What is reflective insulation and how is it different from traditional insulation?
What reflective insulation is, how it stops the radiant heat fiberglass and cellulose let through, the three product types, where a foil layer pays off most, and how it works alongside your existing insulation for year-round savings.
Reflective insulation is any building product that uses a low-emittance foil surface, facing an air gap, to reflect radiant heat instead of absorbing it. Emittance is how readily a surface gives off heat as infrared energy. At an emittance of 0.10 or lower (the bar set by ASTM C1313 and C1224), a foil reflects most of the radiant energy that strikes it. A quality foil reaches about 0.05 emittance, which is roughly 95% reflectivity measured per ASTM C1371.
This sits apart from bulk insulation. Fiberglass, cellulose, and spray foam slow conduction by trapping still air in a high-resistance material. Reflective insulation reflects radiant heat across an air gap. The two address different parts of the heat-transfer problem, and they work best together.
What reflective insulation is
Reflective insulation covers a range of products, all sharing one trait: a low-emittance surface (usually aluminum foil) that faces an adjacent air space and reflects long-wave radiant heat. Long-wave radiant heat is the infrared energy a warm surface gives off. The foil reflects most of it back toward the source instead of soaking it up and re-radiating it onward.
The qualifying threshold is consistent across the standards. The U.S. Department of Energy and ASTM both require emittance of 0.10 or lower and reflectance of 0.90 or higher for a material to count as a radiant barrier. A DOE-qualifying low-emittance surface reflects most of the radiant energy that hits it.
The air space is part of the mechanism. The foil only reflects when it faces an open gap. Press it flat against a solid and it conducts heat through the contact, the same way any thin metal sheet would. That single requirement separates a working reflective layer from a decorative one.
How reflective insulation works: radiant heat, emittance, and the air gap
Heat moves three ways. Conduction is heat traveling through a solid by direct contact. Convection is heat carried by moving air. Radiation is infrared energy crossing a gap from a warmer surface to a cooler one, with no contact and no air movement needed.
Radiation is the path reflective insulation targets. Radiant heat travels through air without warming the air itself, which is why a roof deck at 130 to 150 F can bake an attic even when the air sits still. A low-emittance foil re-emits almost none of the infrared energy that strikes it. At 5% emittance, about 95% of that energy reflects back toward the source, tested per ASTM C1371.
The air gap is mandatory. DOE specifies at least a 1-inch air space for foil stapled to rafters. When pressed flat between two solids, the foil conducts heat rather than reflecting it. Dust settling on the surface also raises emittance over time, which is one reason a perforated, airflow-friendly foil holds up better in a real attic.
Bulk insulation works on the other two modes. Fiberglass batts like R-13 batts and blown cellulose resist conduction and convection by trapping still air in a high-resistance matrix. They do little to stop radiant heat crossing an air space, which is the gap a foil layer closes.
The payoff is measurable. Oak Ridge National Laboratory recorded up to a 50% reduction in summer daytime attic-floor heat flow with foil stapled to rafters in large-scale climate-simulator testing, and DOE cites 5% to 10% whole-house cooling savings in warm, sunny climates.
A foil at 5% emittance reflects about 95% of the radiant heat that strikes it, tested per ASTM C1371. The two values are complements for an opaque foil.
DOE specifies at least a 1-inch (2.5 cm) air space facing the foil for rafter installs. Without a gap the foil conducts heat instead of reflecting it.
Peak summer daytime heat-flow reduction at the attic floor measured in a large-scale climate-simulator test with foil stapled to rafter undersides.
The three types of reflective insulation
The category breaks into three product types. They differ by what sits behind the foil and by where each one fits. The Department of Energy groups reflective products by their backing material, which can be kraft paper, plastic film, polyethylene bubbles, or cardboard.
| Type | Core material | Air gap needed | Typical assembly R-value | Best application |
|---|---|---|---|---|
| Bare foil radiant barrier | None (foil sheet or foil-faced fabric) | Yes, framing supplies it | R-4.1 to R-14.5 attic; R-5.54 to R-10.85 wall | Attic rafter staple-up |
| Foil-faced bubble | Polyethylene bubble layers between foil faces | Built-in, plus a facing gap | R-9.2 to R-22 in metal buildings (Double P2 reaches R-22.5) | Metal buildings, floors, wall retrofits |
| Foil-faced foam board | Rigid foam (e.g. polyiso) with foil facing | Yes, for the radiant benefit | Labeled foam R-value plus radiant gain | Continuous wall, under-slab, crawlspace |
The three reflective insulation types differ by core and air-gap requirement. Bare foil is the attic standard; bubble and foam-foil suit metal buildings, floors, and continuous wall insulation.
