Types of Insulation

In general, the lower the K value of an insulating material or system all other things being equal, the better, however there is usually a trade-off between K values and other factors such as cost. For example, aerogels offer a better U value than virtually any other material, but are generally too expensive for bulk applications. If, however you have limited space, and are insulating pipes containing high temperature or cryogenic fluids, then the extra cost could well be justified. (Aerogels were used in the re-entry heat shield of the space shuttle).

The cost of thermally insulating materials can vary enormously, and the choice of insulation takes into account the balance between cost and other desirable factors.

Given the terrible events at Grenfell Tower, there is now a much more intense awareness of the need for non-flammable insulation materials or systems in certain situations. Many of the rigid sheet insulation materials on the market are made of foamed plastics such as polystyrene polyurethane and polyisocyanurate which even with fire retardant additives can burn or off-gas toxic substances if exposed to enough heat. Other materials exist such as foam glass, ceramic fibres, and aerogel which are inherently not flammable and which are inert at temperatures up to several hundred centigrade. Such materials will not degrade, catch fire, or give off toxic gases in a fire.

The insulation selected for a particular job can differ depending what is being insulated – a flask of liquid nitrogen will be insulated differently to a residential wall, which in turn will be insulated differently to the walls of a waste to energy incinerator or blast furnace.

Insulation comes in loose fill such as blown cellulose fibres, flexible such as rolls of batts of mineral wool, and rigid sheet / foam boards / moulded shaped rigid insulation. Each has its own place and advantages. It is important that in the required application, the insulation materials insulate without leaving gaps. (Gaps in insulation can lead to cold bridging which in turn can lead to condensation if the cold spot is below the dew point of the air in the room).

Cold Bridging Defined

Cold bridging is caused by a colder element in the structure allowing coldness to pass through the structure much quicker when warm moist air is present in the property, often caused by things like having a shower or a bath, cooking or washing, particularly if you are drying washing on the radiators. This is also caused by the general climate which results in condensation on the element.

Some insulation materials are completely vapour sealed such as synthetic foam boards allowing no transfer of moisture, whilst others are vapour permeable allowing moisture to pass through. This latter type are better suited to older construction types where the building is constructed without a DPC or vapour barriers as they do not trap moisture in the walls. In some cases, such as natural wool insulation, the wool acts as a humidity buffer helping to prevent the wall becoming either damp, in which case mould could potentially grow, or too dry which might result in cracks in the interior finish.

Some insulation materials derive from plant or animal sources – such as recycled paper, straw bale, hemp fibre, cellulose fibre boards, recycled cotton fibre, and natural wool, and expanded cork. Most of these have some susceptibility to rot, fungi, or being bio-degraded in situ if they become damp. Others are synthetic deriving from glass, rock, ceramics and plastics – which will not rot, but which in some cases can become damp and so be infiltrated by fungi. Insulation selection can have an impact on the end of life recycling, reuse or disposal of construction materials, and hence on the building's sustainability.

A few insulation materials have high strength and can be employed in structural or semi structural situations – such as foam glass boards which have high compressive strength and can be used in decking rooftop car parks as insulation for the floor below, and concrete with foam glass aggregate which can be used for structural walls. On the other hand, certain insulation materials are not even structurally stable under their own weight and tend to compact and sag over time losing much of their insulation value. Loose glass fibre and some other similar materials when used in cavity wall insulation applications can sink under their own weight – resulting in loss of insulation performance in the upper parts of walls and potential cold bridging.

Some older foam insulation materials were originally blown using chlorofluorocarbons CFCs (Infamous for the Ozone hole), and also very powerful greenhouse gasses. These have been banned for many years now, and most manufactured insulation materials have very little or no potential to release greenhouse gasses during or after manufacture.

This form of insulation depends on several mechanisms for its effectiveness. The system consists of alternating thin layers of aluminised foil which acts as a radiation barrier (opaque to Infrared), and foam which creates a separation between foil layers and which holds still air. In order to get the best out of multi-foil layers they are best installed in the middle of a cavity with a minimum 25mm air gap on either side of the multi-layer foil – which air gap plays an integral part of the insulation system – without the air gaps, you will not get the advertised U values for your construction.

These panels work like vacuum flasks and consist of an air tight coating over a rigid frame, and first have the air extracted from them in a vacuum chamber then are sealed so that there is near vacuum inside the panel. The panel coating holds the air out, and has a radiative barrier in the form of an aluminised coating. These panels have exceptional insulation properties for their thickness, but are easily susceptible to damage – one screw in the wrong place, or a careless scratch with a tool and the vacuum is broken resulting in loss of insulating properties. The system is also rather expensive, so is likely to be used only in specialist applications calling for exceptional insulation in a thin layer where the panel can be installed without risk of damage.

Some materials used in insulation have potential to give off toxic gases when heated or burned in a fire, or in other ways create a hazard – e.g. Asbestos. Asbestos is highly fire retardant and in certain forms is an excellent thermal insulator, however all forms have been banned from use in new construction and refurbishments since 2000 due to its ability for inhaled fibres to trigger serious lung disease and cancer in the long term.

Older buildings often contain asbestos – in buildings built between 1920 and 1950 asbestos is often found - sometimes as an insulation, or on buildings up to 2000 in a variety of guises for other purposes. Where asbestos is present in good condition, it is best left in place and not disturbed, it should however be inspected periodically to check its condition.

If asbestos is deteriorating or damaged by knocks, abrasion, water etc, and shedding or likely to shed fibres, or if renovations are intended which may disturb asbestos then either removal or encapsulation by professional specialists is required.

As can be seen, product selection can be complex with a variety of factors relating to safety, suitability for the application, and cost, needing to be considered.