In the pursuit of creating highly energy-efficient buildings, we often focus on the big-picture items: high-performance windows, thick layers of wall and roof insulation, and airtightness. These are the U-values we specify and calculate, representing the bulk of a building’s thermal performance. However, even a building with outstanding U-values can dramatically underperform if the details—the junctions between elements—are not properly considered. This is the problem of thermal bridging.
Imagine building a watertight boat. You use the strongest, most impermeable materials for the hull, but you fail to properly seal the seams where the panels join. No matter how good the hull material is, the boat will leak. Thermal bridging is the energy equivalent of that leak. It is a small, often overlooked detail that can have a huge effect on a building’s overall heat loss, comfort, and compliance.
This article will take a technical look at the critical issue of cold bridging at the foundation, explaining the science behind it, the metrics used to measure it (Psi and Chi values), and why it represents one of the most significant performance gaps in modern construction. Most importantly, we will demonstrate how a well-designed insulated raft foundation doesn’t just mitigate this problem—it eliminates it entirely.
What is Thermal Bridging? A Path of Least Resistance
A thermal bridge, or cold bridge, is a localised area in the building envelope where the resistance to heat flow is significantly lower than in the surrounding materials. It is, quite simply, a path of least resistance for heat to escape. These bridges typically occur at junctions between different building elements (e.g., wall-to-floor, wall-to-roof), where a highly conductive material penetrates the insulation layer, or where the continuity of the insulation is interrupted.
While we often focus on the transmission of heat through the main fabric elements (measured by U-values in W/m²K), thermal bridges represent an additional, and often substantial, source of heat loss. According to the Building Research Establishment (BRE), thermal bridging can be responsible for up to 30% of a well-insulated building’s total heat loss [1]. This is the “hole in the dam” that undermines all our other efforts.
Quantifying the Leak: Understanding Psi (ψ) and Chi (χ) Values
To properly account for thermal bridging, we need to move beyond U-values. The heat loss from these junctions is quantified using two key metrics:
- Psi (ψ) Value: This measures linear thermal transmittance and is expressed in W/mK (Watts per metre per Kelvin). It quantifies the additional heat flow per metre of a linear junction (like a wall-floor junction or around a window) that is not accounted for by the U-values of the flanking elements. A higher Psi value indicates a worse-performing junction.
- Chi (χ) Value: This measures point thermal transmittance and is expressed in W/K (Watts per Kelvin). It is used for point-like thermal bridges, such as a steel column penetrating an insulated wall or a fixing that passes through the insulation layer.
Think of it this way: if U-values measure the heat loss through the main surfaces, Psi and Chi values measure the extra heat loss that occurs at the seams and penetrations. For a building to be truly energy-efficient, these values must be minimised.
To put this in context, UK Building Regulations set maximum allowable Psi values for key junctions. For a wall-to-floor junction, the maximum permitted value is a poor 0.16 W/mK. Using default or accredited details might improve this to around 0.08 W/mK. However, a high-performance detail, such as that achieved with an insulated raft foundation, can achieve a Psi value of 0.01 W/mK or even lower—a staggering 94% reduction in heat loss at that junction compared to the regulatory maximum.
| Junction Detail | Typical Psi (ψ) Value (W/mK) | Performance |
| Poorly Detailed Junction | > 0.16 | Non-Compliant |
| Regulatory Maximum | 0.16 | Bare Minimum Compliance |
| Accredited Construction Detail (ACD) | ~ 0.08 | Standard Practice |
| Enhanced Construction Detail (ECD) | ~ 0.04 | Good Practice |
| GreenRaft Insulated Raft | < 0.01 | High Performance |
Table 1: Comparison of typical Psi values for wall-to-floor junctions.
A Quantified Example: The Real-World Impact of Psi Values
To truly understand the dramatic impact of thermal bridging, let’s consider a quantified example. We will compare the total fabric heat loss of a simple, well-insulated building using two different foundation details:
Building Specification:
- Dimensions: A single-storey building, 10m x 8m (Perimeter = 36m).
- Fabric U-Values: Walls (0.18 W/m²K), Floor (0.15 W/m²K), Roof (0.11 W/m²K), Windows/Doors (0.8 W/m²K).
First, we calculate the heat loss based only on the U-values of the main fabric elements. This is the heat loss before accounting for the additional losses at the junctions.