The first type is a bare foil radiant barrier, such as a perforated foil radiant barrier. It carries no foam or bubble core, only a reflective sheet facing an open air space. This is the product category covered in depth by the complete radiant barrier guide, so this article keeps the attic overview brief and stays on the category comparison.
The second type is foil-faced bubble. A polyethylene bubble core between two foil faces creates enclosed air spaces. Assembly values rise once a facing air gap is present, and Double P2 Double Bubble Double Foil reaches R-9.7 to R-22.5 in metal-building roof assemblies. It suits metal buildings, floors, and wall retrofits. For how engineered bubble foil differs from plain packing bubble wrap, see our bubble wrap insulation guide.
The third type is foil-faced foam board, such as foil-faced polyiso. The rigid foam core carries a real labeled R-value, and the low-emittance foil facing adds radiant benefit when it faces an air gap. It fits continuous wall insulation, under-slab, and crawlspace work.
This article focuses on the bare foil and foil-faced bubble types. Foil-faced rigid foam belongs to continuous-insulation practice and behaves more like a labeled bulk product.
Reflective insulation vs traditional insulation: different problems, different tools
Bulk insulation and reflective insulation solve different parts of the same problem. Bulk insulation (fiberglass batts, blown cellulose, open-cell spray foam) resists conduction and convection by trapping still air. R-value measures that resistance, and building codes mandate it. A code-level fiberglass batt like the R-13 wall product handles the conductive heat flow that a foil layer leaves untouched.
Reflective insulation closes the radiant gap, measured by emittance. On its own it adds little conductive resistance. So the two are partners. DOE and ORNL consistently recommend using both, because each handles a heat-transfer mode the other leaves open.
Assembly U-value is the number that ties them together. U-value is the combined heat flow through every layer of an assembly: the air films, air spaces, bulk insulation, and reflective layers. Adding a low-emittance foil layer lowers the U-value even when the bulk insulation stays the same, because it removes the radiant share of the heat flow.
The classic measurement comes from an ORNL test-house study in 1986. A radiant barrier added on top of R-19 fiberglass cut peak ceiling heat flux by 39% and the cooling load by 21%, with electrical consumption down 17%. The gain came from layering the foil over existing insulation, which makes the case for an additive approach.
Buyers are protected when they read the numbers. The FTC R-Value Rule requires a seller to substantiate any R-value claim, so a stated assembly figure has to hold up to the test basis behind it. The agency enforces the rule: the Federal Trade Commission obtained a court order requiring one insulation marketer to pay $350,000, the largest civil penalty in a home-insulation case. Credible research supports roughly 5% to 12% cooling savings in hot climates, a meaningful and realistic figure.
Where reflective insulation helps most
Reflective insulation pays back wherever a hot surface radiates across an air gap to a cooler one. A few applications stand out.
- Hot-climate attics (DOE Zones 1 to 3): roof-deck temperatures regularly exceed 130 to 150 F, and bare foil stapled to rafter undersides meets the air-gap requirement on its own. This is the most documented application for payback.
- Metal buildings and post-frame structures: steel skin heats fast and radiates across the interior air space onto people and equipment below. Foil-faced double-bubble products suit this because they create their own enclosed air space.
- Floors over unheated crawlspaces or garages: foil facing the crawlspace air space cuts the radiant exchange between a cold floor and the conditioned room above.
- Anywhere a hot surface radiates across an air gap: garage doors, cargo van roofs, and under-roof decking in pole barns all fit the same pattern. Our garage door insulation cost guide prices out the garage-door case.
- RVs, campers, and vans: a thin reflective layer blocks radiant roof heat in summer and reflects cabin heat back in winter without adding much weight. Our RV insulation guide covers it zone by zone.
- Greenhouses: reflective foil on the opaque north wall bounces light and heat back toward the plants without shading the glazing. See our greenhouse insulation guide.
In hot, sunny climates the cooling case is strongest. Florida Solar Energy Center field research (FSEC-CR-1231-01) documents 8% to 12% annual air-conditioning savings in a typical Central Florida home, where the attic accounts for about 22% of total cooling load. The benefit compounds when cooling ducts run through that hot attic.
For metal buildings, Double P2 Double Bubble Double Foil suits the job, rated R-9.7 to R-22.5 in metal-building roof assemblies once it spans the purlins with an air gap. For floors over crawlspaces, P2 Single Bubble Double Foil is rated R-9.3.
Mixed and cold climates still benefit. The payoff shifts from summer cooling savings toward year-round assembly improvement and condensation control, which the next section covers.
How reflective insulation complements bulk insulation, year-round
Reflective insulation is a layer that improves an assembly’s overall U-value by removing the radiant share of heat flow that bulk insulation cannot reach. DOE, ORNL, and FSEC all frame it as an addition to code-level insulation, working alongside whatever batts or blown fill are already in place.
