Why 50mm of External Wall Insulation Isn't Enough

When it comes to external wall insulation, 50mm of EWI is not enough. The impact made by that thickness will be almost negligible. Different variants of external wall insulation boards will impact the overall impact but even for the most thermally efficient, 50mm is not enough. In this blog, we will discuss the benefits of thicknesses in excess of 100mm as well as the potential issues with lower thicknesses.

Why 50mm of EWI is not enough in U-value terms

As detailed above, different external wall insulation boards have different thermal properties. This is due to the different U-values attributed to them.

U-value is often referred to as thermal transmittance, and is the rate of transfer of heat through a structure. This value is consequently divided by the difference in temperature. In simple terms, the better insulated the property is, the lower the U-value is. A low U-value results in slower loss of heat and less need to heat more often.

At EWI Store, we offer 3 different forms of external wall insulation boards. As you can see in the table below, 50mm of EWI is not enough for any of the boards. Therefore the reason it is not enough is because it does not allow the property to reach the crucial value of 0.3W/m²K.

For Rockwool and EPS, the value achieved is approximately half of what is required. Kingspan K5 Kooltherm is a space saving system. As a result of this benefit, 50mm of K5 gets very close to achieving the desired U-value. However, 50mm of EWI is still not enough to achieve the industry standard.

Solid brick walls have a U-value of around 2.1-3W/m² and adding 50mm of expanded poly­styrene (EPS) will take the U-value down to 0.5W/m²k, which is a sub­stan­tial improve­ment, but 100mm takes it down to just 0.27W/m²k.

So why not insulate with just 50mm?

There are several reasons for insulating with more than 50mm. Primarily, building regulations dictate that if you are planning to install EWI on over 25% of the external walls, you are legally obliged to use a thicker version of your chosen insulation board.

The other simple answer as to why insulate with more than 50mm is the savings. Your heating system, whether gas or electric, will work less hard. As a result, you will use less of your energy supply, resulting in bigger savings. With 50mm of EWI, the reduction of your heating bills will be at around 30%. When you up the thickness to 100mm, the savings can rise to 50%. With the current energy prices climbing constantly, it is crucial to increase savings as much as possible.

What's the cost difference between 50mm and 100mm?

The major savings come from the labour and adhesive costs. The installation process is usually priced up depending on the size of the property, not the thickness of the insulation installed. Therefore, 100mm of insulation will not cost you any extra for installation.

The adhesive you use is also effective irrespective of the thickness of insulation. Our Premium Basecoat is particularly great for installing all of our insulation boards. You do not need to apply extra layers of adhesive to attach thicker insulation boards, so the costs remain the same. The reinforcement layer and render finish also stay exactly the same, and do not require extra materials.

The price of the individual boards also increases at the same rate. You do not pay any sort of premium for thicker boards. The fixings constitute a minor cost increase, but ultimately, you may only end up spending a couple of hundred pounds extra for far better thermal performance. For this reason, 50mm of EWI is not enough.

The issues with only 50mm of EWI

The issues with installing only 50mm of EWI extends beyond satisfying building regulations. A lower thickness of insulation will result in your walls feeling colder. This is not only a comfort issue but an indication that the walls are not working effectively as the heat stores they should be. Well insulated walls will hold the heat for a longer period. In turn, this results in savings on heating bills. Your heating needs to be turned on less frequently and for shorter periods, as the residual heat keeps your rooms warm.

Cold bridging is another issue that can arise when you install only 50mm of EWI. Cold bridging is a form of thermal bridging wherein cold spots are created in the thermal envelope where the external wall insulation is insufficient.

Houses in the UK suffer exponentially in comparison to European counterparts. Cold spots are frequent as the insulation is insufficient and in most cases, not promoted by the Government. To address this, the UK Government has brought forward new legislation targeting low income housing. A £1bn grant fund is allocated towards the installation of loft insulation. Unfortunately, loft insulation has a low impact on overall thermal comfort. The flip side is that it is easy to install and relatively cheap. A report on UK homes raises the issue:

Report author Dr Salvador Acha, from Imperial College London (ICL), said: "Studies show the UK’s 28.6m homes are among the least energy efficient in Europe and lose heat up to three times faster than on the continent, making people poorer and colder.

“At a time of increased energy bills and inflation, people in the UK can’t afford to lose energy due to inefficient housing, but unfortunately policy in this area has been nil for many years. With the climate crisis, and the fact our homes account for 30% of the UK’s total greenhouse gas emissions, the planet can’t afford this lack of action either."

The benefits of installing 100mm+ of EWI over 50mm of EWI

The thermal comfort of installing 100mm+ of EWI is the major benefit. However, with thicker insulation, you also eliminate areas of cold bridging. Insulation boards like Rockwool will also offer acoustic insulation for the property. Whichever way you approach the subject, the benefits of 100mm+ are obvious. Your property needs a thick thermal envelope.

The improvements in home insulation all lead towards the continued push for decarbonisation. Unfortunately, the UK is also trailing in that department; 'While Norway has all but fully decarbonised heating across the country. Around 85% of buildings in Norway are heated using electricity. Of these buildings, 92% are produced by hydropower and 6.5% from wind.' (Alice Clifford, London World)

If you have any other questions about the benefits of installing more than 50mm of insulation, leave us a comment or give our sales team a call!