| Insulation | K-value |
|---|---|
| Glass/rockwool | 0.04-0.032 |
| Sheep's wool | 0.04-0.035 |
| Loose EPS beads | 0.04-0.033 |
| Loose Cellulose | 0.035 |
| EPS board | 0.035-0.032 |
| XPS board | 0.029 |
| PIR board | 0.022 |
| Phenolic board | 0.020 |
| Multi-foil sheets | ? |
| Polyurethane foam | 0.023 |
Glass wool roll
Glass wool is made from a spun molten glass and sand mixture and is very flexible and because it is also cheap, it is used in a lot of places. The best places to use it are where space is not an issue, so within joists in an attic or below suspended timber floors. It has a Lambda (K) value of 0.04 to 0.032 depending on the product specified. This can be used in the form of a roll or a semi-rigid sheet (also called a batt). It can also be loose fill if it is blown into a cavity (although this is becoming less popular because the polystyrene beads are more resistant to compression and moisture ingress). It is also wrapped in plastic and formed into tubes to create fire barriers around windows and doors and within walls. Popular brand names are Isover by Saint Gobain and Earthwool by Knauf, as well as similar products by Superglass and URSA.
Mineral wool roll
Rock wool is similar to glass wool (and often called mineral wool) but be aware that these are often confused. It is formed from molten basalt rock, and is denser than glass wool so gives better acoustic performance, has better fire performance and moisture resistance. It performs the same as glass wool, with a Lambda (K) value of 0.035, but is around double the price of glass wool. It can also be used in the form of a roll or a semi-rigid sheet (also called a batt). Popular products include Knauf’s Earthwool and Rockwool.
Sheep’s wool roll
A product that often catches people’s imagination is sheep’s wool. This is a similar insulator to rock and glass wool, with a K-value of 0.035 to 0.04 W/mK, but it is a completely natural material, without the need for energy intensive manufacturing processes like melting glass or wool! The only factory processes are cleaning and adding a pest and rodent prevention treatment. Unfortunately it is in limited supply as you have to wait for it to grow, and so it is more expensive than the other wool products, and can cost 3 to 6 times the amount of artificial wool depending on the performance. One big bonus is that it is not an irritant to skin or eyes, and so goggles and gloves do not need to be work when it is being used.
Loose polystyrene beads
E
xisting cavity walls should be insulated with blown beads of polystyrene (EPS). This performs similarly to mineral wool with a Lambda (K) value of 0.040, and is more suitable for cavities as mineral wool can slump under weight and be damaged by water ingress. A binding agent is often added as it is blown into the cavity to ensure that they stick together when they dry, so that there is no risk of them all flooding out when you next install an extractor fan! These are generally white in colour, although grey ones are also available which have graphite added to improve the Lambda (K) value to 0.033.
Loose cellulose
Or recycled newspaper, is a fairly recent introduction to the insulation market. It performs similarly to mineral wool in terms of heat loss, with a slightly better Lambda (K) value of 0.035 W/mK, and as recycled newspaper it is a waste product rather than a new (and oil-based) product, which gives it excellent environmental credentials. One issue is that because the product is not yet established, there is not the same amount of test data and technical support than for the more commonly specified materials, but this is definitely one to watch. It has flame and damp retardant ingredients added to the product during processing so that it does not create a fire risk, and is not subject to rotting if it gets damp. The flame retardancy is effective enough to allow you to hold a handful of it and apply a blow torch without it or your hand burning! The main manufacturer in the UK is Thermofloc which replaces Warmcel which closed last year. Although it is no longer made in the UK, the price has gone down significantly since it has been imported from Eastern Europe. As it is still an unusual choice you have to shop around to purchase it and may well need to factor in a delivery cost for it as it is not readily available at trade merchants.
Expanded polystyrene board
Polystyrene is also available as a board. In this form it is most commonly used below a slab because as a board it will not be damaged by the weight of the building above, and it is a lot cheaper than the other better performing insulation boards. However, as the more expensive boards are thinner they can reduce the amount of excavation needed, and so still give a saving so this should be assessed for every project. It is also highly water repellent, allowing it to be used below the DPC whilst retaining it’s insulation properties. It is worth noting that this should be protected from a naked flame, as it is flammable. It achieves a Lambda (K) value of 0.035, or it is available in a grey form with graphite added it achieves 0.032. Manufacturers include Jablite, Arbet, Kay-Metzler & Styropor.
Extruded polystyrene board
T
his process of manufacturing polystyrene insulation improves the thermal properties of the board, allowing it to achieve a Lambda (K) value of 0.029 W/mK. This board is often coloured rather than white to distinguish it from expanded polystyrene, and it has a denser cell structure which is visible if you look at the two products together. Popular brand names include Kingspan Styrozone, Knauf Polyfoam, Styrodur by BASF, as well as products from Celecta, Sundolit and Marmox. This is generally only used on large projects for underfloor insulation, so it is not available in most trade merchants.
