Fiberglass insulation typically has a thermal conductivity in the range of about 0.030 to 0.045 W/(m·K) at room temperature, depending on density, fiber diameter, and product form. That number sounds precise, but in practice it shifts with conditions your walls and attics actually face: temperature swings, humidity, compression during installation, and even how the fibers are oriented. Understanding what drives those shifts matters more than memorizing a single value, because the conductivity printed on a product label describes ideal lab conditions that your building may never replicate.
Three Ways Heat Gets Through Fiberglass
Fiberglass does not stop heat transfer through a single trick. Heat moves through a fiberglass blanket by three routes simultaneously: conduction through the solid glass fibers themselves, conduction (and a small amount of convection) through the still air trapped between fibers, and infrared radiation passing through gaps in the fiber network. The total, or “effective,” thermal conductivity you see on a spec sheet is really the sum of all three contributions acting in parallel, much like parallel resistors in an electrical circuit.
At typical building temperatures, the air-conduction component dominates. Still air has a thermal conductivity around 0.025 W/(m·K), and because fiberglass is mostly air by volume, this sets a rough floor on how low the material’s conductivity can go under normal atmospheric pressure. The solid-fiber contribution is smaller but grows as you pack more glass into the same space. Radiation, meanwhile, plays a relatively minor role near room temperature but becomes increasingly important as temperatures rise, because radiative heat transfer scales steeply with temperature.
The practical takeaway is that the effective thermal conductivity of fiberglass is not a fixed material property in the way that, say, the thermal conductivity of solid aluminum is. It is a composite number reflecting the interplay of all three heat-transfer paths, and changing any condition that shifts the balance among them changes the number you measure. Mathematical models treat these components as additive, combining conduction and radiation into a single effective conductivity value.
Why Density Has a Sweet Spot
You might assume that packing fiberglass more tightly always improves its insulating ability, since denser material means more glass blocking the path of heat. In reality, adding density helps up to a point and then starts to hurt. At low densities, radiation passes too freely through large open gaps between fibers, so the radiative component of conductivity is high. As you add more fiber, those gaps shrink and radiation gets blocked more effectively. But the solid glass fibers themselves conduct heat, and eventually the rising solid-conduction component overtakes the falling radiation component.
The result is a U-shaped curve with a broad minimum. Laboratory testing shows this minimum sits in the density range of roughly 50 to 80 kg/m³ for standard glass fiber insulation, right around the point where the conductivity through the glass fibers equals the radiative conductivity.1NETZSCH Analyzing & Testing. Effect of Density on the Thermal Performance of a Glass Fiber Insulation Material Most commercial fiberglass batt insulation for residential walls sits below that optimum, somewhere around 10 to 30 kg/m³, because the cost and weight of denser batts outweigh the modest performance gain for typical building applications. High-density industrial boards, by contrast, push closer to the sweet spot when maximum thermal resistance per unit thickness is the goal.
Fiber diameter also matters. Thinner fibers create more surfaces per unit volume, which blocks radiation more effectively without adding as much solid-conduction pathway. Research confirms that the effective conductivity depends on both insulation density and fiber diameter, along with temperature.2International Journal of Applied Glass Science. Effective thermal conductivity of fiberglass insulation Manufacturers can therefore tune performance by adjusting the fiberization process to produce finer fibers rather than simply cramming in more glass by weight.
How Temperature Shifts the Numbers
At everyday building temperatures, fiberglass conductivity rises gently as the temperature goes up. This is mostly because the air trapped between fibers conducts heat a bit better when it is warmer, and the radiative component grows as well. For a residential wall or attic that sees temperatures between roughly 0 °C and 40 °C, the change is modest and already factored into the R-value stamped on the packaging (which is tested at a standard mean temperature, usually around 24 °C in North America).
The relationship is more dramatic at extreme temperatures. In industrial pipe insulation or furnace linings operating at several hundred degrees, the radiation component can become the dominant pathway, and the effective conductivity may be several times higher than the room-temperature value. At the other extreme, in cryogenic applications, researchers have measured thermal conductivity of fiberglass-epoxy composites increasing linearly with temperature down into the range of superfluid helium, near absolute zero.3Cryogenics. Thermal conductivity and Kapitza resistance of epoxy resin fiberglass tape at superfluid helium temperature That linear behavior at cryogenic temperatures is useful for designing supports and thermal straps in particle accelerators and space hardware, where fiberglass composites serve as structural elements that also need to limit heat leakage.
For most readers concerned with buildings, the key point is that fiberglass performs somewhat worse in hot attics and somewhat better in cold winter walls compared to the single R-value on the label. The effect is small enough that building codes do not require temperature correction for normal conditions, but it is real.
What Moisture Does to Performance
Moisture is one of the most underappreciated enemies of fiberglass insulation performance. Water conducts heat roughly 25 times better than still air, so when moisture replaces the air in the pore spaces of the insulation, the effective thermal conductivity climbs. How much it climbs depends on the form the moisture takes and how much is present.
