Was the space occupied for your measuring time? i.e. were occupants opening and closing windows? I assume no, but it is a fundimental for how you model "natural ventilation" (which I try to keep mentally seperate from "natural infiltration").
For a passive house the effects of infiltration, especially in summer, are probably not the issue here. Infiltration is also less (usually) when mechanical fans are on. I aggree thermal break modelling of either walls or slab as well as the thermal mass effects thereof is probably not the root of the shift.
Brent's suggestion of the air node capacity was worth a try as it is thermally a more direct coupling, but as you point out, the main problem here is shift, not amplitude of spike. More accuracy for the slab modelling of a naturally ventilated building has been known to have a large effect in warmer climes and in your case I would do this even if it is also probably not the root of the shift.
Solar distribution model is a question. The basic model ("FullExterior") throws all the solar on the floor surface and redistributes the reflected amount area weighted to the rest of the surfaces thereafter. If you are using say an adaptive convection algorithm (several to choose from here), you may find surfaces like walls may not be recieving the energy of the solar in, in the same proportions as they would in reality. The vertical surfaces however normally have a much higher convection coefficient attributed to them (either by the algorithm or user defined) and could possibly play a larger role in speeding up heat transfer from surface to air node. If by accident more energy than in reality is on the floor and ceiling surface with smaller convections coeff.s, it may explain a retarded heat transfer to the air node.
So I find the combination of solar radiation distribution model and convection coefficient assignment modelling, may play a role here.
Facinating thread, also what Ned said about how SLAB reacted to his measured data. PS keep in mind you may have to itterate a couple of times with SLAB, but don't overkill it, as it is probably not the root issue.
Keep us posted,
Jean
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