A Guide to Flooring Insulation by Celotex

Floor insulation is often overlooked in building developments with more focus typically placed on insulating walls and roofs. After all, if heat rises, how much impact can the floor really have on keeping us warm? This article explores the crucial role of floor insulation contributing to sustainability, finances and health, as well as best methods for insulating various floor constructions in both new and existing buildings to comply with building regulations. 

How does floor insulation support sustainability?

Floor insulation plays a crucial yet often overlooked role in reducing carbon emissions. While walls account for around approximately 35% of heat loss and roofs approximately 25%, floors can contribute up to 15%. Proper insulation helps minimise this loss, significantly improving energy efficiency, helping to reduce a building’s overall environmental impact.

Floor insulation was not a building regulation until the mid-1990s. However, with the UK’s goal of achieving net zero carbon by 2050, every aspect of heat loss, including floors, are now under increased scrutiny. Installing floor insulation with reliable thermal performance is essential in helping to reduce emissions in both new residential and commercial buildings.

Floor insulation also helps to improve the sustainability of existing buildings by addressing a common source of heat loss. Many older buildings have uninsulated suspended timber floors, which can lead to significant energy waste. Properly insulating these floors helps reduce heat loss and lower carbon emissions across the UK’s housing stock.

How does floor insulation choice affect sustainability?

Natural insulation materials such as sheep’s wool, hemp and wood fibre may appeal to those seeking sustainable options, as they typically have a lower manufacturing energy demand compared to products such as stone wool and glass wool. 

On the other hand, chemical-based insulation materials including PIR, Phenolic, EPS and XPS, are not naturally occurring but can offer lower lambda values. This means less material is required to achieve the same level of insulation helping to offset their higher energy-intensive production. 

The choice of insulation material also depends on the type of floor. Flexible insulation offers more natural options when installed between floor joists in a suspended timber floor. However, for applications beneath concrete screed, where compressive strength is essential, natural options are more limited.

The sustainability of insulation is a complex matter, factoring in the energy used during manufacturing, the energy savings after installation, and its overall embodied carbon. For large projects, specifiers may use sustainability assessment tools to compare insulation products directly, while smaller refurbishments are often influenced by customer preferences for natural materials.

Why is thermal performance a consideration for floor insulation?

By insulating the floor, temperatures remain more consistent throughout the space, reducing the need for extra heating. This not only improves comfort but also helps lower energy bills, increase overall efficiency, and save money.

Each type of insulation has a different thermal conductivity, known as its lambda value, measured in W/mK. A lower thermal conductivity indicates better performance, as it slows heat transfer through the insulation layer. Rigid boards with closed-cell structures, such as PIR, Phenolic, and XPS offer generally provide better thermal performance per unit thickness compared to flexible insulation, which typically has an open structure. 

A floors thermal performance is measured by its U-value in W/m²K, with lower values indicating better insulation. The overall U-value depends on the floor’s size and shape, which determines its P/A ratio, the ratio of the floor’s perimeter to its area. This ratio, combined with the insulation’s lambda value, helps calculate the required insulation thickness required to achieve the desired thermal performance.

PIR, Phenolic and XPS rigid floor insulation boards are ideal for use with underfloor heating (UFH). As the floor insulation sits underneath the pipes it ensures that the heat is directed upwards into the room and limits the loss of heat into the ground below. The floor screed heats up and acts as a giant radiator giving even distribution of heat across the room. UFH is a popular choice in modern new-build properties, especially when combined with heat pumps. The increased surface area of heat radiation suits the lower flow temperatures supplied by the renewable heat pump technology.

What are the required U-values for floor insulation?

In England, Wales, and Northern Ireland, retrofitting an existing suspended timber floor the notional target requires a U-value of at least 0.25 W/m²K. 

For new thermal elements added to an existing building, the notional required U-value improves to 0.18 W/m²K. In Scotland, the typical target U-value permitted for both cases is stricter at 0.15 W/m²K. 

