About Thermal Bridge Calculator
The thermal bridge calculator evaluates linear thermal transmittance (psi-value) at building junctions per EN ISO 10211 and EN ISO 14683. It is used by building physicists, Passive House designers, and energy assessors to quantify heat loss at junctions and to check the surface temperature factor against condensation and mould risk.
Select a junction from the catalog, set the insulation thickness and conductivity, and the tool returns the psi-value with a severity classification, the temperature factor fRsi, and the resulting annual heat loss for the junction length.
How It Works
- Look up the junction psi-value from the catalog, interpolated for the chosen insulation thickness and conductivity.
- Classify the psi-value as negligible, acceptable, significant, or severe against thermal-bridging thresholds.
- Compute the temperature factor fRsi from the minimum interior surface temperature to assess condensation risk.
- Multiply psi by junction length and heating degree days to estimate the annual heat loss impact.
Worked Example
A junction with psi = 0.05 W/mK over a 1 m length and an fRsi of 0.80 is classed as acceptable and exceeds the 0.75 condensation threshold, so there is no mould risk.
Formulas
- Linear thermal transmittance
psi = L2D - sum(U_i * l_i)- Temperature factor
fRsi = (Tsi_min - Te) / (Ti - Te)- Annual heat loss
Q = psi * L * HDD * 24 / 1000
Standards & References
- EN ISO 10211
- EN ISO 14683
- EN ISO 13788
Frequently Asked Questions
What is a psi-value?
The psi-value (linear thermal transmittance, W/mK) is the extra heat loss per metre of a junction beyond what the plane elements already lose, capturing the thermal bridge at the connection.
How is the temperature factor fRsi used?
fRsi = (Tsi_min - Te) / (Ti - Te) measures how warm the coldest interior surface stays. Per EN ISO 13788 an fRsi at or above 0.75 indicates no mould risk; below 0.72 condensation and mould are likely.
When is a psi-value considered acceptable?
This tool classifies psi below 0.01 W/mK as negligible (Passive House target), 0.01-0.10 as acceptable, 0.10-0.30 as significant, and above 0.30 as severe.
Is the thermal bridge calculator free to use?
Yes, it runs entirely in your browser at no cost and stores none of your input data.