Each opaque surface within an EnergyPlus model is assigned a construction. This construction is made up of Materials that are with fully defined with Density, Thermal Performance, Specific Heat, etc. or a No Mass Material which only includes the Thermal Performance.
For ease of assigning specific Thermal Performance internal surfaces within the model are assigned a No Mass Material that reflected the defined R-value or U-value. However, including mass within the model is important to help the Heat Balance simulations resolve.
To introduce mass into the building models and improve reliability of the simulation a standard Internal Mass Material is added to each interior surface. This includes the Ceilings, Interior Floors, and Interior Walls. The thermal mass added has the follow properties
Thickness = 3.93 in (0.1 m)
Conductivity = 13.86 Btu*in/hr*ft2*R (2.0 W/m*K)
Density = 187.28 lb/ft3 (3000 kg/m3)
Specific Heat = 0.238 Btu/lb*R (1000 J/kg*K)
Thermal Absorptance = 0.9
Solar Absorptance = 0.7
Visible Absorptance = 0.7