Reducing our use of fossil fuels is the most important factor in tackling climate change, and one of the largest uses of fossils fuels is in heating our homes. Better insulating our houses has massive potential for reducing our CO2 emissions, and our household bills, whilst improving comfort levels.
Insulation may be regarded as any material which significantly reduces the heat transfer between two objects – in this case the objects are elements of a wall separating spaces, or the spaces themselves.
Essentially, it is air pockets that arrest heat transfer through a material – if we could have vacuums enclosed within our walls there would be no heat lost whatsoever! However as yet this is not possible and so most insulation materials are in fact lots and lots of air pockets separated by a material with an inherently low thermal conductivity.
Insulation is measured by its k-value, or thermal conductivity, measured as Watts per m2 Kelvin (W/m2K).  – the lower the number the better, The u-value of a certain thickness of insulation is achieved by multiplying its k-value by its thickness.
Insulation products are generally available as boards, batts (like a quilt), or loose-fill – specification will depend upon what needs insulating, and also which material is favoured.
There are many different insulation products on the market, but they all fall into one of three main categories, which loosely have similar properties:



Organic insulation materials are generally those with the highest k-values (0.037 – 0.040W/m2K) and highest prices – however they are natural, largely chemical free, hygroscopic, made from renewable materials, and sequester CO2 within your walls. They are reusable and compostable.

Animal Fibre (Sheep's Wool); Typical k-value = 0.039W/m2K

Comes largely in batts or rolls with a polyester binder added and treated with borax for fire and insect resistance. It is suitable for use between rafters, joists, and timber studs in ‘breathing’ wall construction – it has excellent hygroscopic properties that help to moderate internal atmospheres.

Sheep’s Wool has many environmental benefits as an insulation material – it is a waste product from renewable, local sources that sequesters CO2, and is reusable and recyclable.
However it contains an oil-based binder (polyester) that prevents it from being composted and raises its embodied energy. It is also treated with borax, which has a suspected reproductive toxin that could affect installers, and will leach if permanently or intermittently kept damp. Imported wool may contain pesticides.
As with all batts, thermal conductivity is negatively affected by compaction.

Cellulose (Recycled Paper); Typical k-value = 0.039W/m2K

The recycled newspaper is usually treated with a mixture of borax and boric acid to provide fire, insect, and fungi resistance. It is available loose for pouring or spraying between rafters and joists and in timber 'breathing' wall construction, as well as slabs for fitting within metal or timber frames.
At least 75% is from recycled sources, and the original is from renewable sources that sequester CO2 (trees!). It has a very low embodied energy, is recyclable and compostable, and hygroscopic.
On the down side, newsprint production can produce hazardous waste, and approximately 20% of the total content contains borax – see above. There are possible risks of odour and formaldehyde from off-gassing printing inks, and from the inhalation of paper dust during installation.
Thermal conductivity can be negatively affected by compaction or settlement. 

Cork; Typical k-value = 0.040 – 0.050W/m2K

Cork insulation is made from bark that is harvested every 25 years per tree. Cork granules are expanded and then pressed into blocks using high temperature, pressure and the tree's natural lignum.
Cork insulation is used primarily in flat roofs and insulated render systems, but is useful in any situation that requires dimensional stability and resistance to compression.
Cork comes from managed woods that once primarily fed the wine industry. Its production helps to sustain communities in poorer agricultural areas and maintain local indigenous wildlife.
Like all organic materials, cork sequesters CO2, it is reusable (if not adhesive or render-coated), recyclable as loose fill, and compostable. It is waterproof and naturally resistant to insect and rodent attack (except wasps!)

Plant Fibre (Hemp, Flax, Cotton); Typical k-value = 0.038 – 0.040W/m2K

Plant fibre slabs are made from hemp, flax, or cotton (or potentially a mix), combined with wood fibres and a polyester binder. It is then treated with borax for fire, insect, and fungi resistance.
Plant fibre insulation is used in breathing wall construction, ventilated pitched roofs, ceilings, and floors.
Plant fibres are renewable materials that sequester CO2, are hygroscopic, can contain recycled material, are reusable, recyclable, and can be composted.
They also contain non-renewable material however (polyester binder), and the use of pesticides and fertilisers during the growth of the plant contribute to climate change and can have a negative environmental impact. They also contain borax – see Animal Fibre (above) for more information. These products are generally imported from the continent, which adds slightly to their embodied energy.
Thermal conductivity can be negatively affected by compaction or if damp. 