Scenario A: Standard Details (Poor Psi Values)
In this scenario, we use the poor, but compliant, Psi value of 0.16 W/mK for the critical 36m wall-to-floor junction. This represents a standard strip foundation detail that just meets the regulatory backstop.
- Heat Loss from Junctions (Thermal Bridging):
- Wall/Floor Junction: 36m × 0.16 W/mK = 5.76 W/K
- Other Junctions (corners, windows, roof): Let’s assume a conservative total of 4.5 W/K.
- Total Thermal Bridge Heat Loss: 5.76 + 4.5 = 10.26 W/K
Scenario B: High-Performance Details (GreenRaft Solution)
Here, we use the same building but specify a GreenRaft insulated raft foundation, achieving an excellent Psi value of 0.01 W/mK for the wall-to-floor junction.
- Heat Loss from Junctions (Thermal Bridging):
- Wall/Floor Junction: 36m × 0.01 W/mK = 0.36 W/K
- Other Junctions (same as above): 4.5 W/K.
- Total Thermal Bridge Heat Loss: 0.36 + 4.5 = 4.86 W/K
The Result
| Heat Loss Component | Scenario A (Standard Details) | Scenario B (GreenRaft Solution) |
| Wall/Floor Junction Heat Loss | 5.76 W/K | 0.36 W/K |
| Total Thermal Bridge Loss | 10.26 W/K | 4.86 W/K |
By simply changing the foundation detail from a standard, compliant strip footing to a high-performance insulated raft, we have reduced the heat loss from the wall-to-floor junction by 94%. More importantly, the total heat loss from all thermal bridges across the entire building has been more than halved (a 53% reduction).
Translating Psi Values into Effective U-Values
Another powerful way to conceptualise this impact is to see how the thermal bridge degrades the performance of the adjacent building element. While the floor itself has a specified U-value of 0.15 W/m²K, the heat loss at the perimeter edge effectively worsens its overall performance. We can calculate the effective U-value of the floor by distributing the junction’s heat loss over the floor’s area (80m²).
Scenario A (Standard Details):
- Additional Heat Loss (W/K) per m² of floor: 5.76 W/K ÷ 80m² = 0.072 W/m²K
- Effective Floor U-value: 0.15 (specified) + 0.072 (from bridge) = 0.222 W/m²K
Scenario B (GreenRaft Solution):
- Additional Heat Loss (W/K) per m² of floor: 0.36 W/K ÷ 80m² = 0.0045 W/m²K
- Effective Floor U-value: 0.15 (specified) + 0.0045 (from bridge) = 0.155 W/m²K
| Performance Metric | Scenario A (Standard Details) | Scenario B (GreenRaft Solution) |
| Specified Floor U-value | 0.150 W/m²K | 0.150 W/m²K |
| Effective Floor U-value | 0.222 W/m²K | 0.155 W/m²K |
| Performance Degradation | 48% Worse | 3% Worse |
This is the crucial takeaway for any specifier. In Scenario A, the poorly detailed junction makes the floor perform 48% worse than its specified U-value. The thermal bridge has effectively turned a good floor into a mediocre one. In contrast, the GreenRaft solution results in a negligible 3% degradation, preserving the integrity of the specified design.
This example clearly demonstrates that even in a building with excellent U-values, the choice of junction detail has a monumental impact on the real-world energy performance. The additional heat loss from the poorly detailed foundation is not a minor rounding error; it is a significant performance gap that can be, and must be, eliminated with proper design.
The Critical Junction: Cold Bridging at the Foundation
While thermal bridges can occur anywhere in the building envelope (e.g., around windows, at roof eaves, through lintels), the wall-to-floor junction is arguably the most critical and most frequently mishandled detail. In traditional construction, a concrete strip foundation is placed in the ground, and a blockwork or masonry wall is built up from it. The ground floor slab is then cast inside these walls. This creates a direct, uninterrupted path for heat to flow from the heated internal slab, through the inner leaf of the wall, down into the concrete foundation, and out to the cold ground. This is a classic and severe thermal bridge.
The consequences are significant:
- Massive Heat Loss: As noted by the Energy Saving Trust, this junction alone can be responsible for around 15% of the total heat loss from a well-insulated home [2].
