EcoRenovator - Homemade Retangular HVAC Ducts

Howdy guys I got a quick way to save some big bucks on redoing the old or putting in new duct work. I needed to replace an old undersized/under-insulated 8" round metal duct in my attic. It was trying to run 4 registers. So I priced some metal rectangular truck to come off my main plenum, 24 feet long 10" x 12" was a whooping $675! That's not even installed. So I started looking into what would be needed to do the job right. Here is what I came up with. $150 total cost.

Parts list (all from Home Depot)

Owens Corning FOAMULAR 3/4 in. x 4 ft. x 8 ft. R-4 Tongue and Groove Insulating Sheathing-37L - The Home Depot

GAF 0.5 in. x 4 ft. x 8 ft. EnergyGuard PolyIsocyanurate Insulation Sheathing-1S01 - The Home Depot

Master Flow Water Based Mastic Half Gallon Tub-WBA50 - The Home Depot

3M 17.6 oz. High-Strength 90 Spray Adhesive-90-24 - The Home Depot

So I took the foam boards to the table saw and cut my pieces out.
http://ecorenovator.org/forum/attach...1&d=1434503182

I cut the PolyIsocyanurate (with vapor barrier) smaller to be 3M spary glued to the FOAMULAR .
http://ecorenovator.org/forum/attach...1&d=1434503182

http://ecorenovator.org/forum/attach...1&d=1434503182

http://ecorenovator.org/forum/attach...1&d=1434503182

This is some crazy awesome stuff! Its water based too so it washes up really easy.

http://ecorenovator.org/forum/attach...1&d=1434503182

I the coated EVERY seem VERY well with the mastic and glued them together. I then came back and coated the outside of each seem as well.

http://ecorenovator.org/forum/attach...1&d=1434503182

All done. I just have to make up 2 more and replace the old with the new. This should give me a R valve of over 7. Finished inside size of 13" x 9".

http://ecorenovator.org/forum/attach...1&d=1434503182