Before Will Rietveld created the above chart, my testing showed that low down fill jackets with lots of sewn-through seams increase their insulation value SIGNIFICANTLY by wearing a windshirt over them. This phenomenon becomes less important as the number of sewn through quilt lines goes down or the fill amount goes up.
In advance of testing backpacking jackets for a three part article he would later write, Will exchanged numerous emails with me to discuss testing procedures. One of the tests he agreed to was a windshirt over down insulation augmentation test. The windshirt tests did not involve wind. The windshirt benefit would be even greater in that environment.
Note that the outside IR temperature for a perfect insulator would be ambient temp. Will tested near 60F ambient.
Note that outside IR temperature for non-existent insulator would be the temperature of the heating pad (Will didn't provide it but it is typically in the 91F - 95F range).
Note that Will is only measuring relative warmth because of Newton's Law of Cooling which is logarithmic instead of linear. As the outside jacket surface temperature goes higher the insulation heat loss always increases faster than the temperature increase. If a heated object is allowed to cool, the rate of cooling at any instant is proportional to the difference between the object's temperature and the ambient (room) temperature. In other words, an object cools faster at first, while it is hot, and the rate of cooling slows down as the temperature of the object approaches the ambient temperature. The function that relates the object's temperature to time is a negative exponential function.
So, what was achieved by putting a windshirt over the hooded Flash was a DRAMATIC system insulation improvement even without the added benefit of blocking wind.