Reflectix Article Data Crunch
Here's the Rmd for the November 2018 Brew Your Own article that I wrote about using Reflectix for kettle insulation when brewing-in-a-bag.
The article appears on the BYO website (paywalled):
https://byo.com/article/insulation-for-single-vessel-brewing/
Tl;dr: Reflectix makes a difference, though not as much as I thought; also two layers is better than one, but not by much.