- Comfort Issues: The cold bridge results in cold interior floor and wall surfaces, creating discomfort, draughts, and an environment that feels colder than the thermostat reading suggests.
- Condensation and Mould: The cold interior surfaces can fall below the dew point of the indoor air, leading to surface condensation. This moisture creates the perfect breeding ground for mould, with serious implications for both occupant health and building durability.
- Regulatory Non-Compliance: Under current Building Regulations (Part L), continuity of insulation is mandatory. Designs that fail to address this thermal bridge properly will struggle to achieve compliance, especially as standards tighten towards the Future Homes Standard in 2026.
The Problem with Thresholds
A related and equally problematic thermal bridge occurs at door thresholds. Here, the need for a level, robust, and accessible entrance often leads to a break in the floor insulation. A standard metal or concrete threshold can create a direct cold bridge from the outside in, resulting in a cold spot right at the entrance, leading to condensation and significant localised heat loss. This detail must be specifically addressed with a thermally broken solution.
The Solution: A Continuous Thermal Envelope from the Ground Up
The only way to truly solve the problem of cold bridging at the foundation is to rethink the junction entirely. Instead of trying to patch a fundamentally flawed design, we must adopt a system that is inherently free of thermal bridges. This is precisely what an insulated raft foundation achieves.
By placing a continuous layer of high-compressive-strength, low-conductivity XPS insulation beneath the entire concrete slab and wrapping it up around the slab edge, the foundation is transformed. The system works by:
- Bringing the Slab Inside: The entire concrete raft slab is brought inside the building’s thermal envelope. It is now a thermally protected element, just like the insulated walls and roof.
- Creating a Continuous Upstand: A vertical upstand of insulation at the slab edge creates a continuous thermal break between the concrete slab and the external ground and rising wall. Heat simply has no conductive path to escape.
- Eliminating the Junction: The problematic wall-to-floor junction is effectively designed out of existence. The wall insulation now overlaps with the foundation insulation, creating a seamless, unbroken line of thermal protection.
This approach doesn’t just reduce the Psi value; it fundamentally changes the physics of the junction, resulting in the exceptionally low Psi values seen in high-performance systems like GreenRaft.
GreenRaft: Engineered to Eliminate Thermal Bridges
At GreenRaft, our insulated raft foundation systems are engineered from the ground up to eliminate thermal bridging. Our solutions include:
- High-Performance XPS Insulation: We use XPS with superior compressive strength and moisture resistance, ensuring long-term thermal and structural performance.
- Engineered Edge Details: Our systems include a proprietary insulated upstand that guarantees a continuous thermal break at the critical slab edge, delivering market-leading Psi values.
- Thermally Broken Threshold Solutions: We offer specific, engineered details for thresholds, incorporating thermal breaks that ensure the continuity of insulation is maintained even at doorways, preventing cold spots and heat loss.
By specifying a GreenRaft system, you are not just specifying a foundation; you are specifying a complete, engineered solution to the problem of thermal bridging at the base of the building.
Conclusion: Detail is Everything
In the drive for energy efficiency, it is no longer enough to focus on the U-values of the main building fabric. The 30% of heat loss that can occur through thermal bridges is a performance gap we can no longer afford to ignore. The wall-to-floor junction represents the most significant of these leaks, and traditional construction methods are fundamentally ill-equipped to solve it.
By embracing the principles of continuous insulation and adopting modern, engineered solutions like the insulated raft foundation, we can eliminate this critical thermal bridge at its source. For specifiers and engineers committed to delivering buildings that are truly comfortable, healthy, and energy-efficient, paying attention to this detail is not just best practice—it is everything.
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References
[1] Building Research Establishment (BRE). (n.d.). Thermal Bridging. Information retrieved from various BRE publications and guides.
[2] Energy Saving Trust. (n.d.). Heat loss in the home. Information retrieved from various EST publications and guides on domestic energy efficiency.
[3] Zero Carbon Hub. (2016). Thermal Bridging Guide. Retrieved from https://greenbuildingencyclopaedia.uk/wp-content/uploads/2016/10/ZCH-ThermalBridgingGuide-Screen_0.pdf [4] BEAT Solutions. (n.d.). Thermal Bridge Modelling & Psi Values Explained. Retrieved from https://www.beatsolutions.co.uk/psi-values-explained