The thermal numbers make this concrete. In attic and roof assemblies, the system R-value of a perforated foil radiant barrier ranges from R-4.1 to R-14.5. In wall assemblies it ranges from R-5.54 to R-10.85. These figures are calculated using the air-film methods in the RIMA, AIRAH, ASHRAE, and ISO 6946 standards and describe the full assembly, including its air space.
System R-value, both seasons
A reflective layer earns its number as part of an assembly that includes an air space. A perforated foil radiant barrier in this assembly adds R-4.1 to R-14.5 in attic and roof assemblies and R-5.54 to R-10.85 in walls, calculated per RIMA, AIRAH, ASHRAE, and ISO 6946 air-film methods. In summer the foil cuts radiant heat gain from a hot roof deck; in winter it reflects radiant heat from the conditioned space back toward the attic-floor insulation, reducing downward heat loss.
Cold and mixed climates raise a moisture question, and a perforated foil answers it. RB+ is perforated to a 6.29-perm rating per ASTM E96, where perm rating measures how readily water vapor passes through a material. A higher perm rating lets attic moisture escape rather than collect behind the foil. That keeps roof-deck temperatures more stable and limits the frost-buildup-and-melt cycle that causes attic rain, the condition where moisture condenses and freezes on cold decking, then melts and drips.
In DOE Zones 6 to 8, DOE recommends more bulk insulation as the primary attic upgrade. Reflective insulation performs best in walls and crawlspaces there, often as foil-faced rigid foam with an adjacent air gap. In every climate the foil works with the bulk insulation already installed, with no change to framing or floor insulation.
Summer cooling
The foil reflects radiant heat off a hot roof deck before it loads the attic-floor insulation, cutting cooling runtime and peak demand in hot climates.
Winter heat retention
It reflects radiant heat from the conditioned space back toward the attic-floor insulation, slowing downward heat loss across the assembly.
Moisture control
A perforated 6.29-perm foil lets attic humidity escape and stabilizes roof-deck temperatures, reducing condensation and attic-rain risk in cold and mixed climates.
Reflective insulation and R-value: what the numbers mean
A bare radiant-barrier sheet has no standalone R-value. R-value measures resistance to conductive heat flow, and foil is a near-perfect conductor. Its job is to cut the radiant share of heat transfer, which R-value does not capture. Measured as part of an assembly, the system R-value of a perforated foil radiant barrier ranges from R-4.1 to R-14.5 in attic and roof assemblies and R-5.54 to R-10.85 in walls, calculated per RIMA, AIRAH, ASHRAE, and ISO 6946 air-film methods, as the RB+ thermal table shows.
Products with enclosed air spaces carry labeled assembly R-values because the trapped air films supply most of the resistance. As an industry-wide figure, a polyethylene bubble core alone measures only about R-1.0 to R-1.1 of conductive resistance, but the assembly value rises once an air space is present. The number you see depends entirely on what is being measured.
Read the figure as a labeled assembly R-value calculated per RIMA and ASHRAE, and never compare it directly to a fiberglass batt’s standalone rating. The FTC R-Value Rule exists because some marketers have advertised assembly figures as product R-values, which crosses into deceptive territory.
Radiant Barrier RB+: the reflective insulation this guide recommends
Radiant Barrier RB+ is the primary reflective insulation Radiant Barrier USA makes for attic rafter installs. It is a perforated aluminum foil sheet over a heavy-duty woven core, built for DIY staple-up with no special protective equipment. The perforations let moisture escape freely, making the foil safe for cold-climate attics, because humidity passes through the foil rather than collecting behind it.
For metal buildings and floors, the bubble-core options are Double P2 Double Bubble Double Foil (R-9.7 to R-22.5 in metal-building assemblies) and P2 Single Bubble Double Foil. The full attic walkthrough is in the complete radiant barrier guide.
Radiant Barrier RB+
A perforated aluminum foil sheet with a heavy-duty woven core that reflects 95% of radiant heat at 5% emittance, tested per ASTM C1371. Perforations give it a 6.29-perm rating (ASTM E96) so attic moisture escapes instead of getting trapped behind the foil. Staple it to the underside of rafters and the system R-value ranges from R-4.1 to R-14.5 in attic assemblies and R-5.54 to R-10.85 in walls, with no change to framing or existing floor insulation required.
- 95% reflectivity at 5% emittance (ASTM C1371): blocks the radiant heat that batts and foam alone cannot stop
- 6.29-perm perforated woven construction (ASTM E96): moisture escapes freely so the attic stays dry year-round
- System R-4.1 to R-14.5 in attic/roof assemblies and R-5.54 to R-10.85 in walls, with no extra framing depth needed
- Class A / Class 1 fire rating, fiber-free and non-carcinogenic: DIY staple-up with no special PPE required
Not sure which product fits your space? Contact our team and we’ll size it for your building.
