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You have two options:
- Model it as a crawlspace.
The crawlspace (or any floor type) is assumed to be exposed to a constant, monthly average outdoor temperature (Heat loss = Floor area / Floor R-value x temp diff).
- Model it as a North Wall.
The North Wall, like all other walls, is assumed to be exposed to an hourly, orientation-dependent, sol-air temperature profile.
To add additional convection losses due to wind:
Additional convection from wind must be taken into account through lower composite R-value of this "floor" or "north wall." Of course, all that wind convection can really do is to increase the outdoor film coefficient, so a safe thing to do might be to ignore it altogether and use as an overall R-value of the "floor" or "wall" the sum of indoor air resistance and the composite construction R-value.
Another effect of wind is additional air infiltration, at least in a low pressure system. The typical leakage area of a crawlspace would likely be a low estimate of the true leakage area of a suspended floor like that. So doubling that floor's leakage area might be a reasonable approach. Of course this will have any detectable effect only if (1) A/I is included in loads (typical for low pressure systems) (in the SYSTEM Outside Air Data Form) and (2) if the Design A/I in BUILDING is *NOT* used for monthly A/I calcs (in the BUILDING Infiltration Data Form).
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