An OpenStudio residential model works well otherwise, but all attempts to use an F-Factor construction to add perimeter slab insulation are unsuccessful. In the model's construction set, an internal source construction for radiant heat yields the same plausible results whether it is in the top line (sc., box) for floors, "exterior surface constructions," or the bottom/third line, "ground contact surface constructions." When any F-Factor construction is placed in the top line and the internal source construction in the bottom floor line, the model runs, but the results are the same as if the F-Factor construction wasn't present at all. In this case the spaces tab/surfaces sub tab are the same as when the F-Factor construction simply isn't included. When an F-Factor construction is placed in the bottom line, ground contact, the model warms up, but then fails. In this condition, in the spaces tab/surfaces sub-tab, the F-Factor construction has replaced the internal source construction for all floor surfaces. Everything is green, thus defaults. The outside boundary is ground, as it always is when just using the internal source construction. At this failure, I see: * Severe * ZoneHVAC:LowTemperatureRadiant:SurfaceGroup = ZONE HVAC LOW TEMPERATURE RADIANT VARIABLE FLOW 13FLOORS does not have any surfaces listed. (13 is one of the zones of the model). I have files from both the successful run (internal source construction without F-Factor) and the failure, in case they might be helpful. I have read all the related questions, but still do not understand this.

Regarding the F-Factor construction itself, am I correct that what it calls "area" is the whole area of the slab . . . and is "perimeter exposed" the simple perimeter of the whole slab? I assume the F-Factor is the value of the insulation used.

Thanks for any light you can shed on what I do not understand here.