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Loft Insulation vs External Wall Insulation

The best strategy for insulating a home is to take a holistic approach. This means that you create a thermal envelope around the whole property. As such, you need to consider the loft, external wall, floor insulation, and double or triple glazing. However, to completely wrap a whole property is an expensive venture. Whilst external wall insulation is the most costly of the endeavour, it is so for an excellent reason. The impact on your energy savings is highest through the installation of EWI. Therefore, in this blog, we are pitting it against the cheapest option, loft insulation. What is their payoff? Which impacts your property value? It’s all covered below.

What is loft insulation?

Loft insulation is a method used to prevent heat loss through a building’s roof by adding an insulating layer in the loft or attic space. It is a highly effective way of saving energy and reducing heating bills, as around 25% of heat in an uninsulated house is lost through the roof.

There are three main types of loft insulation:

  1. Blanket (matt) insulation: This is the most common type and is also called batt insulation. It comes in rolls of materials like mineral wool, glass fibre, or foil-backed felt, which are laid between the joists or rafters in your loft. Therefore, it’s relatively easy to install and can often be done as a DIY project.
  2. Loose-fill insulation: This is an option that involves lightweight materials such as cellulose, mineral wool, or recycled newspaper. These materials are simply poured to fill gaps in irregularly-shaped areas or around obstructions in your loft. While it’s easy to top up existing insulation with loose-fill materials, achieving a consistent depth can be more challenging, and it can be disturbed by drafts.
  3. Blown-fibre insulation: This type of insulation is typically made of mineral wool or cellulose and is literally blown into place by professionals using specialist equipment. It is useful for insulating hard-to-reach areas or topping up existing insulation.
Loose fill loft insulation
Loose fill loft insulation

What is EWI?

External wall insulation (EWI) is a layer of insulation applied to the exterior of a building. It works by reducing heat transfer through walls. In turn, helping to decrease energy consumption, lower heating bills, and enhance the property’s overall thermal comfort.

External wall insulation can be especially useful in older buildings without cavity walls, although it can also be beneficial for more modern buildings. In addition to providing insulation, external wall insulation can also help protect the structure of a building. It can also significantly improve a building’s exterior appearance.

Here are the primary types of external wall insulation:

  1. Mineral Wool (or Rockwool) Insulation: Mineral wool insulation is made from molten glass, stone or industrial waste that is spun into a fibre-like structure. This type of insulation is fire resistant, provides good sound insulation, and is breathable, allowing moisture to escape, which can be beneficial in damp climates. However, it is generally thicker than other types of insulation for the same U-value.
  2. Expanded Polystyrene (EPS) Insulation: EPS insulation is a lightweight, rigid, plastic foam insulation material produced from solid beads of polystyrene. It’s a popular choice due to its excellent balance of insulation efficiency, cost-effectiveness and ease of installation. It’s also water-resistant and durable, although it’s less breathable than mineral wool and less fire-resistant.
  3. Phenolic Foam Insulation: Phenolic foam offers the highest level of thermal performance of the commonly used insulation materials. It is approximately twice as effective as polystyrene or mineral wool for the same thickness). It is a rigid panel that can be used with a variety of finishes. However, it is generally more expensive than other types of insulation and needs careful installation to prevent moisture penetration which can reduce its effectiveness.

Loft and external wall insulation impact U-values and R-values

Understanding U-values and R-values is key to comprehending insulation effectiveness. Here’s a brief recap:

  • The R-value gauges the resistance to heat flow through a certain thickness of a material. Higher values denote superior insulating effectiveness.
  • On the other hand, the U-value assesses the heat transfer rate through a material. Lower U-values suggest enhanced thermal performance.

Broadly, insulation materials with high R-values have low U-values, indicating superior performance in staving off heat loss or gain.

When it comes to loft insulation and external wall insulation, their impact on the U-value of a building is significant.

For instance, a typical uninsulated loft has a U-value of about 2.5 W/m²K (watts per square meter per degree Kelvin difference). When you install 270mm of loft insulation (the recommended depth by the Energy Saving Trust), the U-value can reduce to about 0.16 W/m²K, a significant improvement in thermal performance.

In the case of solid wall houses, an uninsulated wall can have a U-value as high as 2.1 W/m²K. But when you apply external wall insulation, this can plunge to as low as 0.30 W/m²K, or even better, depending on the insulation system used. This translates to a tremendous enhancement in the wall’s insulation properties.

Cost, payback, savings, and property value

Investing in insulation, whether loft or external wall insulation, comes with an upfront cost but also offers long-term savings on energy bills. The exact costs, savings, payback periods, and potential increase in property value can vary based on several factors, including the size and type of property, its location, and the specific insulation material and method used.

Loft insulation

The cost of loft insulation depends on the method used. You can expect to pay between £100 for DIY installation of blanket insulation to £350-£500 for professional blown-fibre insulation.

The savings from loft insulation can be significant, with Energy Saving Trust estimating annual savings of around £120 to £215 for a detached house in the UK.

The payback period for loft insulation can be as short as two years. This makes it one of the quickest payback periods for energy efficiency investments.

While loft insulation may not directly increase the property value in the same way that visible improvements like a new kitchen would, it can make the property more attractive to potential buyers or renters due to lower energy bills and increased comfort.

External wall insulation

External wall insulation is more expensive, with costs typically ranging from £8,000 to £22,000. This includes the additional finish which improves your home’s appearance and possibly its value.

External wall insulation can provide yearly savings of approximately £500+ for a detached house in the UK, depending on the pre-existing wall type.

Due to the higher initial cost, the payback period for external wall insulation is longer. It may take between 13 to 25 years.

External wall insulation can significantly increase the value of a property, particularly for older properties with solid walls that were previously inefficient and hard to heat. The improved external appearance can also add to the property’s appeal.

Loft insulation vs external wall insulation

Loft insulation and external wall insulation each play a pivotal role. They provide energy loss reduction, shrink your carbon footprint, and trim energy bills.

Although less expensive and with a shorter payback period, loft insulation primarily curtails heat loss through the roof and is typically more potent during the colder months. It is a commendable starting point for energy efficiency, but its impact is restricted.

On the flip side, external wall insulation, though costlier initially, insulates the entire structure. Therefore, reducing heat loss in winter and maintaining a cooler environment in summer. It also enhances the building’s appearance, potentially escalating its value, and safeguards the walls against weather conditions and dampness.

Evaluating from a holistic standpoint, loft insulation might appear to be the front-runner owing to its cost-effectiveness. However, external wall insulation often emerges as the superior choice because of its comprehensive coverage, year-round energy efficiency benefits, and potential to augment the property’s value and aesthetics. Therefore, if you’re seeking an all-encompassing, longer-term solution, external wall insulation makes a compelling case as the better investment.


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