Reflective foil insulation and fiberglass insulation both appear in home energy projects, but they work by completely different mechanisms and are useful in different situations. Conflating them is one of the more common points of confusion in home energy. This guide covers what each actually does, where each works, and where they're better used together.
The mechanisms
Fiberglass (and cellulose, mineral wool, foam): bulk insulation
Bulk insulation works by trapping tiny pockets of still air or gas. Still air is a poor conductor, so a thick layer of trapped air slows conductive heat flow. The R-value of fiberglass batt (roughly 3.0 to 3.5 per inch) measures this conductive resistance.
Bulk insulation works in any direction, in any climate, against heat moving either way (in or out). It's the default for walls, ceilings, and floors.
Reflective foil: radiant barrier
A radiant barrier is a layer of low-emissivity material (typically foil-faced) that reflects radiant heat rather than absorbing it. It works specifically against radiant heat transfer — the kind where heat moves through space as infrared radiation, like sunlight hitting a roof.
Radiant barriers don't have meaningful R-value on their own. Their effect is measured as a radiant-reflectance percentage and depends on having an air gap adjacent to the foil surface. Foil pressed against another material does nothing useful.
Where reflective foil works
The dominant use case is hot-climate attics. In the southern US, summer sun heats the underside of the roof deck to 140 to 160°F, and the radiant heat then radiates down through attic insulation into the living space below.
A radiant barrier installed under the roof rafters reflects the radiant heat back up — keeping the attic and ceiling below noticeably cooler. Real-world studies in hot climates show attic temperature reductions of 10 to 20°F and cooling bill reductions of 5 to 10 percent with proper radiant barrier installation.
Radiant barriers also see use in:
- Metal buildings (where roof-panel radiant heat gain is severe)
- Garage doors (especially in mixed and hot climates)
- Crawlspaces and basements to reflect heat back into or out of the space
In cold climates, the benefit is much smaller. Heating dominates, and bulk insulation (which handles conductive heat loss) does most of the work. Radiant barriers in cold-climate attics can help retain winter heat radiated by the ceiling below, but the effect is modest.
What about reflective 'bubble' insulation?
Some products include a layer of reflective foil laminated to bubble-pack plastic ('reflective bubble insulation'). These look like they should be highly insulating but the actual R-value is low — typically R-1 to R-2 for the material alone, often referenced as 'R-8' or 'R-10' by manufacturers assuming a specific air gap on each side of the product.
The marketing R-value depends on the test setup: with a 3/4 inch air gap between the foil and the hot surface, the assembly reflects heat and the trapped air provides modest R-value, but the test condition has to match your install exactly to get the rated performance.
For practical purposes:
- Reflective bubble products can be useful where space is tight and you can install them with proper air gaps (e.g., inside duct wrap, water heater jackets, behind radiators).
- They should not be used as a substitute for proper bulk insulation in walls, ceilings, or floors. They cannot deliver the R-value bulk insulation does at similar cost.
Where fiberglass (and other bulk insulation) wins
Anywhere you need substantial R-value in a wall, ceiling, or floor:
- Attic insulation (R-30 to R-60)
- Wall cavity insulation (R-13 to R-23)
- Floor over unconditioned space (R-19 to R-30)
- Basement or crawlspace walls (R-10 to R-15)
Bulk insulation is general-purpose: it works against conductive heat flow in any season, any direction, hot or cold climate.
Where they pair
A productive pairing for hot-climate attics is:
- Radiant barrier under the roof deck to cut radiant heat gain
- Fiberglass or cellulose loose-fill on the attic floor (R-49 preferred) to slow conductive heat flow into the living space
The two together substantially reduce total cooling load. A radiant barrier alone with poor attic insulation doesn't fix much; high-R-value attic insulation alone in a hot climate adds cost without the radiant barrier benefit. Together, they address both heat transfer mechanisms.
For cold climates, focus on bulk insulation first; a radiant barrier is a minor addition only if you have already maxed out the bulk insulation.
How to think about it
If you're only getting one of the two: get fiberglass (or another bulk insulation). Bulk insulation is the universal answer.
If you're in a hot climate and you have a moderate existing attic insulation level already: a radiant barrier is a high-value add — typically cheaper than another 6 inches of loose-fill, and tackles a heat flow path loose-fill doesn't address well.
If you're starting from low insulation across a hot climate: add the bulk insulation first. Use the insulation calculator to estimate savings and payback before committing to material choices.