How are the "U-factor no film" and "U factor with film" calculated by EnergyPlus for a slab-on-grade modeled as a Building Surface Detailed (with a Construction:InternalSource) with Site:GroundTemperature:Building surface object entered?
I'd like to be able to add insulation to a slab modeled with the BuildingSurface:Detailed and Construction:InternalSource object to the level specified by ASHRAE 90.1 2013, which is defined in terms of an equivalent F-factor, or a certain length of insulation (which seems to be measured from the slab edge). So far, I've added a horizontal layer of insulation, and looked at the resulting U-factors for the construction.
Since I am comparing this model to another energy model with a Slab with F Factor, I'd like to be able to compare the "U-factor no film" for this slab to "U-factor no film" calculated for a Slab with F Factor.
But the "U factor no film" calculated for the "slab with f factor" seems to be consistent with the "U-effective" defined for the Slab with F Factor (Equation 3.175 in the Engineering Reference), from my back calculations.
Q=areaUeff(T, air, out-T,air,in)=(T,air,out-T,air,in)(P,expF-factor)
This makes me think that the two U-factors would not be equivalent, since I assume that the U-factor for the "regular" slab (defined without the f-factor) would be calculated with a simplified 1-D approach, with the "outside" temperature being the ground temperature, not the air temperature.
I would appreciate any thoughts on how to either add insulation to a BuildingSurface:Detailed with a Construction:InternalSource (where I can add layers, but not use one of the GroundDomain:Slab objects, as far as I know) to the level specified by ASHRAE 90.1, or interpret the calculated U-factors.
