This effectively acts like a mirror reflecting heat back to where it came from.
Foil backed bubble wrap insulation.
Foil faced bubble wrap is composed of either one single bubble or two double bubble layers of mylar air bubbles sandwiched between two metalized surfaces.
Prodex total is composed of 13 64 inch approx.
I have heard some say that the insulation could be installed loosely to maintain the gap.
3 16 or 5mm closed cell polyethylene foam covered on both sides with 0012 inch 0 03mm aluminum facing.
Some insulation materials don t work 100 on trapped air.
Double bubble reflective foil insulation 24 inch x 10 ft roll industrial strength commercial grade no tear radiant barrier wrap for weatherproofing attics windows garages rv s ducts more.
These combine a bit of trapped air technology along with using the foil material to reflect radiant heat.
Worth mentioning to get the equivalent r value to what the manufactures claim on the packaging the installer has to use spacers under the insulation to keep the prescribed air gap between the ducts and the foil.