Wood Fibre; Typical k-value = 0.040W/m2K

Wet-formed wood fibre board insulation is largely made from saw mill waste. The wood is chipped and then soaked in water before being pressed and dried. No bonding agents are added - the fibres are held together by the wood's natural lignum.
Wood fibre board insulation is typically used in breathing wall construction, but also as in roofs as insulated sarking. It is particularly good for use in external or internal retrofitted solid wall situations, as it will allow any moisture built-up in the existing wall to escape. It also has a certain thermal mass and so can dampen lag times, meaning it is ideal for use in lofts which may tend to get hot during the day.
Wood fibre boards are made from waste material from renewable sources that sequester CO2. They are hygroscopic and have thermal mass, are reusable, recyclable, and compostable.
On the down side, they have a relatively high embodied energy (due to the manufacturing process) when compared with other organic insulation materials, but still very low when compared to mineral-based materials, and tiny compared to oil-based materials. Quite simply it is the Don.

Wood Wool; Typical k-value = 0.038W/m2K

Wood wool insulation is very similar to wood fibre, except that it is looser and comes in batts, needing polyolefin fibres for binding, and an ammonium phosphate based fire retardant - both of which add to its environmental impacts. Because of the manufacturing process, it has roughly half of the embodied energy of wood fibre insulation however.
Note: Not to be confused with Wood Wool Cement boards (0.05W/m2K) which are lightweight rigid boards made of slithers of wood with a cement binder, ideal for inexpensive internal refubishment and plastering, and fire and soundproofing. They will 'breathe', but are not hygroscopic as such.

Wood Wool Cement Boards; Typical k-value = 0.05W/m2K

 Wood Wool Cement boards are lightweight rigid boards composed of slivers of wood bound with a cement binder.

They have been used for decades, and are an ideal and inexpensive internal refubishment material that can be directly plastered, and offer good fire and sound proofing. They will 'breathe', but are not hygroscopic as such.



Mineral insulation materials are mid-range options in terms of price and performance (generally 0.03 – 0.04W/m2K). They are natural materials that have generally been through energy-intense processes and thus generally have a higher embodied energy than organic materials. They do not sequester CO2, and although not strictly hygroscopic, they will allow moisture/air flow to some extent – although this is to their detriment as an insulant. They are reusable and recyclable.

Aerogel; Typical k-value = 0.013W/m2K

Aerogels are the world's lightest solid materials. Aerogel insulation is made from silica and 99.98% air - it is produced by extracting the liquid component of a silica gel through supercritical drying, allowing the liquid to be slowly dried off without causing the solid matrix to collapse.
Very much the new kid on the block, aerogel blankets are beginning to appear as a component in laminate panels bonded to plasterboard, wood fibre reinforced gypsum board, plywood, and chipboard. The panels are distinguished by their outstanding insulation properties for little thickness.
Aerogels display relatively high compressive strength, are water impermeable, and inherently resistant to fire and rot.
Silica-based aerogels are not known to be carcinogenic or toxic, however they are a mechanical irritant to the eyes, skin, respiratory tract, and digestive system. Small silica particles can potentially cause silicosis when inhaled. It is recommended that protective gear, including respiratory protection, gloves and eye goggles be worn whenever handling aerogels.
They are very expensive.

Cellular/Foamed Glass Insulation; Typical k-value = 0.037 – 0.048W/m2K

Foamed glass insulation is formed by crushing glass, mixing it with carbon, and heating it to 1000ºC, causing the carbon to oxidise and form bubbles. Foamed glass has a relatively high compressive strength and good moisture impermeability, making it suitable for high-load situations such as retaining walls, car parks and green roofs.
Foamed glass typically contains 60% of post-consumer glass waste, and can be crushed and recycled itself. It is inherently resistant to fire and rot.
Its environmental impacts and emissions are associated with sand quarrying, glass manufacture, and its high embodied energy.

