In order for radiant barrier to have
any benefit requires an air gap. Foil sandwiched between two other materials has no ability to reflect infra-red heat. With an inch of air between the foil facer and a hotter surface on the top side it'll deliver the equivalent of ~R6 when there's a 30F temperature difference (somewhat less at lower delta-Ts, somewhat higher at higher delta-Ts.) If it's a foil suspended horizontally with an inch or more of air gap on both sides (RB between the rafters in a vented attic, for instance) the R-equivalent @ 30F delta, hot side up is closer to R10. With cold side up (winter conditions) it's less than half that. The effect increases somewhat with bigger air gaps, but shrinks rapidly once the gap is 3/4" or lower, and most vendors (smartly, from a marketing point of view) never provide ASTM test data for gaps less than 3/4" where the performance is abominably low.
For a typical sample of how various types of single & multi-layer RB products fare at different gaps & orientations see
this or
this.
As long as there's at least on air gap facing a low-emissivity surface there's some benefit, but it's hard to measure.
More ventilation won't dramatically change the attic temp if the insulation is at the attic floor unless you power-vent it, but if you power-vent it it will usually draw more conditioned space air into the attic, sucking more outdoor air into the conditioned space and raise the AC costs (it's usually better to just naturally draft it, even if the attic stays hot). Along the gulf coast a vented attic is far more prone to moisture issues (condensation & mold along the joist tops) than an air tight sealed attic. Sealing the attic brings the dew point of the attic air down to that of the conditioned space. Even if it's a few degrees hotter up there as a result, the lower infiltration drive can lower the overall AC load by lowering the latent load in the house. If you have ducts up there, it's important that they're well sealed if it's a vented attic (less so, in a sealed attic), not just insulated, since the air handler itself will generate infiltration drives if the ducts are leaking.
Most of the time in a '60s vintage house adding blown rock wool or cellulose insulation is more effective (and more cost-effective) than adding RB, even with ducts in the attic. If yours is a typical R19 batts between the joists situation, doubling that or more (up to R50) with blown higher density fiber that burys the joist tops by 3" or more is usually a good mid to long-term investment compared to what's in your 401K and a better investment than any radaint-barrier product. (None of it has payback if you're planning to flip the place in short years though.) The best time to consider RB options is if you live in a place thats hotter than hell (south TX, check
), and you have ducts & air handlers in the attic, and you don't have sufficient space to bulk up the floor insulation to over R40. Code min in TX is R30, and cost effective always in a 25 year present-value analysis, but from a comfort point of view 1.5x code min is usually worth it, and usually worth it financially as well if you use goods that are opaque to infra-red (blown cellulose or rock wool) and can cover the joist tops/truss chords (for lower thermal bridging at the framing.)
To compare the economics of RB vs. bringing it up to code min (if it isn't already) with or without ducts in the attic,
see the chart on page 5 and the tables on p.6. In the Austin simulation if you started out with uninsulated ducts above the insulation and R19 batts between the joists the economics of adding both insulation + RB are quite compelling, but if the ducts are insulated & sealed (or not in the attic) RB is worth maybe $15-40 year if you're at R30, $110/yr if you're only at R19.