In this episode of “If Walls Could Talk,” W&C Editor John Wyatt talks with Tom Harris of Tom Harris PUR Consulting. In this discussion, we chat about getting “back to basics” on one of the most common—and most misunderstood—terms in construction: R-value.

At its core, Harris explains, insulation is installed for two main reasons: to manage condensation and to reduce the energy needed to maintain comfortable indoor conditions across different climates. R-value is the metric the industry and all three major building codes use to measure how effective insulation is at doing that job.

What R-Value Really Means—and What It Doesn’t

Tom Harris explains that R-value only measures conductive heat transfer, overlooking convection and moisture effects that significantly impact real-world insulation performance.

But here’s the catch: R-value only measures one type of heat transfer—conduction.

To make it relatable, Harris paints a picture most people recognize. Imagine sitting around a campfire and grabbing a metal poker that’s been resting in the flames. The heat travels from the fire, through the metal, and into your hand. That’s conduction. Materials that conduct heat poorly are what we call insulation, and their resistance to conductive heat flow is what R-value measures in a lab under controlled conditions.

The problem? Heat doesn’t move in just one way.

“There are actually three ways heat is transferred,” Harris says: conduction, convection, and radiation. R-value ignores the latter two entirely.

Convection, for example, is the heat you feel when you hold your hand above a campfire or a stove burner. In wall assemblies, convection shows up as air movement inside insulation—particularly in fibrous materials like fiberglass, mineral wool, and cellulose. When these materials reach certain thicknesses and densities, warm air can rise inside the cavity while cooler air sinks, creating what’s known as convective looping.

As insulation gets thicker to meet higher code-mandated R-values, this looping can actually increase—reducing real-world thermal performance instead of improving it.

“So we increase the R-value requirement, use thicker fibrous insulation, and unintentionally make it perform worse,” Harris notes.

Add air movement and moisture into the mix, and the gap between lab-tested R-values and actual installed performance grows even wider. That’s why modern codes now require air barriers and vapor retarders. Insulation, especially fibrous insulation, must stay dry and protected from airflow to work as intended—factors the standard R-value test simply doesn’t capture.

Harris points to past and ongoing third-party research, including studies often referred to as “Thermal Metric” studies, which evaluate insulation performance under real-world conditions like air pressure differences and varying indoor humidity levels. These studies consistently show that installed performance can differ significantly from rated R-values.

So what’s being done about it?

According to Harris, the spray foam industry is taking a proactive role. Organizations like the Spray Polyurethane Foam Alliance and the Spray Foam Coalition are supporting a large-scale, third-party research effort known as the A-Value (Assembly Value) Study. Unlike R-value, A-value looks at the thermal performance of entire wall assemblies—not just insulation material tested in isolation. More information on the study is available at Sprayfoam.org.

Harris sums up the main drawback of the current R-value system simply: it only accounts for conduction, ignores air movement and moisture, and allows testing at temperatures that favor certain materials—like fiberglass, which performs best at a mean temperature of 72°F.

“The game is rigged,” he says bluntly.

As the industry continues to push for more accurate ways to measure insulation performance, Harris and Wyatt agree that better education and better metrics are essential. The goal isn’t just higher numbers—it’s buildings that actually perform better where it matters: in the real world.