You know the answer is it depends - what are you trying to accomplish, why, how much are you willing to pay for it... Is it shady, do you have a tile roof... http://thehtrc.com/2012/building-science-radiant-barriers
With that once all air sealing, insulation work etc... is done properly, probably not though it can help with certain issues & might still be worth it. Now if you are redoing your whole roof I see no issues with adding the radiant sheathing though I would probably opt for a cool roof treatment thus negating that
I know we have a few members here that like spray on (i.e. they sell it) & that can honestly help overcome some longtime issues with others and generally is easier to install until you hit baffles & insulation which you would need to redo
In new construction, I used to specify TechShield roof sheathing (OSB with RB already installed), which cost about 10 cents extra per sq.ft. of roof (may no longer be accurate). Then I read a large study of Energy Star homes in Houston included homes with ducts in the attic, sealed to Energy Star standards. As expected, those with radiant barriers cost less to cool than those without RB's, after adjusting for other factors. However, the difference was surprisingly small, on the order of 3% of cooling energy IIRC. In addition to verified duct leakage, one would imagine that these homes had better-than-code air sealing and ceiling insulation, all of which reduces potential savings from the RB. Bottom line: even at only 10 cents extra, radiant barriers were only justified in the hottest climates, such as Houston, Miami, etc.
There's obviously no way to retrofit a radiant barrier for that cost (unless the roof sheathing is being replaced for other reasons). Some argue the Houston study isn't a fair comparison for older homes with leaky and poorly insulated ceilings and attic ducts. However, I would argue that fixing those problems offers more bang for the buck. I've seen retrofit RB quotes for upwards of a dollar or sq.ft !! At that price, I can't imagine it would ever come close to payback.
The HP Forum Archives (see link in main menu bar) has a number of discussions on radiant barriers, including links to the above referenced Houston study, as well as a study by FSEC on retrofit radiant barriers in Florida.
I had in mind the RB film that attaches to underside of the rafters.
duct work is located in the crawl space, so other than low head room in attic 4/12 hip, no other obstruction.
A couple of issues with that - working around the trusses is a pain for that especially to get everything right & it gets to be real tight real quick. The biggest though is dust settling on the sheets over time which has been shown to reduce the effectiveness of the barrier. Plus I second David's thoughts - no ducts no need. Do what I suggested first, fix the air sealing & insulation (which includes baffles) and you have no need
If no ducts in attic, I can't imagine any rationale for installing RB. If the objective is to reduce ceiling (cooling) loads, fix the ceilings.