When calculating a U-value JPA Designer displays an error message which states:
This calculation method is not applicable because the ratio of the upper to lower limit exceeds 1.5
JPA Designer uses the combined method of U-value calculations set out in BS EN ISO 6946:2007. It calculates two different resistances for the construction, the first is calculated by considering the heat paths through the construction (the upper limit resistance), the other by considering the resistances of layers (the lower limit resistance). The final resistance – which gives the U-value – is the average (mean) of the upper and lower limit resistances.
The standard states that the calculation method is not valid where the ratio of upper to lower limit resistances is greater than 1.5. That usually happens where one layer of insulation is bridged by a material with a high thermal conductivity: usually steel, but sometimes very dense concrete columns or beams. JPA Designer checks the ratio in each calculation and displays the error message when the ratio exceeds 1.5.
The error message will appear if the layers and bridging of an element with light steel framing have been entered, but the element has not been defined as being a light steel frame construction in the U-value Construction dialogue. Once the element is correctly defined as light steel frame the error message will not appear.
Unfortunately, there is no simple solution if the error message appears with dense concrete bridging insulation. The calculation result will offer a guide to the overall performance but there is no guarantee of its accuracy. The only way of obtaining an accurate result would be to undertake two or three dimensional numerical modelling, using a program such as THERM.