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Qb = U x HTDh x Area

Where:

 

 Qh        = Heat flow out of surface, Btuh
 U        = Below grade U-Value (heat transfer coeff.) from Table 4A Manual J 8, Btuh/ft²·°F
 HTDh        = Winter temperature difference (inside temperature - outside temperature), °F
 Area        = Net area of surface, square feet (ft²)

 

Ref: Manual J 8 - Page 3-2

 

Note:        Below grade HTM values, defined by the equation HTMh = U x HTDh, are determined from the winter temperature difference, material type, and insulation.