What is the psi value?

The psi value (Ψ, linear thermal transmittance) is the additional heat loss per metre length of a linear thermal bridge, expressed in W/(m·K). It is the heat loss you miss if you only add up the U-values of walls, floors and roofs.

Thermal bridges occur mainly at junctions: wall to foundation, around windows and doors, at continuous balcony slabs, at the eaves or parapet, and at lintels. These junctions often run for many metres, so the psi value adds up quickly.

How to calculate with it

Additional heat loss (W) = Ψ × length (m) × temperature difference (K)

Example: a 10 m junction with Ψ = 0.5 W/(m·K), at a 20 degree difference between inside and outside, gives 0.5 × 10 × 20 = 100 watts of extra loss for as long as that temperature difference lasts.

For point thermal bridges (a wall tie or a penetration) there is the chi value (χ) in W/K.

How psi values are determined

• Numerical calculation. The detailed method models the junction in two or three dimensions according to ISO 10211.
• Default values. ISO 14683 gives default psi values for common junction types. National energy calculation methods often include their own, usually conservative, defaults.
• Accredited or reference details. Many countries publish standard details with known psi values that you can adopt.

Default values are safe but unfavourable. Well-designed, calculated details usually give a better energy result.

Worth knowing

• Lower is better. A psi value can even be negative. This comes from the convention of using external dimensions, so the plane elements already overestimate the loss at a corner.
• Psi value and condensation are separate issues. The psi value is about energy loss. For condensation and mould risk you look at the temperature factor (f_Rsi) of the detail.

How to reduce the psi value

1. Keep the insulation layer continuous over the foundation, parapet and floor edge.
2. Set windows in line with the insulation and close the insulation tight against the frame.
3. Thermally break continuous elements such as balcony slabs.
4. Insulate around lintels and steel beams in the cavity or on the outside.
5. Use perimeter insulation where the wall meets the ground floor and foundation.

Related terms

• Thermal bridge
• Insulation value (Rd value)
• Perimeter insulation
• Dew point
• Lintel

Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.