External Wall Insulation: Everything You Need To Know

External Wall Insulation (EWI), which is also referred to as ETICS (External Thermal Insulation Composite Systems) are suitable for both new and old houses, and help significantly save money as well as greatly improve the thermal comfort of your home and protects the environment by reducing the heating energy requirements. However, only correct application guarantees optimum results!

As we know, heat moves from warmer to colder areas. Therefore, on cold days, heat from inside the building seeks to get outside. On warmer days, the opposite occurs. The heat from outside seeks to get inside. Insulating the external walls prevents heat from transferring through the walls thereby significantly optimizing the energy consumption of the building. When the external insulation surrounds the structure, the dew point will occur on the external surface of the wall, thereby eliminating the risk of condensation appearing on the internal surfaces.

In order to understand how external insulation works, we can compare it with a “tea-cozy”. Both function in the same way, a tea cozy is a cover for a teapot that insulates it, keeping the contents warm. External wall insulation works the same way since it is one continuous layer of insulation that has wrapped the whole house. EWI reflects the interior temperature back inside, keeping the room warm or cold, preserving/ maximizing its thermal mass and eliminating thermal bridges (the transfer of heat from the inside to the outside through junctions).

Is external wall insulation an option worth pursuing?

External wall insulation is expensive but effective. Indeed, the initial cost of installing external wall insulation is high, but the benefits are instantly noticeable. It is one of the quickest energy-payback projects you can do, resulting in lower heating and cooling bills. You’ll save from 10% to 50% on heating and cooling bills, depending on where you live, what type of heating system you have, and how much insulation you add.  It is easy to apply external insulation, creating robust structures that are warm, dry and healthy, the benefits are numerous. Some types of external insulation (for example Mineral wool) improve wheatherproofing and sound resistance of the building. Also, EWI works as a barrier which protects the external walls of your home against the influences of weather, expanding their lifespan.

How does external wall insulation work?

External Wall Insulation (EWI) is accomplished by directly applying insulation panels on the external walls of the building before finishing (and covering it) with a rendering system for impact and weather protection. Both solid walls and cavity walls can be insulated externally.

External insulation can be anchored to walls using mechanical fixings (often with plastic heads, as metal conducts heat) or mortar dabs, or a combination of both. Depending on the type of external insulation used, changes to the original wall finishes – as in the case of pebble dash – are sometimes needed to ensure a smooth application. 

Once the external insulation board is applied, finishes are available to create and replicate all sorts of looks, including dash and brick. Those seeking a brick finish have a number of options: renders designed to create an authentic brick effect, synthetic brick slips, or slips cut from real brick.

What is thermal resistance?

The thermal resistance of a material is a measure of how resistant it is to the transfer of heat across it. The thermal resistance of building materials is expressed as the R-value. The higher the R-value, the greater the resistance. All materials have some resistance to heat loss; bust most structural materials such as concrete, aluminum and wood have relatively low R-values. Insulation materials have higher R-values and are designed specifically to reduce the rate of heat loss through the building shell. R-values are determined by material type, thickness, and installed weight per square foot, not by thickness alone. All materials having the same R-value, regardless of type, thickness, or weight, are equal in insulating power.

While the R-value is the measure of resistance to heat flow, the inverse of R-value, called U-value. U-value is a measure of the number of BTUs that will flow through a square foot of the material for each degree Fahrenheit difference in temperature from one side to the other in an hour. (Btu/ft² F hr). A low U-value meaning that only small amounts of heat will pass through, thus keeping your home warmer during the winter months or colder during the summer months for longer. This is the term used in most heat loss calculations. A wall system with a high R-value has a very low U-value because U=1/R. The lower the U-value, the less heat moves through the material.

Which insulation should I use?

The wide variety of insulating materials and finishes available give designers and builders plenty of technical and aesthetic options. Below we list some of the most common external insulation materials: expanded polystyrene (EPS), extruded polystyrene insulation (XPS), mineral wool, phenolic foam (PF) and Wood fibre.

Expanded polystyrene (EPS)

Extruded polystyrene or expanded polystyrene (EPS) is the most popular and less expensive insulating material used in exterior insulation systems available on the market at this time.

The white EPS has been the mainstream in external insulation for years and has a thermal conductivity of 0.037 to 0.040 W / mK.

More recently it has been replaced by gray EPS, which is enhanced with graphite and has a higher conductivity value of about 0.031 W / mK. In fact, it means you have the same performance as the white EPS by applying a thinner gray EPS profile.

EPS is easy to install. It is lightweight, easy to cut and durable. It is also water resistant, it can get wet during installation without suffering any loss of thermal value. Because it’s so common, you can get it almost any thickness you want.

It is a flammable material (category E / F), but with the addition of appropriate combustion retarders the material is made up of category B with very low flame spread. Although, it has very reduced sound-insulating properties and therefore cannot be used as a soundproofing insulation.

Extruded polystyrene insulation/ XPS

The production of extruded polystyrene began in the 1940s. The polystyrene foam extrusion process results in a material with uniformly small, closed cells, which give a higher rigid foam, making it highly compressible and having a thermal conductivity of about 0.034 W / mK. XPS has a natural resistance to rain, snow, frost and water vapor, and is an extremely stable material that retains its initial thermal insulation performance and physical integrity in exposed conditions over long durations.

