# Slab insulation for Construction:Internal Source and Slab U-factors

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? (Matt, @mdahlhausen, I know I asked you about this yesterday, but looking at the U-factor calculations again gave me some doubts.)

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=area*Ueff*(T, air, out-T,air,in)=(T,air,out-T,air,in)*(P,exp*F-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.