Insulation Radiant Barrier or Bubble Foil vs Spray Foam in a Metal Building
Bubble foil, foil-faced bubble wrap and any other insulation radiant barrier do one narrow job: they slow heat that travels as radiation across a gap. Spray foam does a different job, stopping heat that conducts through a material and air that leaks through cracks. Whether a reflective layer helps in a steel building depends almost entirely on the air space it faces, so the two products are not rivals so much as answers to different questions.
This page sorts out what the reflective products can honestly do under a metal roof or on metal walls, what the federal rule says about their R-values, and the one case where a spray foam crew will not simply cover them over.
What an insulation radiant barrier does (and what it does not)
A reflective surface has low emittance: it gives off little radiant heat and bounces most of what lands on it. A roof heated by the sun radiates downward into the building, and a shiny layer facing that downward heat can turn much of it back. That is the whole mechanism. It does nothing about heat that conducts through a solid touching it, and it does not seal air leaks.
The U.S. Department of Energy's radiant barrier fact sheet (written in 1991 and archived by Oak Ridge National Laboratory) is candid about the limits of the evidence. It says there is no standardized method for testing how well radiant barriers cut heating and cooling bills, and that the field tests were mostly in warm climates, with very little done in places colder than Knoxville, Tennessee (DOE/CE-0335P). If your building is in a colder climate than those test sites, treat any promised percentage with caution. The sheet is about house attics and is old, so we use it for the physics, not for steel-building numbers.
Three more points from the same sheet explain why the products disappoint in practice:
- Dust. Dust on foil raises its emissivity and lowers its reflectivity, so a surface that collects dust loses benefit over time. A downward-facing surface under a roof is less likely to collect it than a floor.
- Existing insulation. Radiant barriers cut the most heat flow when there is little other insulation, and much less when there is a lot.
- Summer first. The reduction in winter heat loss is smaller than the summer gain.
The Department of Energy's consumer sheet on home insulation reaches a matching conclusion in one line: a radiant barrier is especially useful in hot, sunny climates (DOE Energy Saver, 2021). A steel shop you heat in January is the opposite case.
Bubble foil, single and double: what the product is
Bubble insulation for metal buildings usually means a roll of plastic bubble sheeting with an aluminum-coated face on one side (single bubble) or both sides (double bubble). The DOE consumer sheet lists reflective rolls as one of the standard insulation types, next to blankets, sprayed foam and rigid board. The foil faces are the reflective part. The air trapped in the bubbles adds only a small insulating layer of its own, so the foil's performance still comes down to what it faces.
The terms overlap in sales listings. In this page, "radiant barrier" means a reflective surface alone, and "bubble foil" means the bubble-and-foil sandwich. Both rely on the same physics.
The R depends on the air space
The Federal Trade Commission's R-value rule is the clearest statement of how reflective products are measured. Under 16 CFR 460.5, a single-sheet reflective material is tested for emittance, and the R-value is then read from a table in the ASHRAE Handbook that depends on the emittance, the air space and the direction of heat flow. A multi-sheet reflective system, or a single sheet outside the table's conditions, is tested as a whole panel instead. A material with a reflective facing can be tested with its air space or without it (16 CFR 460.5(b)-(d)).
Put plainly, a reflective R-value is not a property of the foil alone. It belongs to the foil plus a gap of a stated size, with heat moving in a stated direction. The rule is written for home insulation (16 CFR 460.2), so it does not bind a steel-building quote, but it is the standard method any honest label follows. An R number on a roll with no air space and no direction of heat flow next to it is incomplete.
How big is the gain? Martin Holladay of GreenBuildingAdvisor wrote that the R-value of an air space between 1/2 inch and 4 inches thick is about R-1, and that a shiny foil face can lift it to roughly R-2 or R-2.5 (GBA Q&A, 2014). That is a building-science answer, not a Kansas test, and it applies here because the behavior of a still air gap is the same everywhere. It is also a long way from the R-values people read into stacks of bubble layers.
Assembly scenarios: where the foil sits
The table below is our own reading of how the physics above plays out under a steel roof. It is a set of scenarios, not test results.
| Where the foil is | Does it face an air space? | What the sources support | What it leaves open |
|---|---|---|---|
| Pressed flat to the underside of the panel (no gap) | No: the foil touches the steel | The FTC method ties reflective R to the air space beside the foil, so a layer with no gap has little to reflect across | A sealed foil face can keep damp air off the steel, as the trade group's FAQ describes (RIMA) |
| Held below the purlins with a gap above it | Yes, between foil and roof | Gap of about 1/2 to 4 inches is about R-1, foil face lifts it to about R-2 to R-2.5 (Holladay, above) | The gap must stay a gap: sag, torn seams and fastener holes change it |
| Draped across the purlins, sagging into the bays | Partly, and uneven | The air space varies along the span, so no single R applies | Where the foil sags onto the panel it behaves like the first row |
| Foil face pointing up, as on a floor or ledge | Yes, if there is a real gap | The DOE sheet says dust on an exposed face cuts its benefit | An upward face collects dust fastest, which is why roof-side use is the common case |
The first row is the easiest way to hang a roll, tight to the panel with tape. By the FTC method's own logic it is also the row where "double bubble" has the least to work with.
