Full Fill Insulation: Your Guide to Pros, Cons, Costs & Regs

This guide will help you understand full fill insulation, including its advantages, potential drawbacks, costs, and regulations. Whether you're choosing insulation for your home or staying informed about industry standards, let's explore the pros, cons, price, and rules of full fill insulation together to make informed decisions about your insulation needs. 

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Full fill insulation is a highly effective method for cavity walls. This technique involves filling the cavity between the inner and outer walls with a dimension board (450mm/455mm), securing no gaps or voids. Full fill insulation on a cavity wall creates an unbroken thermal barrier that improves the building's energy efficiency. To achieve optimal performance, it is important to follow specific installation techniques. This includes using insulation materials that precisely fit the cavity dimensions and offer high thermal resistance.

One key benefit of full fill insulation is that it effectively eliminates potential pathways for heat transfer and minimises air infiltration. This results in a significant improvement in energy efficiency, making it a highly desirable method for insulating cavity walls.

When choosing between full fill and partial fill cavity insulation, weighing the differences in installation, cost, and effectiveness is crucial. Partial fill cavity insulation is easier and quicker to install, making it more cost-effective due to reduced material and labour requirements. However, it leaves a gap between the insulation and masonry, leading to air gaps and decreasing overall insulation effectiveness. These gaps can result in heat transfer and air infiltration, reducing energy efficiency. 

On the other hand, full fill insulation fills the cavity, eliminating gaps and creating a continuous thermal barrier. Although this method is more expensive and requires careful installation, it significantly enhances insulation effectiveness and energy efficiency. While partial-fill insulation offers a cost-effective installation, full fill insulation provides superior thermal performance and long-term benefits.

Cavity closers are still necessary with full fill insulation, as they prevent heat loss, moisture penetration, and mould growth around doors and windows. Using cavity closers with full fill insulation is essential for maximising energy efficiency and ensuring the overall integrity of the insulated cavity wall.

Full fill cavity insulation is popular among homeowners who want to improve their homes' energy efficiency and comfort. Although it provides various advantages, full fill cavity insulation problems can occur. 

The regulations for full fill cavity wall insulation are designed to provide flexibility while providing specific recommendations and requirements to guarantee optimal insulation performance and maintain the structural integrity of the building:

Insulation efficiency is measured by its "U-value" in building regulations. This value indicates the heat loss or gain rate and is expressed in watts per square metre per Kelvin (W/m2k). Lower "U-values" indicate better insulation performance. Insulation is recommended to have a 0.18W/m2K u-value.

Building regulations may limit full fill cavity insulation based on cavity size and wall construction. For example, very narrow cavities may not be suitable for full fill insulation, as the cavity width should be at least 50mm. Most standard regulations require cavity widths of 100mm to 150mm, and the insulation installed should be at least 270mm thick.

Building regulations regarding insulation values may vary slightly in England, Scotland, Wales, and Northern Ireland. In some regions of the country, filled cavity wall insulation might not be recommended due to the risk of moisture penetration. Regions with high rainfall or coastal areas exposed to wind-driven rain are especially vulnerable to moisture ingress, making full fill insulation less suitable.

We recommend checking your government's website for more detailed information.

Materials selected for full fill cavity insulation should be carefully chosen to ensure compliance with the current UK building regulations for thermal performance. These regulations guarantee that the insulation meets the minimum standards for energy efficiency, helping to improve the overall thermal performance of the building. 

When selecting the suitable material for full fill cavity wall insulation, it's crucial to consider options with superior thermal performance and a perfect fit for this specific application. Here are some of the best materials that meet these criteria:

Mineral Wool Insulation such as rock or glass wool is recognized for its high thermal resistance and fire-retardant properties. It efficiently fills the cavity, providing excellent sound and thermal insulation. Mineral wool is also non-combustible, making it a safe choice for cavity wall applications. This material cost on average £25 – £30 per m2. 

Phenolic Core Insulation or phenolic foam, is an excellent choice for thermal performance and fire resistance. Its high R-value per inch of thickness makes it one of the most efficient insulation materials available. Its lightweight yet rigid structure and outstanding moisture resistance make it suitable for full fill cavity wall insulation in various environments. It costs on average £10 per m2. 

Polyisocyanurate core (PIR) insulation is a proven choice for full fill cavity wall insulation. It provides superior thermal efficiency and fire resistance. With a closed-cell structure, full fill PIR cavity insulation offers excellent moisture resistance and dimensional stability. PIR insulation boards are lightweight and easy to install, making them popular cavity wall insulation in new build properties. This material costs on average £5 – £15 per m2. 

Hemp full fill insulation is a sustainable and environmentally friendly insulation material made from the fibres of the hemp plant. It is a renewable resource that requires minimal water and no pesticides to grow, making it an eco-friendly choice for insulation. Hemp offers excellent thermal and acoustic properties, effectively keeping homes warm in the winter and cool in the summer while also reducing outside noise. It costs an average of £20 to £40. 

When choosing the most suitable material for full fill cavity wall insulation in a particular project, extensive research and consideration of factors such as climate, building design, budget, and local building regulations are crucial. Asking for advice from insulation professionals can also offer valuable insights and personalised recommendations to meet the project's specific needs.

When considering full fill cavity insulation in the UK, it's important to note that the typical cost ranges from £475 to £800. However, this cost can vary based on the size of the property. For example, a small flat may cost around £350, while a larger detached house can cost up to £1,500. Several factors influence the cost, including the type of insulation material, installation expenses, and the property size. It's advised to request quotes from multiple providers before deciding to guarantee you get the best value for your investment.

Deciding whether full fill cavity wall insulation suits your property depends on several factors that require careful consideration. Firstly, your property must have cavity walls, as full fill insulation is specifically designed for this type of construction. Additionally, it's essential to balance your insulation needs with other factors such as budget, building regulations, and regional considerations.

Although full fill cavity wall insulation offers numerous advantages, it's important to consider potential limitations. Due to the risk of moisture penetration, it may not be advisable in regions with high rainfall or wind-driven rain. Evaluating your location's climate and any local regulations or recommendations is crucial before opting for full fill insulation.

Finally, the decision to use full fill cavity wall insulation should be made after carefully considering your specific needs and circumstances. Seeking advice from insulation professionals can provide valuable insights and help you determine the best insulation solution for your property.

Considering insulating your home? Let GreenMatch handle the hard work for you. Skip the hours of online research and appointment scheduling – just fill out our quick online form in 30 seconds. You'll then receive up to 3 free quotes from vetted installers near you. Take the first step now by clicking the button below.