At moderate humidity levels, the effect is surprisingly gentle. One experimental study found that raising the moisture content of a low-density fiberglass sample (27 kg/m³) from zero to about 16 percent by weight at 24 °C increased the thermal conductivity by only around 5 percent.4Construction and Building Materials. The variation of thermal conductivity of fibrous insulation materials under different levels of moisture content That is a relatively small penalty, and at typical indoor humidity levels the degradation is even less. But conditions matter. Higher operating temperatures combined with higher moisture content compound the effect, because both independently push conductivity upward.
The behavior at high relative humidity is more nuanced than a straight line. At moderate humidity, water vapor slowly displaces dry air in the pore spaces, raising conductivity gradually. Interestingly, some experimental data show that fiberglass conductivity can actually plateau or even dip slightly around 70 percent relative humidity, because the moist air layer creates a thin thermal resistance against the pore walls. When humidity reaches 100 percent, though, vapor condenses into liquid water within the pores, and conductivity spikes sharply.5Energy and Built Environment. Experimental study on the influence of temperature and humidity on the thermal conductivity of building insulation materials This is why vapor barriers and proper ventilation in attics and wall cavities are so important: they are not just protecting the fiberglass from mold or rot, they are protecting its thermal performance.
How Compression Kills R-Value
Fiberglass batt insulation is sold with both an R-value and a specified thickness at which that R-value applies. When a batt gets squeezed thinner during installation, whether by being shoved behind wiring, compressed into a too-narrow cavity, or stacked with heavy objects on top, its R-value drops. This is one of the most common real-world performance losses and one that many homeowners and even some contractors underestimate.
The underlying reason connects back to the density story. Compressing a batt raises its density per unit thickness, which does reduce radiative heat transfer through the thinner layer. But the total thermal resistance depends on both the conductivity and the thickness of the material. While the conductivity per inch may improve slightly at higher density, you have lost inches, and the net result is a lower total R-value for the installed assembly.6Journal of Thermal Insulation. The Effect of Compression on the Material R-Value of Fiberglass Batt Insulation Think of it like a down jacket: squashing it thin makes each remaining inch of down a marginally better insulator, but you have far less jacket left, so you end up colder.
The practical advice is straightforward. If a cavity is too shallow for the rated thickness of a batt, use a batt rated for that cavity size rather than cramming in a thicker one. An R-13 batt compressed into 2.5 inches does not deliver R-13 performance. This matters most in older homes being retrofitted, where stud depths can vary.
Batts, Blown-In, and Loose-Fill
Fiberglass comes in several physical forms, and the form factor affects its thermal performance. Batts are pre-cut blankets designed to friction-fit between studs or joists. Blown-in-blanket (BIB) systems use a machine to spray loose fiberglass into a cavity behind a mesh, creating a custom-fit fill. Loose-fill fiberglass is simply poured or blown into open attic spaces.
Laboratory comparisons show that blow-in-blanket fiberglass products have apparent thermal conductivities a few percent higher than the corresponding loose-fill version of the same material.7Journal of Thermal Insulation and Building Envelopes. A Thermal Resistance (R-Value) Correlation for Blow-in-Blanket Mineral Fiber Insulation The difference is small, but it exists because the blowing process and the settled orientation of the fibers create a slightly different microstructure than a factory-formed batt or a gravity-settled loose fill. In practice, blown-in products often outperform batts in real buildings because they fill irregular cavities more completely, leaving fewer air gaps. The slight conductivity penalty is more than offset by better coverage.
Loose-fill in attics faces its own issue: settling over time. As gravity compacts the layer, thickness decreases and the installed R-value drops. Installers typically blow in extra material to account for this, following manufacturer-specified settled-thickness guidelines. If you are checking old attic insulation, measuring its current depth and comparing to the settled-thickness chart for that product gives you a reasonable estimate of remaining R-value.
How Fiberglass Holds Up Over Time
One genuine advantage of fiberglass over some competing insulation types is its long-term stability. Glass does not off-gas blowing agents the way closed-cell foams do, so there is no inherent mechanism for the thermal conductivity to drift upward as the material ages. Previously published experimental work has shown that fiberglass and extruded polystyrene (XPS) insulation retained about 97 percent of their R-value after several weeks of exposure to elevated temperature (about 66 °C) and 90 percent relative humidity. By comparison, aerogel blankets lost around 15 percent and polyurethane foam lost roughly 28 percent under the same conditions.8Elsevier (Energy and Buildings). Experimental study on the effects of humidity and temperature on aerogel composite and foam insulations
That resilience comes from the fact that the glass fibers are chemically inert under building conditions and the insulation is open-celled, meaning any moisture that enters can also leave once conditions change. Foam insulations trap blowing agents inside closed cells; as those gases slowly leak out and are replaced by air, the foam’s conductivity creeps upward. Fiberglass skips this problem entirely. Its thermal conductivity at year 20 is essentially the same as at year one, assuming it stays dry, uncompressed, and intact.
