Insulation Academy
Insulation is one of those ubiquitous techniques that is always around, always impinging on our work, social & domestic activities...and yet for most of the time is hardly noticed. Insulation is a passive product; once installed it works efficiently, quietly and continually, usually out of site enclosed with in an element of construction. As such, it is perhaps taken for granted or, worse still, given a level of importance which is far beneath its value.
Insulation comes to the fore when new design of buildings, plant, equipment or process is being considered. It's at this stage that the right specification must be made since any shortfall in thickness or error in the type and application detail will prove costly or impossible to rectify at a later date.
Why insulate?
There are many reasons why insulation is used - these are just some...- To comply with mandatory legislation (e.g. Building Regulations)
- To reduce heat loss/heat gain and hence energy consumption
- To raise comfort levels
- To reduce running cost
- To reduce pollution (greenhouse gases, acid rain)
- To provide condensation control
- To reduce the risk of freezing
- To reduce heating plant capacity
- To control process temperatures
- To control surface temperatures
- To provide acoustic correction & noise control
- To provide fire protection
Thermal Insulation
A thermal insulation material is one which "frustrates" the flow of heat. It slows down the rate of heat loss from a hot surface and slows down the rate of heat gain into a cold body. Insulation cannot stop the loss or gain of heat completely.
Insulation itself does not generate heat but feels warm to the touch because the rate of heat loss from your hand is slowed down. The heat is generated by your body. Likewise, in buildings, heat loss is slowed down by using insulation and this can be used to...
- reduce the initial heat load applied (smaller heating systems, less use of heating systems) thereby saving energy, reducing costs & pollution
- to raise internal temperatures (increase comfort, condensation control, freezing control)
- or a combination of both
Most insulants comprise a solid component which forms a matrix to trap pockets of still air. It is the still air which actually insulates. These pockets are also called voids or cells. Some products are formed by using an inert gas to create bubbles in a molten matrix and in these cases the cells are filled with the inert gas not air.
Principles of insulation
Heat transfers
When a hot surface is surrounded by an area that is colder, heat will transfer until both are at the same temperature. This transfer of heat takes place by one or more of 3 methods:
1. Conduction
A process by which heat flows by molecular transportation along or through a material or from one material another. Conduction requires contact between 2 materials for heat transfer to occur. It can occur in solids, liquids or gases.
2. Convection
A process that can only occur in liquids and gases which involves a change in density of the liquid or gas resulting from a change in temperature. If in contact with a solid, heatloss would initially be through direct contact(conduction) but resulting density change of the liquid or gas would cause it to be displaced and a continuous flow(or convection) would result.
Forced convection is when the liquid or gas is displaced at an accelerated rate by artificial means (such as wind). The rate of heat transfer is substantially increased in such cases.
3. Radiation
A process by which heat is emitted and transmitted across space as energy. It requires no contact or medium such as air to transfer and readily occurs across a vacuum. All bodies emit radiant energy and the rate of amission is governed by temperature difference, the distance and the emissivity of the surfaces.
How insulation works?
In order to perform effectively, insulation must restrict heat flow by any and preferably all three methods of heat transfer. Most insulants (but not all) adequately reduce convection and conduction by having a cellular structure. The radiation component is reduced by absorption into the body of the insulant and can be further reduced by the application a bright foil on the outer facing of the product.
To reduce heat transfer by conduction, an insulant should have a small ratio of solid volume to void volume. Additionally, a thin wall matrix, a discontinuous matrix or a matrix with minimum point contact are all beneficial. Conduction across the voids can be further reduced by the use of inert gases rather than still air.
To reduce heat transfer by convection, an insulant should have a cellular structure or a high vold content. Small cells or voids inhibit convection within them and are less prone to transfer energy to adjacent cells.
To reduce heat transfer bt radiation, insulation is placed in close contact with the hot surface. Radiation may still penetrate an open cell material but rapidly absorbed within the immediate matrix and energy is converted to conductive or convective heat flow.
Insulation Science
Thermal conductivity (λ)
Defines a materials ability to transmit heat, so the lower the figure the better the performance. Called lambda, it is measured in watts per square metre of surface area for a temperature difference of one Kelvin per unit thickness of one metre(W/mK). For the Construction Products Directive and CE marking, the term lambda 90:90 is used which signifies a particular treatment of the results. However, in construction, materials are not used at unit thicknesses of one metre and so another measure of performance is commonly adopted, known as R value.
Thermal resistance (R)
Defines a materials ability to resist heat flow, so the higher the figure the better the performance. Called R value, it is calculated by dividing the thickness (in metres) by the thermal conductivity (l90:90). For the Construction Products Directive and CE marking, the term R90:90 is used. This is not only a result of using l90:90 in the calculation but also requires a rounding down of the value to the nearest 0.05.
Example: 170mm Crown wool R value =0.170/0.044
= 3,86
= 3,85 (zaokrąglone do dołu)