Energyplus 101: Part 2 - Construction Materials

Alan Jackson bio photo By Alan Jackson Comment

In the last part we covered the creation of geometry. In the example videos we chose to use the default construction profile for our surfaces within OpenStudio. It is now time to modify those construction templates with ones that resemble the actual design specified construction performance.

Here is how the default exterior wall (typical mass wall) from Open Studio is written to the Energyplus IDF file:

Exterior Wall,           !- Name  
M01 100mm brick,         !- Outside Layer  
M15 200mm heavyweight concrete,  !- Layer 2  
I02 50mm insulation board,  !- Layer 3  
F04 Wall air space resistance,  !- Layer 4  
G01a 19mm gypsum board;  !- Layer 5

Our example uses lightweight residential construction so we would like to replace all the exterior walls with the following construction information in the IDF file.

Exterior Wall,     !- Name  
F08 Metal surface,       !- Outside Layer  
I02 50mm insulation board,  !- Layer 2  
04 Wall air space resistance,  !- Layer 3  
G01a 19mm gypsum board;  !- Layer 4

You can see that we’ve given the new construction the same name as the default construction “Exterior Wall”. What this allows us to do is have Energyplus recognize that we are replacing all the instances of “External Wall” with the new construction properties. This eliminates having to go back into OpenStudio and re-assign the walls to our new construction definition. The important thing to remember thought is once you are done either remove the old definition or comment it out by adding an exclamation ! to the beginning of each line if you don’t want to delete.

Below is our new material definition. As you can see we are free to define the individual material layers of our construction as long as we have the required material properties: conductivity, density and specific heat) All of these should be readily available to architects and engineers. Energyplus offer typical ASHRAE constructions within the /dataset/ installation folder.

Here is the new definition we want to add ‘F08 Metal surface’. The other materials I02, F04 and G01a are already defined from the default wall so we don’t want to re-enter them. Remember having two instances of a definition with the same name will confuse Energyplus and cause errors! The definition can be placed at the end of your IDF it is not important to place near the other material definitions. In fact on thing to note is if you prefer to use the IDF editor it will typically sort all your definitions on save with the default setup.

F08 Metal surface,       !- Name  
Smooth,                  !- Roughness  
0.0008,                  !- Thickness {m}  
45.28,                   !- Conductivity {W/m-K}  
7824,                    !- Density {kg/m3}  
500;                     !- Specific Heat {J/kg-K}`

I wanted to include as a side note the way that gas layers are defined as they are a little different than a typical opaque material. You can see they only require the resistance properties and are defined using Material:AirGap.

F04 Wall air space resistance,	  !- Name  
0.15;  !- Thermal Resistance {m2-K/W}`

Thats it! This should give you enough information to create all the construction surfaces you require for your building. If you have nay questions please leave a comment or send me a message.