| U | = | 1 | ||||
| ro+ R1+ R2 . . . ri | ||||||
| Where: | ||||||
| U | = | the thermal transmittance of the assembly. | ||||
| ro | = | outside air film resistance. | ||||
| ro | = | .17 for all exterior surfaces. | ||||
| ri | = | inside air film resistance. | ||||
| ri | = | 0.61 for interior horizontal surfaces, heat flow up. | ||||
| ri | = | 0.92 for interior horizontal surfaces, heat flow down. | ||||
| ri | = | 0.68 for interior vertical surfaces. | ||||
| R | = | 1 C |
= | X K |
= | measure of the resistance to the passage of heat for each element. |
| C | = | conductance, the heat flow through a specific material of specific thickness. | ||||
| K | = | insulation value of a material per inch. | ||||
| X | = | the thickness of the material in inches. | ||||