External Wall Insulation – Quick, Durable, Long-lasting

Blog By Dermot Moore

Quick, durable and long-lasting are three crucial qualities of external wall insulation (EWI):

  • Quick – Instalments and repairs are straightforward and cost-effective. 
  • Durable – Endure local environmental conditions. 
  • Long-lasting – The material quality should ensure that minimal maintenance is required.

EWI is time effective, essentially wrapping an extensive package. It does not affect the home interior during installation; indoor space remains the same. Although EWI generally requires planning permission, it is an ideal option and ‘generally preferred‘ for rock or solid block walls. It can be the deciding factor in a successful retrofit, where it is risky to prise apart stone walls. EWI is also the primary insulation in retrofits for such reasons. 

Thermal Effect

Costs sustained by heating reduce when using EWI, as it prevents excessive heat loss. In addition, choosing the most suitable material for EWI will effectively reduce thermal bridges, ‘also known as cold bridging and refers to any area of a building which has a significantly higher level of heat loss or transfer than the immediate surrounding areas.’ Thermal bridges can cause up to 30% heat loss from a building. 


The best EWI should be breathable to capture air and prevent stagnation. If insulation is airtight, it disrupts interior breathability, increasing the chance of respiration complications. Also, where there are cracks in airtight insulation, condensation can form, which may cause mould growth.


An external insulating layer is ideal for weatherproofing a building, particularly in the UK. The damp climate and inclement weather conditions put UK homes under immense stress. The repercussions include tendencies for mould, algae, frost damage and crystallisation of stone, brick and concrete. EWI can counter this, but you must first know where the building’s dew points are. However, this is material dependent, as dew points are of less concern if insulation is permeable, not airtight. 

The Right Materials

Dew points lead the conversation on to which materials to use. And would you believe it? Hemspan says hemp and lime. Hemp insulation is encompassing in addressing key issues that EWI must challenge. Using hemp means EWI can be erected quickly, is durable to climatic forces and promotes long-lasting structural performance. 

What Lambda Reveals

Lambda represents the heat conductivity of a material, also depicted as the formula W/m.k. Thermal conductivity of 0.008 W/mk indicates near total airtightness, while the opposite is around 0.061 W/m.k. Hemspan insulation falls approximately in the middle at 0.039 W/m.k, which provides ideal permeability to suit breathability, heat maintenance, and moisture/vapour control. So treat a low W/m.K with caution.


The increasing popularity of EWI highlights a few issues. 

  1. Modern construction techniques could be more effective and generate substantial additional costs around thermal performance, breathability and dampness. Otherwise, EWI would be unnecessary in such abundance in modern construction. These methods have, in some cases, lowered living quality.
  2. Environmental and climatic processes are more erratic than ever, meaning there is great urgency to reassess construction standards, which leads to the next point.
  3. Particular attention should be given to structural permeability and airtightness, as these aspects heavily influence interior thermal efficiency, breathability and exterior weather resistance.

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External Wall insulation