Building control may allow adjustments where achieving the U-value is not technically feasible, so it’s always advisable to check requirements prior to starting the work. 

Insulation manufacturers can provide U-value calculations based on the exact floor build-up, ensuring the most suitable insulation type and installation method is specified to meet regulatory requirements. 

Why is compressive strength a consideration for floor insulation?

When insulating areas where load bearing is required, such as beneath or above a concrete slab, selecting insulation with sufficient compressive strength is essential. Rigid boards like PIR, Phenolic, and XPS are best suited for these applications.

A structural engineer should assess the expected floor loading to determine the necessary compressive strength of the insulation. XPS typically offers higher compressive strengths than PIR and Phenolic, making it a more suitable choice for high-load applications such as industrial floors.

Why is durability a consideration for floor insulation?

Floor insulation must be durable enough to withstand structural loads and potential moisture exposure, which can arise from surrounding ground conditions, flooding, or leaks from heating systems.

Open-structured insulation materials, such as mineral wool, can absorb water, potentially reducing their thermal performance and structural integrity over time. While EPS insulation has a partially closed-cell structure, it is more permeable than fully closed-cell alternatives and can absorb some moisture, which may affect its long-term durability.

PIR and Phenolic insulation although closed cell, should not be exposed to moisture and are typically installed above a damp proof membrane (DPM) to prevent water ingress. For applications requiring direct ground contact or high moisture resistance, XPS insulation is generally preferred due to its improved water resistance and structural stability.

Why is flexibility a consideration for floor insulation?

When insulating suspended timber floors in existing buildings, the flexibility of the insulation affects ease of installation. Mineral fibre insulation can readily compress and pushed between floor joists, making installation straightforward. Rigid boards can also be used, but they require precise cutting to achieve a tight ‘friction-fit’ between the joists. While this may take longer, using rigid boards such as PIR and Phenolic of the same thickness with lower thermal conductivity result in improved thermal performance and a lower U-value for the floor.

Why is acoustic performance a requirement for separating floor insulation?

In separating and internal floors, insulation is primarily used for acoustic purposes rather than thermal improvement. This is common in apartment buildings, where the floor of one dwelling serves as the ceiling of another. While thermal insulation is not be necessary as these are both heated spaces. Acoustically insulating the floor helps reduce airborne noise transmission, creating a quieter and a more pleasant indoor environment. Flexible insulation materials such as mineral fibre, sheep’s wool and hemp provide good sound absorption and noise reduction. 

How do you select and install the right floor insulation?

Both flexible insulation, such as rock wool and glass wool, and rigid insulation, like PIR, can be specified to achieve the required U-value. The necessary thickness will depend on the insulations lambda value, with lower lambda materials requiring less depth to meet performance targets. 

Install the insulation so that its full thickness remains consistent across the entire span width between the joists. Support for flexible insulation can be achieved using a board, battens, or durable mesh beneath the joists. Rigid boards may be secured by resting them on battens attached to the sides of the joists and secured in place with nails driven into the joists.

Moisture control barriers may be necessary if there is a risk of condensation. It is important to ensure that adding insulation does not create cold bridges within the floor, as these could lead to an increased risk of condensation build-up, potentially causing long-term damage. 

For more detailed information, refer to the government’s ‘Guide to Best Practice’ for retrofitting floor insulation in suspended timber floors.

Where can I found out more about floor insulation solutions?

Celotex has several XPS and PIR floor insulation solutions that are suitable for a wide range of domestic and commercial applications. Celotex provides technical support, including U-value calculations, and project consultations.

David Milner, technical team leader at Celotex, explains the company’s floor insulation offer: “By offering both XPS and PIR flooring insulation, Celotex can help match insulation solutions to specific project needs. We can assist in selecting insulation boards to align with or exceed the performance requirements of different flooring applications.”

Have questions about flooring insulation? Contact Celotex’s experienced team with the details of your project to discuss how they can help.