Comparing EWI & Price Impact

Thermal performance & U-Values

Thermal performance will always be related to thermal conductivity in the external walls. Often termed the λ (lambda) value, it is constant for a particular material, irrespective of the thickness. Thermal conductivity measures the ease with which heat can travel through a material by conduction, conduction being the main form of heat transfer through insulation. To arrive at the U-value, the thermal conductivity of a material needs to be considered in conjunction with the thickness. In essence, the thicker the layer of insulation, the slower the rate of heat loss. Therefore, a higher level of insulation will impact price more. The U-value signifies the heat lost through a given thickness of a particular material. The calculation for the U-value is as follows:

U-value = Thermal conductivity / thickness (measured in metres)

Building regulations state that you need to achieve a U-value of 0.3W/m2K or lower. New builds are constructed with this figure in mind, but retrofitting older properties is more taxing. This is where insulation comes in. Our three systems achieve that U-value through various thicknesses of application. For example, EPS on a solid wall property will require 90mm, whereas Rockwool will require 110mm due its higher thermal conductivity. Other factors impact the U-value, but to a negligible amount.

Comparing our systems

Our three major systems include the BBA Approved EPS and Rockwool, and the space saving Kingspan Kooltherm K5. The best thermal performance can be extracted from the K5 system, as the thermal conductivity is lowest at 0.021W/(m2K). EPS and Rockwool trail behind at 0.032W/(m2K) and 0.036W/(m2K) respectively. As a result of these values, you would require only 60mm of K5 to achieve 0.3W/m2K as opposed to the thickness required for EPS and Rockwool. The phenolic composition of K5 results in this fantastic thermal performance, allowing it to act as a premium space saving option. The system also has high compressive strength, giving it protection against mechanical impact, a potential risk in tight environments.

The tragedy of Grenfell still resonates throughout communities. Fire rating in cladding and insulation has become a hot topic as a result. The best performing system in that respect is Rockwool as it has a fire rating of A1. Rockwool is composed of two slabs of various densities, made from molten volcanic rocks. The material is produced by heating the volcanic rock within a large furnace, then spinning it into fibrous strands and compressing it into boards. The dual density composition also provides acoustic insulation.

The most popular system, mainly due to its cost effective performance, is EPS. Expanded Polystyrene is hydrophobic and highly resistant to the absorption of water but will allow a very minimal amount of water vapour transfer. Expanded Polystyrene is often utilised with a suitable damp proof membrane or vapour control layer to avoid any unwanted water ingress. The fire rating is an F, but when installed inside a fire retardant system, EPS is still a very viable option.

Price Impact

The price naturally varies greatly between the types of insulation due to their qualities and benefits. We also offer the boards in a wide range of thicknesses, which will alter the price. Ultimately, you are always trying to achieve the benchmark of 0.3W/m2K, and we can examine the price impact below:

  • EPS – 90mm required = £8.25 ex VAT per board
  • K5 – 60mm required = £15.80 ex VAT per board
  • Rockwool – 110mm required = £21.50 ex VAT per board

As discussed, EPS is the most cost effective option. However, it lacks the extra benefits of K5 and Rockwool. The Rockwool and EPS systems are BBA Approved, giving you peace of mind regarding their relative performance. Any amount of insulation will help you save money on your energy bills.


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