I looked into glass doors and 99% have air gaps, Zero sealing, only
maybe 1000 $ units looked good to stop air with only R 1
I took 3/4" L angle iron steel from Ace l and screwed them all around
inside my fireplace with Tapcons Then 4" foamboard and stuck 1/2"
stripmagnets on the foamboard , I painted the foamboard the color of the
fireplace. It pops in or out and has 100% airtight seal with R 20 Cost
apx 20 $ and all that cold is kept out.