Frequently asked questions
What is the difference between reflective insulation and a radiant barrier?
Both are low-emittance foil products, but they differ in construction and the standard that governs them. A radiant barrier is the simplest form: a bare foil sheet or foil-faced fabric with no core, installed so a free air space exists on at least one side, governed by ASTM C1313. Reflective insulation is the broader category that also includes products with a bubble or foam core that create enclosed air spaces inside the product itself, governed by ASTM C1224. The practical rule: bare foil on attic rafters is a radiant barrier, while a foil-bubble roll for a metal building wall is a reflective insulation system.
Does reflective insulation need an air gap to work?
Yes, this is non-negotiable. Emittance only matters when the low-emittance surface faces an air space so infrared radiation can travel across it, and DOE specifies a minimum 1-inch (2.5 cm) air gap for rafter-mounted installations. When foil is sandwiched between two solids, heat moves by conduction through the foil and its near-zero thermal resistance makes it effectively transparent to that heat. Bubble-core products create internal air spaces but still need at least one facing air film to hit their rated assembly R-values, which is why installation method is as important as product choice.
Can reflective insulation replace fiberglass batts?
No, because they address different heat-transfer mechanisms. Fiberglass batts like R-13 trap still air to resist conductive and convective heat flow, which is what building codes target, while reflective insulation blocks the radiant heat crossing an air gap that R-value alone does not capture. A hot roof deck radiates energy continuously across the attic, and attic-floor batts absorb that radiation and pass it into the living space. The documented gains come from layering: the ORNL 1986 study found a 21% cooling-load reduction by adding a radiant barrier on top of existing R-19 fiberglass.
Does reflective insulation work in cold climates?
Yes, though the benefit shifts: in DOE Zones 6 to 8 the summer cooling payoff is smaller, and DOE recommends more bulk insulation as the primary attic upgrade. Reflective insulation still delivers real cold-climate value, because it reflects radiant heat from the conditioned space back toward attic-floor insulation, reduces downward heat loss, and helps control condensation and attic-rain risk by stabilizing roof-deck temperatures. For cold climates, foil-faced rigid foam in walls or crawlspace perimeters is often the better primary application. Any attic foil should be perforated to at least 6.29 perms, like RB+, so moisture escapes rather than freezing and dripping.
Does running HVAC ducts through the attic change the payback?
Yes, and it usually improves it. The ceiling R-value figure only describes heat crossing the attic floor, but ductwork sits up in the attic air itself, surrounded by air that can reach 130 F or more on a hot afternoon. That large duct surface area and the wide temperature gap between the hot attic and the cold supply air drive extra heat into the air you paid to cool. A radiant barrier on the rafters lowers the attic air temperature the ducts sit in, so it cuts duct heat gain on top of the ceiling savings, which is why homes with attic ducts often see a larger total reduction than the ceiling R-value alone would predict.
Where can reflective insulation be used in a home?
The main residential locations are attic rafters (bare foil stapled to the underside, facing the attic air space), floors over unheated crawlspaces or garages (bubble-foil with the foil facing down into the air space), wall assemblies (foil-faced house wrap behind cladding where a drainage gap supplies the air space, or bubble-foil inside stud bays), and attic gable walls. Match the product type to the job: attic rafters favor bare perforated foil, while metal-building bays favor double-bubble double-foil. In every case, preserve an adjacent air space of at least 1 inch so the surface reflects rather than conducts.
How do I read the R-value on a reflective insulation data sheet?
Read the footnote and the test basis before you read the headline number. A labeled assembly R-value is calculated per RIMA, AIRAH, ASHRAE, or ISO 6946 air-film methods and assumes a specific air-gap size, heat-flow direction, and temperature, so the same product shows a different figure for an attic (heat flow down) than for a wall (heat flow horizontal). If a contractor's estimate quotes one flat product R-value with no assembly or air-gap conditions stated, ask for the test standard and the assembly it applies to. Under the FTC R-Value Rule, a seller must be able to substantiate any R-value claim, so a number with no test basis is a signal to get the data sheet before you buy.
Reflective insulation is a low-emittance foil surface, facing an air gap, that reflects the radiant heat fiberglass and cellulose let through. It works alongside your existing bulk insulation, lowering the assembly U-value year-round: cutting summer heat gain, reflecting winter heat loss back toward the attic-floor insulation, and helping control condensation. The most common installation error to avoid is pressing the foil flat against a solid, which removes the air gap and turns a reflective layer into a conductor, so plan your install around keeping that gap before you buy.