The caveat is physical damage. Rodent tunnels, water leaks, settling, or compression from stored items in an attic can all degrade real-world performance even while the glass fibers themselves are unchanged. Periodic inspection matters more than worrying about material aging.
Measuring Thermal Conductivity in Practice
The standard way to measure the thermal conductivity of fiberglass insulation is the guarded hot plate method. A sample is sandwiched between a heated plate and a cooled plate, both held at precisely controlled temperatures, and the steady-state heat flow through the sample is measured. This gives the apparent thermal conductivity at a given mean temperature, density, and thickness. Researchers have confirmed this method’s accuracy through calibration against reference materials, though results depend on controlling variables like the emissivity of the hot and cold surfaces, since radiation between the plates passes through the translucent fiberglass.9Journal of Thermal Insulation. Apparent Thermal Conductivities of Fiberglass Insulation
Why does this matter to a non-researcher? Because the R-values on product labels come from these tests, and the tests are performed at a single standard mean temperature with a specific temperature difference across the sample. If your actual conditions differ significantly, so will your actual performance. A fiberglass batt rated R-13 in a lab test at a 24 °C mean temperature will deliver slightly less resistance in a sun-baked roof cavity at 50 °C and slightly more in a cold basement wall at 5 °C. The difference is usually within 10 to 15 percent across the range of temperatures a house experiences, which is why the single-number R-value system works reasonably well for building codes, but it is not exact.
Glass Fiber Cores in Vacuum Insulation Panels
At the high-performance end of the insulation spectrum, glass fibers serve as the core material in vacuum insulation panels (VIPs). By evacuating the air from a sealed panel of glass fiber, you eliminate the gas-conduction component that normally dominates the total thermal conductivity. What remains is the small amount of solid conduction through fiber-to-fiber contact points and whatever radiation leaks through.
The results are dramatic. Researchers have prepared VIPs with specially oriented glass fiber cores (fibers aligned perpendicular to the heat-flow direction) that achieved an initial thermal conductivity of just 1.25 mW/(m·K), roughly 30 times lower than a conventional fiberglass batt.10Energy. Preparation and characterization of vacuum insulation panels with super-stratified glass fiber core material Even conventional micro-glass-fiber VIP cores reach about 4.3 mW/(m·K) at internal pressures below 10 Pa.11Elsevier. Size effect on miniature VIP lifetime decay via multi-scale simulation These values are an order of magnitude below what any atmospheric-pressure insulation can achieve.
The catch is durability. VIPs are only as good as their vacuum seal. Any puncture or slow gas permeation through the barrier film raises the internal pressure, and once gas fills the pore spaces, the panel reverts toward its atmospheric-pressure conductivity. For core structures with large pore sizes on the order of a millimeter, there is no low-conductivity plateau: thermal conductivity rises steadily as soon as internal pressure begins to climb. Smaller pore structures hold their low conductivity over a wider pressure range, which is why manufacturers push for finer fibers and tighter packing in VIP cores. The technology is used in high-value applications like refrigerator insulation, shipping containers for pharmaceuticals, and thin-profile building retrofits where every millimeter of wall thickness counts.
Radiation Control at High Temperatures
For industrial applications above a few hundred degrees Celsius, the radiation component of heat transfer through fibrous insulation can dominate, and ordinary fiberglass loses effectiveness. One approach is to add opacifying particles, tiny solid additives that absorb or scatter infrared radiation, to the insulation matrix. Research on silica aerogel composites (a close cousin of fiberglass-based systems) has shown that doping with silicon carbide or titanium dioxide particles significantly reduces radiative thermal conductivity compared to zirconium dioxide particles at the same loading, with silicon carbide aerogels achieving the lowest total thermal conductivity below about 1000 K.12International Journal of Heat and Mass Transfer. Thermal conductivity of fiber and opacifier loaded silica aerogel composite
The trade-off is that adding solid opacifier particles inevitably increases solid conduction. Getting the balance right requires optimizing particle type, size, and concentration. For fiberglass specifically, some researchers have developed hollow silica-fiberglass composite ceramics that maintain low thermal conductivity while providing mechanical resilience and fire resistance at high temperatures.13Chemical Engineering Journal. Creation of hollow silica-fiberglass soft ceramics for thermal insulation These materials sit in a different category from the pink batts in your attic, but they illustrate how the same basic glass-fiber platform can be engineered across a wide performance range by manipulating the balance among solid conduction, gas conduction, and radiation.
Combined conduction-radiation modeling treats the fibrous medium’s resistance to each pathway as parallel channels and sums them to predict total effective conductivity.14International Journal of Heat and Mass Transfer. Modeling performance of multi-component fibrous insulations against conductive and radiative heat transfer This framework lets engineers simulate how changes in fiber diameter, packing density, or opacifier loading will shift the conductivity curve before committing to a manufacturing run, which is especially valuable for specialty products where physical prototyping is expensive.