As a condensation control
Steel sweats when damp air touches it below the dew point (why the mechanism works). A reflective layer can limit that in two ways, and neither is the one the product ads usually claim.
First, a continuous, sealed foil face acts as a vapor retarder. If the seams are taped and the edges closed, damp air cannot reach the steel behind it, which is why the trade group says reflective insulation below the deck can warm the inside surface and reduce condensation. Second, if the foil has no air-tight seam, damp air gets behind it and condenses on the steel anyway, and the water has nowhere to go but down the back of the roll.
Foam is a different kind of control. A sprayed layer bonded to the steel leaves no gap for damp air to reach, and it does not depend on a taped seam staying closed. The options matrix on metal building insulation options compared shows each type side by side, and the roof-specific case is on foam on the underside of a metal roof.
A reflective layer keeps nothing from moving by conduction. In a heated building that is the heat loss you pay for, so condensation control alone does not make a foil roll a heating solution.
Spraying over bubble: a question for the product's guide
PolyJacks does not remove insulation, and we spray closed-cell foam only. If a steel building already has bubble foil hanging in it, the usual answer is that the foil comes out first, by you or a contractor you hire, and the steel is checked before foam goes on. What comes out and what is checked is on insulating an existing metal building.
The reason is bond. Foam is meant to adhere to clean steel, and a foil or plastic film between the foam and the steel gives it something that is not steel to hold onto. The exception is a product whose installation guide and evaluation report list that exact substrate. We do not assume it.
We also make no flame-spread or smoke claim about any bubble foil product here. Reflective rolls differ by maker, and a claim would need that product's own test report.
Where each product fits
| Building and goal | Reflective layer alone | Spray foam |
|---|---|---|
| Unheated metal building, summer heat is the complaint | Can slow downward radiant heat, with the limits above | Does more, because it also cuts conduction, but adds more than the problem may need |
| Heated shop in winter | Weak: it does not stop conduction through the steel | Strong fit: insulates, seals air leaks, and keeps damp air off the steel |
| Roof drips on equipment, unheated | A sealed face may help; an unsealed one can trap water | Stops the drip if continuous, bonded and thick enough |
| Already hung in the building | Comes out before foam goes on | Not a layer to spray over, unless the guide lists it |
| Building not ordered yet | Specified by the metal building supplier; factory systems differ | Can be sprayed later; decide at order time |
For a pole barn rather than a steel building, the comparison across more materials is on the best way to insulate a pole barn.
FAQ
Does double bubble insulation work on a metal building?
It can slow radiant heat and, if sealed, limit drip, but only when it faces an air space, and its R as installed is low. The R-value method in 16 CFR 460.5 ties a reflective product's R to the air space beside it, and an air space of that kind is worth about R-1 before the foil face adds its gain.
Is a radiant barrier better than spray foam?
They do different jobs. A radiant barrier blocks radiant heat across an air space, while closed-cell foam stops conduction and air leaks in the same layer. In the Department of Energy's own words, radiant barriers are especially useful in hot, sunny climates, and a winter-heated shop is not that case.
Can you spray foam over bubble insulation?
Usually no. The foil comes out first, and you arrange that, because it blocks the foam's bond to the steel. Foam goes over a reflective film only where the named product's guide and evaluation report list that substrate. We have not confirmed that for any product yet, so assume the foil comes out.
Book a free on-site measurement
At the visit we look at what is in the building now, the steel behind it, how the building is used, and whether foam is the right layer at all. If a reflective layer is enough for what you need, we will say so. You get a written estimate after the visit. More on the visit: the free estimate visit.
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Spray foam insulation is a new PolyJacks service line run by its own crew and rig.
Who wrote this: PolyJacks spray foam team. How it was made: drafted with AI assistance from the sources below, then checked against each source.
Sources
- 16 CFR 460.5, R-value tests (FTC R-value Rule), paragraphs (b)-(d), Cornell LII: law.cornell.edu
- 16 CFR 460.2, definitions (home insulation), Cornell LII: law.cornell.edu
- U.S. Department of Energy, "Effect of Radiant Barriers on Heating and Cooling Bills," DOE/CE-0335P, June 1991 (copy of the ORNL fact sheet): inspectapedia.com
- U.S. Department of Energy, Energy Saver fact sheet, Home Insulation (2021): energy.gov
- Martin Holladay, answer in "Insulating a metal barn," GreenBuildingAdvisor Q&A, October 27, 2014: greenbuildingadvisor.com
- Reflective Insulation Manufacturers Association International, Condensation FAQ (trade group; physics only): rimainternational.org
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