# Thickness of internal mass material

Hi everyone

I have a rather simple question, but I haven't been able find the right answer yet. I am doing some research on internal thermal mass and would be using the "InternalMass" section in EnergyPlus.

In the 'engineering reference' I found that for internal mass object that "if both faces are within the zone, this situation can be modeled either by creating another internal mass surface with the reverse construction, or, for symmetric constructions, simply doubling the area of the surface." (3.7 Adiabatic Boundary Conditions)

My question now is, which thickness do I use when such an object has its both faces within the zone? When doubling the area of the surface, do I use the full thickness or only half of the thickness?