Glass Wool Insulation; Typical k-value = 0.030 – 0.040W/m2K

Glass wool batts are made from silica sand, up to 60% recycled glass, limestone and soda ash, with a manufacturing process similar to rock wool.
The insulation is produced in a variety of densities, resulting in varying levels of thermal conductivity. Applications include cavity walls, timber frame walls, roof rafter insulation, loft and suspended floor insulation. 
Glass wool is reusable, recyclable, inherently non-combustible, and resistant to rot.
The emissions associated with the manufacture of glass wool are mostly in energy generation. It can include boron to improve moisture tolerance, which would otherwise affect its thermal conductivity, as does compaction.

Rock Wool Insulation; Typical k-value = 0.030 – 0.040W/m2K

Rock wool insulation batts are made from quarried diabase rock and recycled steel slag. They are produced in a variety of densities, resulting in varying levels of thermal conductivity.  Applications include cavity walls, timber frame walls, roof rafter insulation, loft and suspended floor insulation.
Rock wool can include around 23% secondary industrial waste, or recycled mineral wool. It is reusable, recyclable, inherently non-combustible, and resistant to rot.
Production emissions include carbon monoxide, phenol, and formaldehyde, although a formaldehyde-free binder is available in some brands and UK emissions are within legally defined limits.
Rock wool is no longer classified as a skin irritant, although exposure can cause temporary irritation. Thermal conductivity can be negatively affected by compaction or wetting.

Vermiculite, Perlite 

Vermiculite and perlite are two very different naturally occurring minerals that nonetheless display similar properties – they both have high melting points, but expand greatly when heated, becoming much less dense. Both are produced as small aggregates, up to 10mm in size.
Vermiculite is therefore particularly useful for fireproofing applications, or where insulation is required next to a high heat source, such as furnace linings. In a domestic situation, it can be used to loose fill between chimney flues and the chimney brickwork.
Perlite is generally used as a lightweight aggregate and insulant in cements and renders, ceiling tiles, and firestop mortar.
Both have countless other uses industrially, commercially, and domestically. For example, in horticulture they prevent the compaction of soils, improve aeration and drainage in composts, and are completely stable.
Environmental impacts are largely due to mining and heat treatment in production, although impure vermiculite can contain asbestos, as seen at the Libby mine, Montana, which produced 80% of the world’s supply before the mid-1990s (sold as ‘Zonolite’), after which it was shut due to serious health concerns of both miners and townspeople. Any product containing the Zonolite brand of vermiculite is considered a potential health risk due to asbestos contamination. There is no such risk from pure vermiculite.


Oil-based insulations are the cheapest and most effective at blocking heat transfer (0.02 – 0.04W/m2K). They are also great at blocking moisture, which can be a benefit in certain applications, but not so good for the internal atmosphere if used in your walls, potentially leading to dampness and health issues. They are made from oil, off-gas toxins, and have a large embodied energy, although pay for this easily through the course of their lives. They are potentially re-useable.

Expanded Polystyrene (EPS) Insulation; Typical k-value = 0.032 – 0.040W/m2K

EPS insulation is made from small beads of polystyrene, and boards are essentially produced by putting the beads into moulds and heating to fuse the beads together.
Typical uses of EPS are in walls, roofs and floors. Polystyrene beads are often used as post-fit cavity insulation.
EPS is reusable if in reasonable condition, and recyclable if ground down and added to new boards. EPS has a fairly high compressive strength, is water impermeable, and is inherently resistant to rot and vermin.
However, it is derived from petrochemicals, and therefore has a high embodied energy. It also contributes to resource depletion and pollution risks from oil and plastics production. EPS contains HBCD, added as a fire retardant, which is regarded as hazardous. Hydrocarbons are emitted as part of the production process, although UK emissions are within legally defined limits. The finished product can off-gas styrene.
Its use is wide spread in new-build, basically because it is cheap.

