P1 Junction Psi-Value: Party Wall to Ground Floor

What the P1 junction is

The Standard Assessment Procedure (SAP) lists every junction type in a home in its Table K1, each with a reference code and a linear thermal transmittance (Ψ-value, W/m·K) that adds to the dwelling's heat loss. Most are external-wall details - lintels, sills, eaves, corners - and carry E-codes. The P-codes cover junctions that involve the party (separating) wall, and P1 is the junction where the party wall meets the ground floor.

A party wall separates two heated homes, so very little heat crosses its face. But where it lands on the ground floor it is a different story: the wall and its foundation reach down into cold ground, opening a path for heat to flow out beneath the insulation line and away. That loss is real, it is genuinely three-dimensional, and SAP accounts for it through the P1 junction's Ψ-value.

Why one model isn't enough

For most junctions a single 2D cross-section is all you need, and even the more awkward ones resolve with a single 3D model. P1 is the exception. BR 497 is explicit that these ground-connected junctions "have a complex interaction of heat flows to and through the connecting ground" and that the Ψ-value "cannot be adequately determined from a single model (2D or 3D)" - it has to be extracted from the difference between two 3D models.

The reason is the ground. Heat leaving the foot of the party wall spreads out through a large volume of soil around the foundation before it reaches the outside, and that volume behaves three-dimensionally. You can only isolate the junction's own contribution by modelling the floor twice - once with the party wall and its foundation, once without - and taking the difference. Hence two full 3D models, each carrying a large block of ground.

What the calculation gives you

The linear thermal transmittance (Ψ) of the P1 junction - the extra heat loss per metre where the party wall meets the floor, over and above the plane floor either side. It is built from the two models:

Ψ_c = (Q₁ − Q₂) / [(T_i − T_e) · L]

where Q₁ is the heat loss with the party wall present, Q₂ the heat loss with it removed and replaced by floor, and L the modelled length of junction. For a wall between two properties a further term is added for the strip of floor the party wall stands on (which the floor U-value never counts): Ψ = Ψ_c + (t_wall · U′_f). Because the wall is shared, half the resulting Ψ-value is assigned to each dwelling.

This Ψ-value feeds the building's transmission heat-loss coefficient in SAP - H_TB = Σ(l · Ψ) - exactly like any other junction, but now with the ground heat loss properly counted rather than guessed.

Our method

We model the junction with 3D finite-element analysis, following BR 497 §4.8.1 and the numerical procedure of BS EN ISO 10211:

1. Build two identical 3D models of the ground floor and the surrounding ground from your construction drawings - the first with the party wall and its foundation in place, the second with the wall removed and replaced by a continuation of the floor. Everything else is the same.
2. Include the ground as a large 3D volume, sized per BS EN ISO 13370. By convention the floor is modelled to its characteristic dimension (taken as 8 m for the standard end-terrace built form), with half the floor represented out to a 4 m adiabatic plane and the party wall 2.4 m tall.
3. Replace the external walls with an adiabatic boundary at the edge of the floor, so the Ψ-value reflects only the party-wall/floor junction and not whatever external wall happens to abut it - keeping the result transferable between built forms.
4. Apply the boundary conditions - 20 °C inside, 0 °C outside, and the standard surface resistances from BS EN ISO 6946 (0.13 horizontal and 0.17 downward internally, 0.04 externally).
5. Solve and subtract - refine the mesh until the heat flows are stable, then take Q₁ and Q₂ from the two models and apply the BR 497 formulae above, including the floor-strip term and the half-share to each dwelling. Solid and suspended ground floors are each handled by their own variant of the convention.
6. Apply BR 497 throughout - external dimensions, the agreed ground treatment and surface resistances - so the figure is consistent and audit-ready.

Our models are validated against the benchmark test cases in BS EN ISO 10211 and reproduce the BR 497 worked example for this junction.

The standards we follow

Core method

• BR 497 (BRE) - §4.8.1 sets out the UK convention for the party-wall/ground-floor junction: the two-model method, the ground treatment and the split between dwellings. This is the definition of the P1 calculation.
• BS EN ISO 10211 - the numerical calculation of 3D heat flows that both models rely on.
• BS EN ISO 13370 - heat transfer through the ground, which governs the size and treatment of the soil volume.
• BS EN ISO 6946 - U-values of the plane floor and walls, and the standard surface resistances.

Compliance and companion checks

• SAP (Table K1) - the source of the P1 reference and where the Ψ-value is entered into the dwelling's heat-loss assessment and Part L compliance.
• BRE IP 1/06 & BS EN ISO 13788 - the temperature-factor check for surface condensation and mould, available from the same model where the junction's coldest point is a concern.

What you receive

A short technical report for the junction: the geometry of both 3D models and the ground they include, the conductivities and boundary conditions used, the two heat flows and the derived P1 Ψ-value - with the half-value assigned to each dwelling clearly stated - and the standards and conventions applied in full, so a SAP assessor or building control body can see how the figure was derived and use it in place of a default.

Fee

A P1 calculation is a £250 fixed fee (ex VAT) for a ground-bearing floor on strip foundations, with the Ψ-value for both dwellings in one calculation. Suspended floors, piled or ground-beam foundations, and junctions that change along their length are quoted on request. See the thermal bridging fees, or send your drawings for a quotation.