# One dimensional U-Factor Calculation - How to determine indoor / outdoor film resistances

Can someone clarify the ASHRAE Fundamental 2017 (SI Units) (See Below) Indoor and Outdoor film resistances. The manual states that it comes from Table 3 and in the description of table 3 it states "Table 3 provides effective resistance values for plane (i.e., generally flat) air spaces that are enclosed within an assembly." So the values listed below should be found in the table.

1. How do you determine the direction of heat flow (I'd assume for a wall it's horizontal)
2. What mean temperature do I use
3. What air space to use?

Give the values 0.030 and 0.120 I figured I could reverse out the logic. But 0.120 is not even listed in Table 3?

• I don't have it handy, but my recollection is that indoor air film is a function of which way heat is flowing (to the outside in heating season, inside in cooling season), while outdoor air film is a function of wind speed. Jun 25, 2020 at 21:23
• The mean temperature is commonly the average of the temperatures on either side. Jun 26, 2020 at 8:02