Recognised Construction Details (RCDs) vs Bespoke PSI Values for Masonry Junctions

When targeting compliance with Part L of the Building Regulations, design teams often rely on the Recognised Construction Details (RCDs) to provide pre-calculated Ψ (Psi) values for junction heat loss. These details give peace of mind that standard masonry junctions – like wall-to-floor or wall-to-roof connections – will achieve consistent performance when built to spec.

RCDs are an excellent starting point: they reduce risk, simplify SAP assessments, and help avoid performance gaps.

The limits of recognised construction details

Despite their value, the RCD library is constrained. It only covers a small set of insulation cavity widths (100 mm, 125 mm, 150 mm) and a narrow band of block conductivities:

• 0.11 W/mK
• 0.15 W/mK
• 0.19 W/mK
• 0.28 W/mK
• 0.60 W/mK
• 1.33 W/mK

In practice, UK masonry design rarely fits so neatly into these categories. Architects and contractors frequently work with:

• 50 mm or 75 mm cavities (common in extensions and refurbishments)
• 90 mm cavities (blockwork or hybrid wall types)
• 200 mm+ cavities (low-energy or near-Passivhaus homes)
• Block conductivities outside the six “official” RCD options
• Mixed solutions such as external wall insulation (EWI) or part-fill approaches

When your design falls outside these predefined ranges, you can’t simply pick an RCD Ψ-value. Using default values in SAP instead means accepting a penalty that makes compliance harder and can add unnecessary cost to the build.

Why bespoke Psi values for masonry matter

Even small changes to block conductivity, cavity width, or insulation placement can have a significant impact on thermal bridging performance. Default Ψ-values don’t reflect the real-world behaviour of your junctions, and the result is:

• Higher calculated heat loss
• Stricter compliance targets
• More expensive compensatory measures elsewhere

By calculating bespoke Psi values, you can demonstrate the true performance of your masonry junctions, unlocking compliance without over-specification.

How AutoPSI bridges the gap

At AutoPSI, we provide fast Ψ-value calculations for any masonry junction, regardless of:

• Cavity depth (from thin 50 mm to super-insulated 250 mm+)
• Block conductivity (not limited to 0.11–1.33 W/mK)
• Specific junction conditions (EWI, part-fill, complex reveals)

This gives you flexibility where recognised construction details stop. Instead of relying on defaults, AutoPSI models junctions accurately, reducing calculated heat loss and helping you comply with Part L cost-effectively.

The complete approach to Psi values

• RCDs: trusted, simple, standardised junctions for common cavity walls.
• AutoPSI: bespoke modelling for all the masonry junctions the RCDs don’t cover.

Together, they give you complete coverage for every project type.

With AutoPSI, you can achieve accurate Ψ-values for real-world masonry designs, not just the limited block and cavity options predefined in the recognised construction details library.

Bespoke PSI calculations provide accurate heat loss values, ensuring better energy performance and compliance. Compared to Table K1 defaults, a project-specific Ψ-value calculation offers clear advantages: