These are not competing products, they are different tools. Blown-in insulation slows conducted heat, all year, in both directions. A radiant barrier reflects radiant heat, mostly in summer. If your attic insulation is thin, insulation wins on almost any honest accounting, and it is not close.
What each one actually does
| Blown-in insulation | Radiant barrier | |
|---|---|---|
| Heat transfer targeted | Conduction | Radiation |
| Helps in summer | Yes | Yes |
| Helps in winter | Yes | Very little |
| Measured by | R-value | Emittance |
| Degrades over time | Only if wet or compressed | Yes, if dust settles on it |
| Typical DIY material cost | Higher | Lower |
The order of operations
Insulation first, always, if you are below code depth. A radiant barrier over 4 inches of settled fibreglass is a hat on a man with no coat. Get to your zone's recommended attic R-value first, then consider foil.
The Department of Energy's recommended attic levels run from about R-30 in the hottest zones up to R-60 in the coldest. If your insulation is visibly below the tops of the ceiling joists, you are almost certainly under-insulated.
Then climate decides the second purchase
| Zone | Representative cities | Verdict | Why |
|---|---|---|---|
| 1 | Miami, Key West, Honolulu | Strong | Cooling runs nearly year-round and attic temperatures are extreme. This is where a radiant barrier does its best work. |
| 2 | Houston, Phoenix, Orlando, New Orleans | Strong | Long, intense cooling season. Among the best paybacks in the country, particularly with ductwork in the attic. |
| 3 | Atlanta, Dallas, Los Angeles, Las Vegas | Good | Cooling still dominates the bill. Worth doing, especially on a dark roof with poor attic ventilation. |
| 4 | Nashville, Washington DC, Kansas City | Mixed | Real summer benefit, but heating matters too. Do it only after attic insulation is already at code depth. |
| 5 | Chicago, Denver, Boston, Detroit | Weak | Short cooling season. The same money spent on blown-in insulation will almost always return more. |
| 6 | Minneapolis, Burlington, Helena | Very weak | Heating dominates. A radiant barrier does close to nothing for a winter heating bill. |
| 7 | Duluth, Fargo, Bismarck | Not worth it | Spend the budget on R-value and air sealing instead. |
| 8 | Fairbanks and interior Alaska | Not worth it | Radiant gain is not your problem for most of the year. |
Zones follow the IECC climate zone map used by the US building codes. The verdicts are directional, not a modelled saving for your house: the US Department of Energy reports radiant barriers can reduce cooling costs by roughly 5–10% in hot, sunny climates, and states plainly that they are far less cost-effective in cool ones. Anyone quoting you a precise dollar figure per zone without modelling your roof, your insulation and your duct location is guessing.
Reading that table against your own zone gives the answer directly. In zones 1 to 3, once insulation is adequate, a radiant barrier is a sound next step, particularly if your air handler or ducts are in the attic where they are absorbing radiant heat all afternoon. In zones 5 and up, put the next dollar into more insulation or air sealing.
The case where the answer flips
If your ductwork runs through the attic and your attic insulation is already adequate, a radiant barrier moves up the list. Attic ducts sit in the hottest air in the house and radiantly gain heat from the deck directly. Reducing the deck's radiant output is one of the few things you can do about that short of moving the ducts.
Doing both
They coexist fine. Insulation on the attic floor, radiant barrier stapled to the rafters, and the two do not interfere. Just do not lay solid foil on top of insulation, for the moisture reasons in perforated vs. non-perforated.
Compare both for your attic → Free, no account for your first estimate.
Full background in the complete radiant barrier guide. Related: insulation removal costs if you are clearing old material first.