Extruded polystyrene (XPS) Insulation; Typical k-value = 0.028 – 0.036W/m2K

XPS is made by mixing polystyrene with a blowing agent under pressure and forcing the resulting fluid through a die. It expands into a foam as it emerges, is shaped, cooled and trimmed to size.
XPS is used as EPS might be, except that it is stronger, so more suitable for use where it might be subjected to greater loading and/or impact.
It is reusable if in suitable condition, and recyclable if crushed and added to new boards. It has a high compressive strength, is water impermeable, and inherently resistant to rot and vermin.
Its negatives are as EPS above, plus it can also release small amounts of chlorofluorocarbons, which are greenhouse gases.

Rigid Polyurethane (PUR/PIR) Insulation; Typical k-value = 0.022 – 0.028W/m2K

Polyurethanes are in the class of compounds called reaction polymers, which include epoxies, unsaturated polyesters, and phenolics. According to wikipedia - 'Polyurethanes are produced by reacting an isocyanate containing two or more isocyanate groups per molecule (R-(N=C=O)n ≥ 2) with a polyol containing on average two or more hydroxy groups per molecule (R'-(OH)n ≥ 2), in the presence of a catalyst' - although it's not quite as simple as that... 
Polyisocyanurate foam (PIR) is basically an improved polyurethane - the constituents are slightly different and the reaction is conducted at higher temperatures. PIR is more fire-resistant and more insulative.
Both are supplied as rigid foil-backed boards - applications include roof, wall, and floor insulation (they have good compressive strength), in laminate form in structural insulated panels (SIPs), and as backing to plasterboard and similar.
PUR/PIR boards are reusable if in good condition, and recyclable if ground down and added to new boards. They are water impermeable and inherently resistant to rot, making them useful for damp and wet situations, or underground.
Panels are often produced with tongue & groove, since shrinkage can cause gaps between boards reducing their effectiveness. Effectiveness can also be reduced over the first 3 years due to gas exchange from the cells, although this is usually included in the manufacturer’s declared k-value.
Fully reacted polyurethane polymers are chemically inert, however they are derived from petrochemicals and thus inherently have high embodied energy, and are responsible for resource depletion and pollution risks from oil and plastics production. Decomposition from fire produces trace nitrogen oxides and hydrogen cyanide. On top of this, the production process creates a number of emissions to air and water, and hazardous wastes as defined by EU Directive 91/689/EEC 17. Spray polyurethane foams present health risks.

Phenolic Foam Insulation Typical k-value = 0.022W/m2K

Phenolic foam insulation is made by combining phenol-formaldehyde resin with a foaming agent. Hardener is added and rapidly stirred, which causes an exothermic reaction of the resin, resulting in foaming resin. This then rapidly sets hard.
Much like PUR/PIR, phenolic foam panels are used as insulation in roofs, walls and floors, and as insulation backing to plasterboard and similar. It is inherently flame resistant, has high compressive strength, and is moisture resistant.
Panels are often produced with tongue & groove, since shrinkage can cause gaps between boards reducing their effectiveness.
It can be reused if in reasonable condition, but is not readily recyclable.
As well as all of the usual environmental disadvantages of oil-based products, phenolic foam is produced from phenol formaldehyde - a toxic petrochemical derivative.

Urea-Formaldehyde Foam Insulation Typical k-value = 0.032W/m2K

Urea-formaldehyde foam insulation (UFFI) was invented in the 1930s and seemingly made a great synthetic insulation. It is made by mixing the resin, foaming agent and compressed air, and could be easily injected into walls with a hose. It becomes firm within minutes, but cures within a week. When cured, it is quite brittle, and often has a dry and crumbly texture.
UFFI was used extensively in the 1970s as a wall cavity filler. By the 1980s, concerns began to develop about formaldehyde vapour emitted during the curing process, as well as from the breakdown of old foam.
Because less information was known about the toxic health effects of formaldehyde in the 1970s, extra formaldehyde was often added to the mixture to ensure that the curing process would occur completely. A variety of adverse health effects impacting the eyes, nose, and respiratory system were reported, and consequently its use was discontinued.
The urea-formaldehyde emissions decline over time and significant levels should no longer be present in the homes today. There is no need to use UFFI, as there are plenty of more healthy alternatives available!
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