DOE/Steven Winter Assoc has recently completed a study and built standards for spray foam insulating duct in attics. I attended a webinar where the author, Bill Zoeller presented their findings. It was fantastic:
Early adopters will need to be significantly involved in the process to insure quality, probably having to lead the contractors until these strategies and best practices for implementation trickle to the field.
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You can encapsulate unburried duct. It doesn't meet Challenge Home requirements so you wouldn't design this way, but apparently is a significant improvement for retrofit:
Here's my spray foam picture album for those interested.
Insulating at the ceiling beats insulating at the roof deck, not just because the roof has a greater surface area, but also because of how hot the roof deck gets, as opposed to how hot it is at the attic floor. Spray foaming the ducts and covering them with insulation is the way to go.
Really good designers (and people I admire) seem to avoid absolutes, and the rigidity of thought that absolute thinking creates. I'm trying to learn to be a really good designer, and even eventually be a person admired by others.
It requires "it depends" thinking instead of "this is the way to go" thinking. It requires breaking firmly rooted bad habits.
There are lots of circumstances where spraying the roof deck IS the way to go, it really depends...
Breaking rigid thinking in myself (and others) is really tough.