Usually R-21 means holding heat in well even using a minimal heat source during very cold temps. Reflectix is very thin how does it accomplish this

Reflectix is a reflective type of insulation that depends on how it is installed for total R-value. In that way it is different than mass insulation such as fiberglass. Also to get R-21 you need two layers installed in a 2x8 or 2x10 framing member. The product works by reflecting radiant energy and reducing the chance for convective heat gain or loss. The R-21 is a tested R-value from a third party independent laboratory we do not test our product internally.