Knowledgebase : Materials and Constructions > Opaque
Question: I am simulating a house that has roof rafters with no insulation in between them. I am trying to determine how to define the parameters for the rafters material layer in the roof construction, which is 23% wood rafters and 77% air. How do I best do this? I want a solid baseline design so I can go in later and replace this construction with one that has insulation in between the wood rafters.

Answer: The most common approach for modeling a framed construction is to divide each building surface into two parts, one to represent the framed area and one to represent the insulated (or in this case, uninsulated) area. Because EnergyPlus surface heat transfer is 1-dimensional, some users will set the framed area to something greater than the actual framed percentage to allow for thermal bridging.
Question: I am currently working on a project which use EnergyPlus for PCM (phase change material) performance assessment. I notice that PCM has latent heat, and I am struggling to find a correct field for me to submit the latent heat value for the PCM. would this affect the results whether the input of the latent heat value is there or not?

Answer: The PCM characteristics are entered using MaterialProperty:PhaseChange, as in example file CondFD1ZonePurchAirAutoSizeWithPCM.idf. This objects has inputs to specify a function of enthalpy vs. temperature. The latent heat is represented by a large change in enthlpy over a small change in temperature at the melting point. See figure in Input Output Reference (pdf page 132 in v5.0 docs).

Read about Composite Wall Constructions, IORef page 159 (PDF page 198)

Also the reference construction/materials set: CompositeWallConstructions.idf

For example, the materials specified in the Benchmarks are whole-surface U-values that are meant to include thermal bridging. Of course to build such a building would require different materials than listed for thermal model.

To have these effects in the Benchmark files, the conductivity is increased and/or the thickness is decreased for the insulation materials in the model, compared to what they would be in the real constructions.
Q: Reading the documentation, I'm wondering if the Cavity algorithm is usable for other double facade types or only Trombe wall? In other words, does Cavity implicitly presume that the inner wall is highly solar absorbent and so generate specific convection?

A: The Trombe wall convection algorithm is applicable to just about any vertical cavity with a high aspect ratio and relatively narrow width. I'm not sure if a double facade cavity would meet the aspect ratio requirement. But I do know the Trombe wall algorithm is not picky about whether the inner wall is highly absorbant, or about any particular properties of the walls. Actually the same basic algorithm is used by the window model to calculate the convection between the two panes of a window. The full reference is ISO 15099.