Properly installed XPS boards have a service life comparable with that of the building or structure. The closed cell structure and the treated surface of extruded polystyrene do not retain significant amounts of water so it does not absorb moisture. XPS is widely used in external thermal insulation systems, basements and in the construction of inverted roofs, but has very low sound insulation properties. It is flammable material (category E / F), but with addition of suitable fire retardants material made up of category B with very low flame spread.

Foamed extruded polystyrene is an odorless material, usually blue or light green, with excellent heat-insulating properties, superior compared to expanded polystyrene, but with higher cost of purchase.

Mineral wool

A very popular material, natural mineral wool, is one of the most economical options and has a thermal conductivity in the region of 0.036 W/mK.

The fibrous insulation materials get the best fire proof credentials – they just won’t burn! This is one of the reasons why it is often chosen in  residential homes and commercial multi-storey buildings. Considered to be a bit more difficult to apply because it is difficult to cut and some people feel itchy when they come in contact with the material. It must also be kept dry during installation. If it gets wet, its performance will return as soon as it dries completely.

Mineral wool products have little resistance to water vapor diffusion. The diffused water vapor easily penetrates the insulation material. Mineral wool is fully breathable, any moisture/water vapour in the structure can pass to the exterior surface unobstructed. The high breathability of the material favors the elimination of possible condensation moisture by evaporation. After evaporation the material returns to its original condition without changing its properties. But be careful, breathability is only maintained if the finish is itself breathable. It is also an excellent soundproofing material.

It is available in the market in rolls, batts, boards or it can be customized for use with other products.

Phenolic foam – PF

Phenolic insulation is a rigid foam insulation with a closed cell structure. It is among the most expensive heat insulating materials but also with the best performance, with a thermal conductivity of 0.021 W / mK. The coefficient of thermal conductivity, however, depends significantly on the thickness of the material and the coating material.

The exceptionally low thermal conductivity of closed cell phenolic foam boards means that appropriate insulation efficiency can be achieved with the thinnest possible profile of PF material. Sometimes, it is the only viable option. For example, in narrow passages between houses, a thin profile of phenolic foam can provide the required U value without rendering the passage unusable. Similarly, one of the many building elements you have to consider on renovations is the roof overhang. A thinner insulation profile may obviate the need to extend that overhang.

Phenolic insulation has excellent moisture resistance as a result of low water vapor permeability and 90% closed cell structure. It exhibits very good resistance to compression and can be used for thermal insulation of horizontal surfaces such as roofing, flooring and cavity walls. It must be kept dry during installation. Due to its closed cell structure and coating with waterproof materials it is practically waterproof, does not absorb moisture and does not retain significant amounts of water. If it absorbs moisture on its surface it is not carried inside.

Also, it is fireproof. In case of fire it combines zero or very low flame spread with negligible smoke emission and a very low level of toxic gas emission. The product has excellent sound-insulating properties and is available in pipe sections, blocks, rolls and slabs. It is generally available from 20mm to 50mm thickness, though it is possible to double up. In this case, you must trust trained, certified fitters to  install it. Some fitters say that this material tends to require thicker bases and additional connections than other insulation materials, resulting in increased costs.

Wood fibre

Wood fibers are considered as an ecological heat insulating material. With a thermal conductivity in the region of 0.039 W/mK. However, you do need more of it to deliver equivalent thermal performance. It is fully breathable when used in conjunction with the right finishes and adhesives and is more commonly found on timber frame structures. Although, it’s a little more expensive than mineral wool.

The fibrous structure of the wood allows it to accumulate and store warmth during the day, and then release it at night when the temperature drops, delaying the occurrence of condensation on the surface and thereby making the finished render less susceptible to mould and algae growth.

It has excellent sound insulation properties and is fire resistant with very slow spreading of the fire.  It is also widely used as internal insulation for sound insulation in conservatories, theaters, cinemas, recording studios and mass entertainment venues.

Wood fiber insulation is available as loose fill, flexible batts and rigid panels for all thermal and sound insulation uses.

Getting the right installer

When it comes to installing or upgrading home insulation, it’s best to hire a professional insulation contractor. Proper installation is essential for insulation to perform properly. Knowledge of vapor retarders, air infiltration, ventilation, recessed lighting, and water pipes are just a few of the areas critical to installation techniques. Professional insulation contractors have access to a wide variety of training, are familiar with local codes and regulations, and can offer guidance about the type and amount of insulation to be used. Poor workmanship, poor detailing and poor planning can end up damaging the structure, and seriously compromising indoor air quality.

Once you have chosen an insulation contractor, make sure the contract includes the job specification, cost, method of payment, and warranty information provided by the insulation material manufacturer. When the job is completed the contractor ought to give you a receipt for the insulation installed. This is required by law.

It is important to note that the contract you will sign should list the type of insulation to be used with R-value and where it will be used. Make sure that each type of insulation is listed. Avoid contracts with vague language such as R-values with the terms “plus or minus”; “+ or -“; “average”; or “nominal.” Beware of any contract or verbal offering that quotes the job in terms of thickness only (e.g. “14 inches of insulation”). Remember, it is the R-value — not the thickness — that tells how well a material insulates. When buying insulation, be sure not to get sidetracked by the thickness of the material.

The functionality of the building is guaranteed through the harmonized planning and installation. All the technical rules should therefore have already been taken into consideration at the planning stage. These are important prerequisites for a perfect job.