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  • Elliott Pardington


How much should you use? Which type is best for your project?


Why do we need to insulate our buildings?

Quite simply we insulate our buildings to make it easier to keep them warm and comfortable. Of course, the other way that we can make our buildings warm and comfortable is to turn up the heating and until the 1970’s this was the preferred method. The oil crisis of 1973 made this approach increasingly unaffordable and as a result the introduction of insulation was encouraged for homeowners and mandated for new buildings through the building regulations. The idea of constructing a building without insulation is unthinkable these days, even for most people outside of the building industry.


How much insulation do I need?

The levels of insulation required by the building regulations have increased steadily since first introduced. In the 30 years that I’ve worked in the industry the requirement for external walls has approximately doubled for buildings in England and approximately tripled for buildings in Wales. Whilst mandatory insulation levels for new building works are much better than the insulations levels of the existing housing stock, exceeding the mandatory insulation requirements is still a good idea.

Increased levels of insulation reduce your energy use (good for the environment), reduce your energy bills (good for your bank balance), and increase your comfort (good for your health and wellbeing). Exceeding the mandatory levels probably has greater benefit in England, as insulation levels in Wales already represent an improvement of up to 25% over the English building regulations.


How does insulation work

The purpose of insulation is to slow down the passage of heat from one side of an assembly (wall, roof floor etc.) to the other. Most insulants do this by using the space between cells to trap air, reducing conduction and radiation thorough the assembly. It is important to remember that if the space between cells fills with water the material will no longer offer any thermal resistance and will become completely ineffective. It is important to ensure that insulation materials are suitable for their proposed location.

The other way in which heat can leave your building is by convection and I’ll be putting a separate article together to explain how this can best be reduced.


How is the performance of insulation measured?

In order to compare the thermal performance of different types of materials we look at their thermal conductivity; the measure of a material’s ability to conduct heat. It is often referred to as the ‘k’ value of a material, or the material’s ‘λ’ or ‘lambda’ value.

Thermal conductivity is measured in watts per metre kelvin (or degrees centigrade), commonly expressed as W/mK or W/moc. So, as an example, a material with a thermal conductivity of 1 W/moc and a thickness of 1 metre would conduct one watt of energy for every degree centigrade of difference in temperature between the two sides. By the way, a material with this conductivity would be a very poor insulator.

So the key point to remember here is that the lower the thermal conductivity or k value, the better the performance as an insulator.

Thermal conductivity can vary dependant on the type of installation. For example, a soft insulation quilt installed horizontally over a ceiling, may perform slightly better than when installed vertically in an external wall where it will be subject to some compression under its own weight, leading to a slightly greater density and a reduction in the air pockets that provide the resistance to the passage of heat.

Whilst thermal conductivity is the headline in terms of the performance of insulation materials there are other factors which need to be considered in order to get full understanding of how the material will perform when installed. I’ll come back to these later in the article.


What types of insulation are available?

For the sake of this article it is probably easiest to break insulation down into three main groups:

Flexible insulation quilts and semi-rigid batts

You might typically see this type of insulation in you loft, laid between (and hopefully over) the ceiling timbers. The industry standard is to use a product made from a material such as stone, glass or iron ore slag, which has been heated to a temperature of about 1600oc and spun into a wool blanket. It’s a lot like making candy floss.

In the last 15-20 years natural alternatives such as sheep’s wool have been introduced as environmentally friendly alternatives. Sheep’s wool is almost certainly a less irritant product (I have concerns about the long term health effects of mineral wool fibres). As a by-product of meat production it probably also has a lower carbon footprint.

Flexible insulation quilts are a good choice where insulation needs to fit into gaps between structural or other elements of a building.

The thermal conductivity of insulation quilts varies but is generally in the range of around 0.035 W/mK to 0.044 W/mK.

Rigid insulation boards

There is a wide range of rigid insulation board materials.

The most widespread at the moment is the foil faced Polyisocyanurate (or PIR) foam board. This is commonly known by trade names such as Celotex or Kingspan. PIR foam is an oil based thermoset plastic and this is one of the contributory factors in the fire safety issues encountered at the Grenfell Tower fire of 2016.

Most PIR boards have a conductivity of about 0.022 W/mK, which is why its use is so widespread.

Expanded polystyrene (EPS) and its sister product, extruded polystyrene (XPS) are again oil based plastic products. EPS insulation is commonly known by its trade name, Jablite. XPS insulation has a higher compressive strength and is impermeable to water, making it a popular choice for use below structural floor slabs and in basement construction.

Both EPS and XPS have a thermal conductivity in the region of 0.035 W/mK.

Wood fibreinsulation boards are a by-product of the structural timber industry. Waste wood chips and shavings are mixed with additives and bound or mechanically pressed together to form a sheet or board material. Woodfibre is increasing in popularity, particularly for projects where ‘natural’ materials are preferred to petrochemical based products. It is increasingly used as an external wall insulant (EWI) or internal wall insulant (IWI) in low energy retrofit projects where it’s vapour permeability is a distinct advantage in some scenarios.

Woodfibre typically has a thermal conductivity in the region of 0.040 W/mK.

Blown or sprayed insulation are less common and tend to be used in specialist applications. They will normally be installed by a specialist contractor rather than a general builder.

Cellulose fibre is manufactured from recycled paper, normally from the newspaper industry. Additives introduced during the manufacturing process provide fire resistance. This product is typically used in the sealed external walls or roofs of timber framed houses but can also be used as an alternative to insulation quilts at ceiling levels in traditional housing.

Cellulose fibre typically has a thermal conductivity in the region of 0.038 W/mK.

Expanded polystyrene (EPS) beads are typically used to insulate existing cavity walls. They are usually installed under pressure and combined with binding agent during the installation process.

EPS beads typically have a thermal conductivity in the region of 0.036 W/mK.


Selecting the correct insulation

Your designer will select the most appropriate type of insulation for your project based on several factors.

The first consideration will be the location of the element to be insulated. We will look at issues such as compressive strength, rigidity, moisture resistance, fire safety and vapour permeability. For example, mineral wool insulation quilt would not be suitable for use beneath a floor slab, where it would be almost completely compressed and likely to become saturated.

The second consideration is likely to be the thermal conductivity of suitable products and the space available for insulation. For example, if a roof needs to sit beneath an existing first floor window the depth available for insulation may be restricted. It might be better in this instance to use a PIR insulation board rather than a woodfibre board, as the depth of insulation required to achieve the same thermal performance would be halved.

Buildability and installed performance will be considered – can the product be effectively installed. As an example, cellulose fibre insulation can be installed to completely fill the voids between structural timbers in timber framed walls, whereas cutting a rigid board material between timbers would be very labour intensive and would be likely to leave gaps where board edges abut the structure. The rigid board solution might achieve the same results on paper but the gaps would significantly reduce the thermal performance in real terms. Alongside this, long term performance will be an issue, will the product shrink or slump in the long term, creating gaps that will compromise performance.

Of course, cost is always a consideration. Once a number of products or materials that comply with the performance requirements have been identified, cost can be considered. Alongside cost, availability is also a consideration with some specialist insulants and some standard products in unusual thicknesses being subject to delivery delays on occasion.


Further reading

I’ve really only scratched the surface with this article. If you want more information about insulation you could look at this page on the greenspec website.

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