Depending on the climate, a radiant barrier (the foil) counts as about R-19 by itself. FOr it to work, it needs at least 1/2" airspace on either side (it can't for example be stapled directly on a panel - if is it, then it will conduct and be of little use). Lots of air flow could compromise some of it's effectiveness.
If there aren't any vents to allow air flow under the deck, then the radiant barrier might be plenty. There are various independent studies you can read if you search about the effectivness of radiant barriers, and under what conditions they work. I put one in in my attic, and the attic temp in the summer dropped about 25-degrees on a hot summer day. Even with R40 on the ceiling, prior to this, the ceiling would be hot because everything was heat soaked. After, the ceiling was the same temp as the interior walls.