Reddit reviews Design Polymerics 1030 Water Base Duct Mastic Sealant Pail 0.50-Gallon
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Low to high velocity air duct sealant, Color- GrayUsed for permanently sealing sheet metal and flexible duct system jointsUL-181 listed rigid fiberglass and flexible air ductsWater based non flammable fire & weather resistantConsult your local building codes for approved installation practices
Those ducts look like they were masticed which is perfect. This was the product I was talking about: https://www.lowes.com/pd/Frost-King-0-125-in-Foam-Plumbing-Pipe-Wrap-Insulation/1081449
One side is sticky, but being out and exposed to the elements might take its toll.
My suggestion is this, I warn it will take some work but it will be very worth it, and not that expensive while yeilding the best r-value bang for buck:
Get some foam board adhesive: https://www.homedepot.com/p/Loctite-PL-300-10-fl-oz-Foamboard-Adhesive-1421941/202020476
And get some sheets of foam board: https://www.lowes.com/pd/Kingspan-Insulation-Common-2-in-x-4-ft-x-8-ft-Actual-2-in-x-4-ft-x-8-Feet-R-10-Unfaced-Polystyrene-Foam-Board-Insulation/999972968
Your outside layer of foam board should be this: https://www.menards.com/main/building-materials/insulation/foam-board-insulation/johns-manville-foil-faced-polyiso-foam-insulation-4-x-8/w-n5075/p-1444438921381.htm
To start you should verify 100% that those duct joints are masticed good, and I mean good, all the joints should be full of it and no holes, no pinholes even. if not get this: https://www.amazon.com/Design-Polymerics-Mastic-Sealant-0-50-Gallon/dp/B0085UZBE2
Once they are 100% sealed you can start. IF you want layers of foam board (more r-value the better) the best thing you can do is use an unfaced foam board for your layers. Your outer layer should be the foil faced foam board. To attach your foam board to your ducts use the duct adhesive, you MUST use this specifically, other adhesives can degrade and "eat away" the foam and eventually the adhesive will fail and it will fall off. Obviously you will need to cut the foam board to spec to fit on the ducting. USe the adhesive to attach it to the ductwork, foil face out. You will then want to cover the seams with silicon or foil faced duct work tape, not duck tape. IF you want additional security for them to hold and not fall off or fly off with wind you can use a metal band strap. or self tapping screws with large zinc washers
metal duct tape: https://www.amazon.com/3M-Foil-Tape-3381-Silver/dp/B00A7I5L86/ref=sr_1_6?keywords=speed+tape&qid=1551224134&s=gateway&sr=8-6
metal strapping: https://www.amazon.com/DIVERSITECH-710-001-Galvanized-Metallic-Strap/dp/B00CD6WON0/ref=sr_1_8?keywords=metal+strapping&qid=1551224293&s=gateway&sr=8-8
metal strapping tension locks: https://www.amazon.com/Tension-Triglides-Webbing-Strapping-Adjustor/dp/B01JG09244/ref=sr_1_17?keywords=metal+strapping&qid=1551224293&s=gateway&sr=8-17
self tapping screws (need to be long enough to go through foam AND the metal duct work): https://www.amazon.com/Self-Drilling-Plated-Finish-Undercut-Phillips/dp/B00GYK2S7I/ref=sr_1_3?keywords=3+inch+self+tapping+screws&qid=1551224214&s=gateway&sr=8-3
Now since its outside the air inside will eventually still change to the outside temperature, but not as fast and less bleed off of temperature in the ductwork due to outside ambient temperature making the ductwork sheathing the same temperature.