Foil vs. Spray-On Radiant Barrier: The Material-Class Math
Foil sheet and spray-on coating are sold under one name -- attic radiant barrier -- but they are different material classes with different physics. Foil is a low-emissivity surface, typically rated near 0.03 to 0.05, which means it re-radiates almost none of the heat that reaches it. Spray-on coatings are typically rated near 0.20 to 0.25 emissivity -- still reflective for a paint-like film, but several times more emissive than foil. That rating gap explains most of the performance difference, and the install lanes explain the rest. This guide works the emissivity and dust math, lifespan, and where each class wins.
Orange County, CA · Updated 2026 · Costs shown as ranges — your quote will vary by installer and scope
Emissivity: the rating that separates the classes
| Material class | Typical emissivity rating | Material cost per sq ft |
|---|---|---|
| Foil sheet, one-sided | about 0.03-0.05 | $0.10-$0.50 |
| Foil sheet, two-sided | about 0.03-0.05 | $0.50-$0.90 |
| Spray-on coating | about 0.20-0.25 | $0.65-$1.00 |
Emissivity is the share of absorbed heat a surface re-radiates, scored from 0 to 1 -- lower is better for an attic radiant barrier. Foil's very low rating is why published comparisons treat sheet as the most effective class. Spray-on coatings trade some of that performance for coverage: the film goes wherever a spray gun reaches. Ratings vary by product, so check the data sheet -- the figure to ask for is the tested emissivity. A coating quoted without a rating is a paint quote, not a barrier quote.
Dust behavior: where each class ages well or badly
Dust is the quiet variable in attic radiant barrier performance. A reflective face only works while it stays bright; dust settling on an upward-facing surface dulls it and raises its effective emissivity over time. That is why placement matters as much as material. Foil stapled under the rafters faces downward, sheds dust, and keeps its rating for years. Foil laid flat over attic-floor insulation faces up, collects dust steadily, and is the layout most likely to disappoint. Spray-on coating sits on the underside of the deck, also facing down -- its aging limit is film adhesion and coverage thickness rather than dust.
Install lanes: labor, access, and what can go wrong
Both lanes carry similar published labor -- roughly $0.60 to $1.00 per square foot -- but the work differs. Foil arrives in rolls, gets cut to bay width, and is stapled under the rafters bay by bay: simple, inspectable work that stalls in low-pitch attics where crews cannot reach the eaves. Spray-on goes on with an airless sprayer in a continuous pass, reaches the tight eaves and broken-up deck planes foil cannot, and depends entirely on even coverage -- thin spots are invisible once the film dries. Foil failures are usually layout failures: gaps at ridges, vents smothered, seams left open. Spray failures are usually thickness failures. Either way, get the coverage area and product rating in writing first.
Lifespan: what wears out, and when
Neither class is a short-lived product, but they age on different clocks. Foil sheet has no coating to wear: kept dry and dust-sheltered under the rafters, it commonly serves for decades -- often described as lasting the life of the roof deck -- and damaged bays can be patched one at a time. Spray-on film life depends on deck condition at application and film build; a recoat is a full re-spray rather than a spot repair, and peeling over a dirty or damp deck shortens it sharply. For a home you plan to hold, foil's repairability is worth something real.
When each wins
- Open truss attic, reachable eaves: foil -- lowest emissivity, lowest material cost, repairable bay by bay
- Low-pitch or chopped-up deck: spray-on -- the only lane that reaches every plane evenly
- Two-sided foil where the assembly exposes both faces: the premium buys reflection in both directions
- Dusty inland attic: either class, mounted facing down -- placement beats material in the dust math
- Tightest budget on a simple deck: one-sided foil -- material from $0.10 per sq ft keeps the total near the bottom of the installed band
Frequently asked questions
Which works better, foil or spray-on radiant barrier?
Foil, on the rating that matters: foil sheet typically rates near 0.03-0.05 emissivity against roughly 0.20-0.25 for spray-on coatings, and published comparisons treat sheet as the most effective attic radiant barrier class. Spray-on wins on reach, not performance.
Does dust really hurt an attic radiant barrier?
Yes, on upward-facing surfaces: settled dust dulls the reflective face and raises effective emissivity over time. Foil stapled under the rafters and spray-on film on the deck underside both face downward and stay far cleaner.
Why is spray-on more expensive per square foot?
Material: spray-on coating runs $0.65-$1.00 per sq ft against $0.10-$0.50 for one-sided foil, while labor is similar at $0.60-$1.00 per sq ft for both lanes. The premium buys coverage of tight, hard-to-reach decks.
How long does each type of attic radiant barrier last?
Foil kept dry under the rafters commonly serves for decades and can be patched bay by bay. Spray-on life depends on deck condition and film thickness at application; when it does wear out, renewal is a full re-spray rather than a spot repair.
What should I ask for before signing a spray-on quote?
The product's tested emissivity rating, the target film thickness, and the measured deck area, all in writing. A coating quoted without an emissivity rating cannot be compared against foil on performance at all.