# How does EnergyPlus calculate floor U-values?

I have been looking at the Envelope Summary in the Energy Plus Results for some of my models of domestic buildings and found that the ground floor u-value is very high.

The construction is a simple 10mm of Ceramic Tile and 150mm of Concrete and I would have expected it to be far lower than this. How does OS calculate the U-value? Does OS take into account the exposed perimeter of the floor when calculating this? If so, is there a way of setting the exposed perimeter as my models have party walls with buildings that are not drawn in the model.

Thanks

Surface 29 is a simple solid floor made of: 10mm of ceramic floor tile with a thermal conductivity of 0.8 W/mK 150mm of concrete with a thermal conductivity of 1.7296 W/mK

I have calculated the solid floor U-value using the CIBSE Guide A method and found the U-value to be 1.625 W/m2K. Please see the methods and calculations below.

(CIBSE Guide A)

(CIBSE Guide A)

This question is really about EnergyPlus, not OpenStudio, so I'm going to rename/retag it.

@shorowit okay thanks

I still have not been able to find out how EnergyPlus calculates this floor u-value, I am thinking that maybe I should try to amend the construction to reflect my own calculations using a no mass material on top of the existing structure? Does this sound like something that anyone has done before?

Can you either provide a link to your IDF or share the relevant construction and materials for Surface 29?

Hi @shorowit, yes of course! Surface 29 is a simple solid floor made of: 10mm of ceramic floor tile with a thermal conductivity of 0.8 W/mK 150mm of concrete with a thermal conductivity of 1.7296 W/mK

I have calculated the thermal transmittance of a solid floor using CIBSE Guide A and found the u-value to be 1.625 W/m2K. I will edit the question to show how I calculated this. I am just intrigued to see how EnergyPlus got such a different answer and also I have not set the exposed perimeter in my model, so do not know how it should know this?