Metals are the most familiar examples of poor insulators. Copper, aluminum, silver, gold, and steel all conduct both heat and electricity readily, making them the opposite of what you want in an insulating material. But metals aren’t the only offenders. Several everyday materials that seem like they should insulate, including water, concrete, and even the human body, can fail as insulators under common real-world conditions.
Why Metals Fail as Insulators
Metals conduct electricity because their atomic structure allows electrons to move through the material with very little resistance. In a metal like copper or aluminum, electrons can travel through connected pathways between tightly packed atoms. When voltage is applied, electrons are elevated into these conducting pathways and flow as current. The energy needed to push electrons into those pathways is extremely low in metals, which is why they conduct so easily and insulate so poorly.
The same property that makes metals good electrical conductors also makes them poor thermal insulators. When you touch a metal railing on a cold day, it feels freezing because it rapidly pulls heat away from your hand. Copper has a thermal conductivity around 400 W/mK, and aluminum sits near 235 W/mK. For comparison, a good insulator like wood is roughly 0.1 to 0.2 W/mK. That means copper transfers heat about 2,000 times more efficiently than wood. Steel, with a thermal conductivity around 50 W/mK, is a weaker conductor than copper but still far too conductive to serve as insulation. Concrete reinforced with steel fibers measures about 1.98 W/mK, roughly 20% higher than base concrete, showing how even small amounts of metal compromise a material’s insulating ability.
Water: A Surprising Conductor
Pure distilled water is actually an excellent electrical insulator. The problem is that water almost never stays pure. The moment water contacts dissolved minerals, salts, or other substances, free ions form and carry electrical current. Tap water, rainwater, and ocean water are all effective conductors because of their dissolved content.
This distinction matters for safety. The U.S. Geological Survey notes that water with very large amounts of dissolved ions becomes such an efficient conductor that electrical current will preferentially travel through the water rather than through a human body submerged in it. So while “water” appears on many lists of poor insulators, it’s specifically impure water, the kind you encounter in everyday life, that conducts. Distilled and deionized water used in laboratories can resist current, though even these can contain trace ions.
Graphite and Carbon Materials
Not all non-metals are good insulators. Graphite, a form of pure carbon found in pencils, conducts electricity. Its layered atomic structure allows electrons to move freely along flat sheets of carbon atoms. Carbon fibers and carbon-based fillers have resistivity values in the single-digit ohm-meters, roughly six orders of magnitude (a millionfold) lower than non-conductive materials like dry cement or limestone. This is why carbon-based materials are used as conductive additives in construction composites and why graphite serves as an electrode material in batteries.
Glass, Concrete, and Moisture
Glass and concrete are generally considered decent insulators, but their performance degrades significantly under certain conditions. Base concrete has an electrical resistivity around 9 million ohm-meters when dry, which qualifies it as an insulating material. However, as concrete absorbs moisture, its resistivity drops dramatically. Reducing water saturation to 50% increases resistivity by roughly 55 times compared to fully saturated concrete, and completely drying it out raises resistivity by 600 times. In other words, wet concrete is a far worse insulator than dry concrete.
Glass tells a similar thermal story. While glass is a reasonable electrical insulator, it conducts heat more readily than most people assume. Concrete with glass fiber additions measured 1.65 W/mK in thermal conductivity testing, which is better than steel-reinforced concrete but still nowhere near the performance of dedicated thermal insulators like foam or fiberglass batting. Single-pane glass windows are notoriously poor thermal insulators, which is why double and triple glazing with air gaps became standard in construction.
The Human Body
Your body is a poor electrical insulator. While the outermost layer of skin (the stratum corneum) has a resistivity between 1,000 and 1,000,000 ohm-meters, the tissue underneath is far more conductive. The deeper layers of living skin and dermis have resistivity of just a few ohm-meters, and subcutaneous fat tissue ranges from 10 to 100 ohm-meters. Internal tissues, rich in water and dissolved salts, conduct current readily.
Hair follicles and sweat glands act as preferred pathways for electrical current because they bypass the relatively dry outer skin barrier. Sweaty or wet skin dramatically lowers resistance, which is why electrical safety guidelines are especially strict for wet environments. The moisture inside your body, combined with dissolved electrolytes like sodium and potassium, means that once current penetrates the outer skin layer, it flows through internal tissue with relatively little resistance.
Diamond: A Unique Exception
Diamond breaks the usual rules. It’s one of the best thermal conductors known, with high-quality synthetic diamond films reaching thermal conductivity values of 950 W/mK, more than double that of copper. Yet diamond is simultaneously an excellent electrical insulator, with very high resistivity. This unusual combination exists because diamond’s carbon atoms are locked in a rigid crystal lattice that transmits vibrations (heat) extremely efficiently but doesn’t allow electrons to flow freely. This makes diamond useful in specialized electronics where components need to shed heat without short-circuiting, but it also shows that thermal and electrical insulation don’t always go hand in hand.
Quick Comparison of Poor Insulators
- Copper and silver: Conduct both heat and electricity extremely well. Among the worst possible choices for insulation.
- Aluminum: Lightweight but highly conductive. Used in heat sinks specifically because it pulls heat away from electronics.
- Steel: Lower conductivity than copper but still far too conductive for insulation purposes.
- Tap water and salt water: Dissolved ions make everyday water an effective electrical conductor.
- Graphite: A non-metal that conducts electricity due to its layered carbon structure.
- Wet concrete: Loses most of its insulating ability when saturated with water.
- Human tissue: Internal body tissues have very low resistivity, making the body a conductor once current passes through the skin.
The pattern connecting most poor insulators is the availability of charge carriers. In metals, electrons move freely. In water, dissolved ions carry current. In wet concrete, trapped moisture provides a conductive pathway. Materials insulate well when they lock their electrons tightly in place and contain no free ions or mobile charges. Anything that loosens those electrons or introduces ions into the material shifts it from insulator toward conductor.