Polyisocyanurate board
P
IR is a rigid board material and the sheets often have a foil facing to one or both sides. It has very good insulation properties, with a Lambda (K) value of 0.022 W/mK. Celotex is the main manufacturer in the UK, and all their boards are made from this. It is very similar in chemical make-up to Polyurethane insulation, although the proportions of ingredients are altered. Other manufacturers include Recticel, Ecotherm & Xtratherm.
Phenolic board
P
henolic insulation is a rigid board material and the sheets often have a foil facing to one or both sides. It has the best insulation properties, with a Lambda (K) value of 0.020 W/mK. Kingspan is the main manufacturer in the UK, and most of their boards use this. These are more expensive than PIR and they perform slightly better which gives a marginal decrease in the wall thickness needed to achieve the same U-value, but as the performance increase is marginal, most Contractors find that PIR is the most economic solution. At the moment, Kingspan is the main manufacturer in the UK.
Polyurethane foam
This is a plastic foam formed by combining two materials which react to create the foam. It is commonly used on site in a spray form which allows it to be applied to cavity walls during construction. It achieves a Lambda (K) value of 0.023 which makes it a very good insulator. However it is not a very popular form of insulation because of the mess created and the fairly specialist nature of the installation. It also has an impact on the programme of a project because the cavity walls need to be ready for spraying in a single installation by specialists, whereas mineral wool or rigid boards can be installed by any tradesman and can be done on a piecemeal basis. This is popular in American and Canada, but as yet has not taken off in the UK, and so is not widely available.
Multi-foil sheets
T
his is a fairly modern invention which is made of several layers of reflective foil, sometimes with thin insulation layers between. U-values can always be improved by the addition of foil layers to the inside and outside, so the logical extension of this is to have a material made up entirely of reflective layers, which is a fairly idea, but Lambda (K) values do not apply to this material, as the material is not an insulator in itself, and actually plastic and metal sheets are poor insulators, but they work by reflecting the heat. The theoretical U-values which are possible are 0.19 W/mK for just 25mm of the material, but this has failed to perform up to it’s promise in the industry standard “hot box” test and when tested as part of an overall roof construction it was found to achieve 0.8 W/mK, which is actually the same as what you would get from the same thickness of Rockwool! These poor test results mean that this insulation is not generally accepted by building control departments, so you would need to achieve the minimum requirement with standard insulation products and add this as an additional layer, although given the cost, I am not sure why anyone would.
Which type of insulation is the cheapest?
This is the million dollar question, which is hard to answer because they all perform differently for the same thickness, but I have done a quick study of some of the insulation materials available on the internet and weighted them all in terms of performance. The R-Value is a measure of the insulation offered by a material at a given thickness, so it allows us to compare materials. The R-Value is the thickness in metres divided by the K-Value, i.e. 100mm of Phenolic board is 5.0 as 0.1/0.2 = 0.5. Then we can work out the cost / R-Value by dividing the cost per square metre @ 100mm thick by the R-Value of 100mm of the material. XPS, loose EPS and Polyurethane foam are not included as they are not readily available from trade stockists, and need specialist installation.
| Insulation material | Cost/m2@100mm | K-Value | R-Value @ 100mm | Cost/R |
|---|---|---|---|---|
| Glass wool | £1.41 | 0.044 | 2.27 | £0.62 |
| Mineral wool | £4.17 | 0.040 | 2.50 | £1.67 |
| Sheep wool | £20.74 | 0.040 | 2.50 | £8.29 |
| Loose cellulose | £3.33 | 0.040 | 2.86 | £1.17 |
| EPS board | £6.08 | 0.035 | 2.86 | £2.13 |
| PIR board | £10.07 | 0.022 | 4.55 | £2.22 |
| Phenolic board | £19.21 | 0.020 | 5.00 | £3.84 |
Acoustic insulation
Sound transmission through a floor is a combination of airborne and impact sound which need to be insulated in different ways. Impact sound travels through solid elements so can only be prevented by isolating elements from each other. Floors in houses do not need to perform very well, so insulation is laid between the floors and specialist fixings called resilient bars fix the plasterboard ceiling to the floor joists. This gets an insulation value of 43db. A typical floor for a flat will protect against both with a floating floor made of timber board on a timber batten with a rubber back in as well as a plasterboard sheet between, and then flexible resilient bars are used to fix the plasterboard to the underside of the floor joists which gives a very good insulation value of 56db rw.
Whilst the rigid boards are not effective at providing insulation, wool rolls and cellulose are. Mineral wool is often used in a floor build up to increase the basic insulation of a floor from 36 to 40dB. Cellulose is denser and as a result should perform better. It is not yet widely used so does not have test data, but when used in walls it performed better than mineral or glass wool.