The
R-value is a measure of
thermal resistance used in the building and
construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the
heat flux (
heat transfer per unit area per unit time,
![](http://info.babylon.com/onlinebox.cgi?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=424)
) through it or
![](http://info.babylon.com/onlinebox.cgi?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=1285)
. Thermal resistance varies with temperature but it is common practice in construction to treat it as a constant value.