Insulating a/c ductwork on the roof

When my packaged a/c was installed on the roof they used ductwork that has a thin layer on insulation inside. But then because of leaks they tarred the outside. Of course the black absorbs the heat and warms the air considerably.
How best to insulate? I see the various foam insulation boards. A neighbor suggests going all out with 2" thick. I see you cut with a circular saw. I see you tape the joints with 2-7/8" Weathermate Construction Tape. But the Styrofoam sheathing can only take the UV for 90 days. I have to cover it with something, either white or reflective. What do you all suggest?
I once got an estimate from a local roofer. He wanted $3500 for like 1/2" thick insulation. No way should it cost that much. But I don't how he was going to do the job.
Don. www.donwiss.com (e-mail link at home page bottom).
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I called Dow Chemical. Not unsurprisingly I was asking about an unusual application. He emphasized that anything I put on it can't be solvent based. It must be water based, like latex paint.
Looking at Home Depot I see they have a water based mastic that creates air-tight seals on duct seams and joints.
He says sometimes it sticks out the bottom of walls and people put stucco on it. But again, no solvents!
I supposed I could simply paint with white exterior latex paint.
Don. www.donwiss.com (e-mail link at home page bottom).
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On 8/15/2016 4:38 PM, Don Wiss wrote:

I've been in the foam business for 46 years. I'd use latex paint. I know of applications that are many years old and still in good condition. Any color you desire will work.
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Do you use a circular saw or sabre/jig saw? I see there are special blades for cutting foam: https://www.amazon.com/dp/B00KM0RLUU
I also see a mention of foil faced foam. Then I would use foil faced tape.
I would think Liquid Nails would attach the foam to the ductowrk.
Don. www.donwiss.com (e-mail link at home page bottom).
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On 8/15/2016 8:47 PM, Don Wiss wrote:

Hand saw makes for easy cutting with minimal mess. Electric knife works well too. Not familiar with the blade you show but it looks like it would work.
Pretty much any construction adhesive works well.
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There will be a lot of cutting to cut all the pieces to fit around the ductwork. No way would I use a hand saw.

The electric knife is an interesting suggestion that I had not thought of. Thanks. (I've never owned or used an electric knife.)
If I do go all out and buy 2" insulation, will the electric knife still work? It would be easier to handle than a much heavier sabre saw.
I see it would be hard to spend over $20 on a corded consumer electric knife. And then afterwards I'd have a tool that I might use again.
I see I have a choice of a bread blade or carving blade. I would have no use for a bread blade afterwards. I would think I would want a serrated blade for the foam.
And I find a Heavy-Duty Electric Fillet Knife.
Don. www.donwiss.com (e-mail link at home page bottom).
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Are you using flexible / fiber silver/foil backed insulation? If so. a razor knife and straight edge will do. I'd use foil duct tape and wrap in radiant barrier material(foil)
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What type of foams? Packaging, furniture, industrial? You don't sell any pincore latex, do you?
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wrote:

EPS, expandable polystyrene. Packaging, fish boxes, ICFs. No latex.
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