heat before releasing it. The thermal capacity thus creates a time-lag which is used in traditional/ passive constructions to moderate the external climate.
Insulation is specified according either to meet Thermal Resistance or U-Value requirements.
Targets are usually specified by National Building Regulations
Suitability of Insulants
It is not difficult to find insulation to meet a given target. However, many factors come into play in specifying the type of product that is most suitable in particular:
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